A Perfect Day for Net-metering

Last Friday was a great day for solar power, so great that our house and habits had no way to use the 31.6 kWh generated by our west-facing 7.5 kW solar array.  Charging the car at 1 am only used about 1/5th the power we generated.

Even if we'd started the day with our home batteries empty, we still could not have used all the power we generated.

Fortunately, our solar array is grid-tied and can push the extra power we don't use to our neighbors. Thanks to our net-metering agreement with the utility, our neighbors got to use clean solar power for the afternoon, and we banked the power credit for use on laundry day.

26 families in South Miami received thousands of dollars from the IRS for installing solar panels

I always believed there was no such thing as a free lunch...

until I discovered the Residential Renewable Energy Tax Credit. https://www.energy.gov/savings/residential-renewable-energy-tax-credit
If you put solar panels on a house you own, the IRS will write you a check for 30% of the cost.  

Twenty-six families in South Miami have already taken the free money by installing solar power.

The 30% deal is good for solar systems purchase through the end of 2019, then it’s 26% through 2021, 22% through 2022, then zero.  

The solar system pays back its cost through your electric bills, and keeps paying dividends after that.  https://www.somisolar.com/2019/03/solar-home-economics-day-3-off-grid.html

Even if you have to take out a home equity loan or PACE financing to purchase the solar system, you still make money on the deal.  If you don't have enough income to use the tax credits in one year, they keep rolling over.

We got $5k back on our photovoltaic array, and another $5k on the home battery system.

I’ve heard three excuses for not getting solar:

1) “We have too many trees.”
OK, if your roof is in full shade, that one’s legit, though you can trim the trees a bit.

2) “First we have to fix the roof.” 
So, do it already, before the tax credit expires!  
We had to replace our roof.  We got our roofer to coordinate with our solar installer so the roof was solar-ready.  If your roof needs replacing you should do it before a hurricane does it for you.  You’ll save money on windstorm insurance at the same time.

3) “We’re not sure we’re going to stay in the house long enough.”  
Research shows that you’ll get the money back when you sell a solar house that has no electric bill.  https://emp.lbl.gov/publications/selling-sun-price-premium-analysis

Anyone owning a sunny roof who hasn't take advantage of this tax credit before it runs out is leaving money on the table.  Why would you do that when your neighbors are enjoying the hefty refunds and cheap renewable power?

A successful week off the power grid

I wanted to see if solar + batteries could run our house during a prolonged power outage, so a week ago I turned off the main power switch between the electric meter and our house. 

Simple answer: yes.  

More complicated answer: I want to try it again during the summer when the A/C is running more and the afternoons are cloudier.

With an energy-efficient, 1600 s.f. house, 7.5 kW of solar panels, and two Tesla Powerwall batteries, we did not have to compromise on our use of electricity.  Solar + batteries kept the electric car charged and let us machine-wash & dry the insane number of towels that resulted from three college kids spending much of their spring break at the beach.  

We have never needed to do that much driving or laundry following a hurricane.  My only car trip following Hurricane Irma was a run to South Dade to buy a replacement part for my Honda generator.  The part was back-ordered because everybody else needed the same part.  I mail-ordered the part, so if you want a super-quiet Honda 2000i inverter-generator that's been converted to run on propane, make me an offer.

After a hurricane, we'd be grateful to run the fridge, a few lights, charge the phone, make a cup of coffee for breakfast, and run the air conditioner.  We coveted air conditioning at night, wishing we could close our windows and not be forced to hear or smell the noisy, stinky generators that surrounded us.  Solar + batteries can run our central A/C in style.  

I showed in a previous post that rooftop solar is currently the most reliable and profitable investment you can make, even better than the house it powers.  The average American home-owner doesn't have many opportunities to make money while saving the planet, but solar is one of those.  Solar scores #10 out of the top 100 ways to stop climate change listed by Drawdown.org.

Back on the grid

Last night, I turned the grid power back on.  I have mixed feelings about going back on the grid.  I enjoyed a week of energy independence from our local investor-owned utility; the same utility that has tried for years to run massive power lines through the middle of South Miami; the same utility that sought PR firms to oppose South Miami's solar ordinance; the same utility that spends millions to suppress roof-top solar; the same utility that pollutes the the Southeast coastal Everglades and Biscayne Bay with their hyper-saline cooling canals; the same utility that forces their maintenance costs onto the public in the form of elevated post-hurricane repairs; the same utility that wants to roll back net-metering; the same utility that spends less assisting low income customers than any other urban utility in the nation; the same utility that is building new gas generators when they should be going all-in for renewable power; the same utility that's seeks to extend the operating license on two aging nuclear plants situated on a heavily populated peninsula between two national parks; the same utility that has asked the county to let them cover a biologically productive lake (best fishing around) with solar panels - that's their idea of solar deployment, to fuck-up something biologically valuable, excuse my French.  

So yes, it felt good to turn off the grid power last week, and I felt a twinge of remorse when I clicked it back on last night.

Grid-tied solar has environmental benefits

As an environmentalist, however, I can say it's best to run solar connected to some kind of grid.  Being grid-tied allows us to share our surplus solar power with our neighbors during peak load times, a practice that displaces more carbon pollution than storing our power onsite, at least while most people are depending on polluting utility power.  

The long axis of our house is oriented north-south, so we have long roof surfaces facing east and west.  We chose the west face for solar installation because it shifts power generation into peak load time, that begins in the late afternoon.  Utilities have to make extra power at that time, and the "peaker" generators that supply the peak demand make the most carbon pollution.  By net-metering at the end of the solar day, we can do more good for the planet than if we kept our power onsite for personal use at night.  That's the environmental benefit of net-metering, and to us, it's worth the monthly connection fee we pay to remain on the grid.  Plus, grid power is good back-up to solar power, and net-metering should make our backup batteries last a very long time. 

But, if Florida's investor-owned utilities ever succeed in killing net-metering, we'll turn off the power switch for good and stop paying our electric bill.  Now we know it's possible.  

Is it legal to go off the grid?

People sometimes ask me "Aren't you required to be grid-connected?"  Yes and no.  The government does not want to facilitate squalor, so a house requires utility hook-ups to get a certificate of occupancy.  On the other hand, if you don't pay your utility bills, the utilities will turn off your service at some point.

Thanks Brad

Let me end the week with a word of thanks to our solar muse, Bradley Stark.  Brad was my first friend around these parts to get solar on his house, and he encourage all of his friends to do the same.  He got an electric car and encouraged his friends to do the same.  He signed up for batteries and encouraged his friends to do the same.  Brad provides me with moral support during the dark hours of politics.  He adopted a cat, though he is allergic to cats.  Turns out even an outdoor cat appreciates the environmental benefits of solar panels.

Batteries cost less than a whole-house generator (+ Day #6 off-grid)

After the last hurricane, lots of people told me they'd get solar if they could use it in lieu of a whole-house generator.  I thought I'd use this post to explore the economics of solar + battery versus the whole house generator powered on "fossil fuel" (I hate that term - it's fossil hydrocarbon).

We've been watching the Tesla Powerwall batteries in action this week.  To find out about whole house generators, I consulted my generator gurus, Walter & Rosalie Goldberg.  I remember their generator coming on automatically moments after Hurricane Katrina knocked out the power to their house during dinner.  They insisted we spend the night, but clever me, I insisted we could make it home through a tropical storm (that was the forecast), and proceeded to drive home through a tree-wrenching hurricane so we could feed the dog.  A memorable drive it was.

The Goldbergs currently have a 20 kW air-cooled Generac, with 200A service, and 4 load-shed modules.  The generator is installed on 4” concrete pad with a 1000 gallon buried propane tank out back.  Cost was $15,000 including installation and the first load of fuel.  Liquid cooled generators much quieter, but also cost more.  The Kohler generator costs a bit more than the Generac and has much better customer reviews, but was unavailable where they currently live. 

Fuel cost

Liquid propane is the preferred fuel for on-site reliability.  An 800 gallon refill costs $2400.
The generator runs 20 minutes a week for maintenance purposes, and the fuel cost for that comes to $2.43/week, ~ $125 per year.  Running the house on propane costs $175/day.  If we assume a 10-day outage every 10 years, that's an average of 1-day emergency operation per year for $175/year.

Service cost

Failure to provide adequate service is why so many generators died in the days following the hurricane.  Installer Pat Porzio says, "After 24 to 48 hours of continuous use, get it serviced. After around 10 days, have a professional change the oil and the filter.”
A maintenance plan is essential to keep the generator running. A good plan includes immediate service if something breaks.  Our friends pay $325/ year. Parts & labor are extra, so add another $150/year.  Plus the propane to supply the weekly maintenance runs.

Cost summary

Total upfront cost $15,000
Yearly maintenance $600
Yearly emergency use $175

So one starts with a $15,000 generator and puts in an additional $7,750 over 10 years.
The 10 year cost of owning and maintaining a whole house generator is $22,750.

Price comparison: whole house generator vs. solar + batteries

We saw in a previous post that residential solar is a great financial investment under net-metering and the current tax credit (or break-even without the tax credit).

You don't buy house batteries to make money, but if you wish to keep the house running during a power outage, it's either batteries or the generator.

3 Tesla Powerwalls, have a combined output of a 21 kW generator, equivalent to the Generac in our example above. The batteries cost $23,275 installed, or $16,235 after the 30% tax credit.  That's significantly less than the 10-year cost of owning a whole house generator.

The Generac has a 5-year limited warranty, compared to 10 years for Tesla Powerwalls.  

Tesla Powerwalls should last 20-30 years if you only use them as emergency backup, whereas online reviews show generators lasting 5-10 years.  Do your own math on replacement costs.

Economics alone greatly favors the house batteries over the whole house generator.  Unlike lithium batteries, the generator also produces noise, bad smells, and air pollution (social cost of carbon!), and requires oil changes, fuel deliveries, and will likely needing servicing or fuel after a storm when you are depending on it.

Batteries beat the whole house generator hands down.

Day 6 off-grid

The batteries topped-off their charge and the battery controller turned off the solar array. A technician from Tesla convinced me that the Level 2 charger would work, so I plugged the car into the Level 2 charger and it charged up the car pronto, with the sun providing the largest contribution.  In the late afternoon, I tried out all the appliances to see how much each one drew.  I'm getting bored.

Power use under the microscope (+ Day #5 off-grid)

Here are the electric appliances we've used this week off-grid, and their instantaneous power consumption.  Obviously, the longer an appliance runs the more power it consumes.
Multiply kilowatts (kW) by hours to get kilowatt hours (kWh).

Power demand by appliance:

6.7 kW - Level 2 EV charger
6.0 kW - Clothes dryer
2.6-3.1 kW - Central air conditioner (more on that below)
2.3 kW - Microwave oven
1.8 kW - Dishwasher (startup only, drops to 0.1 kW during wash)
1.0 kW - Level 1 EV charger
0.8 kW - Toaster
0.7 kW - Freezer compressor for refrigerator (Energy Star). Intermittent.
0.6 kW - Clothes washer on spin cycle
0.2 kW - Lights in the evening (LEDs of course)
0.2 kW - TV
0-2.0 kW - demand water heater. Power draw depends on temperature of the water in the solar water heater tank.  Zero if the water’s already hot.

We have a 1953 Chambers stove that runs on propane.

Our batteries

The two Powerwall batteries together provide about 27 kWh of use capacity. 
Even with intermittent light clouds, our solar system generated 27.2 kWh (see below).

The two Powerwalls together can provide 10 kW continuous output, or 14 kW peak.  That's enough that we don't really have to worry about what the family turns on, provided we get some sunshine to recharge the batteries.  One of those 3-day gloomy periods might put the kibosh on heavy use of the clothes dryer.

Level 2 EV charging works!

Today I discovered, that despite my earlier impression to the contrary, we can indeed run Level 2 charging of our Nissan Leaf EV from the house batteries.  Here are screen shots from the phone app during Level 2 car-charging on batteries alone, and on 90% solar power:

Batteries: whole house or just critical load?

We went with whole house, in part to prove it could be done, but ultimately because two Powerwall 2 battery units is the minimum Tesla will install for retrofits.  Also, two Powerwalls are necessary to start up our A/C.  Our house has a 3 ton, 16 SEER, 2-stage, central A/C to cool 1550 s.f. under air.  We keep the house at 79°F, mostly to limit the humidity.  Tesla installed a “Sure-Start” soft-starter device http://www.hypereng.com in our A/C compressor cabinet to reduce power demand when the A/C starts up.  If you are remodeling, consider installing mini-split A/C units to reduce the power requirement - people love them and they're very efficient.

One could make do with less battery capacity if one restricted battery use to supply only “critical loads”.  First decide what you need to use during an outage; for instance a mini-split A/C in the bedroom, the fridge, medical equipment, telecom equipment, and maybe the EV trickle-charger and microwave.  For sure not the clothes dryer or a Level 2 EV charger.  Batteries can be configured to operate a separate critical load panel that powers just the critical devices.  It's less expensive than backing up the whole house.

Day #5 off-grid

Again the house ran fine with no power from the grid.  Despite some light clouds, it was a productive solar day.  I restored the charge in the battery that was depleted by the crazy-heavy use on laundry day.  I could have charged the car, but it had enough power for the next day so I decided to hold off in lieu of filling the house batteries.

How to go solar (+ Day #4 off-grid)

From time-to-time I get cold-called by a stranger who wants to sell me a solar system.  I wouldn't buy anything from a cold-call, much less something with the price and complexity of roof-top solar.  

The good installers stay in business under the same name, and stand behind their work.  They guarantee the roof against leaks caused by their installation, which they can well afford do because a careful, professional solar installation does not create roof leaks.  They do not call random strangers on the phone.

Rooftop solar now costs less than $3/watt installed.  We've seen prices on large installations at less than $2/watt.  The price will vary with the hardware installed, the type of roof, and the size of the system.  Batteries are extra - a lot extra.  Installers also give discounts for group purchases.

The solar co-op makes group purchasing easy

A couple of years ago, local solar aficionados were celebrating the great and glorious defeat of the deceptive solar amendment pushed by FPL and Florida's other investor-owned utilities.  The utilities's sneaky amendment was designed to thwart residential solar, and it went down in flames once the voters learned of the ruse.  The League of Women Voters (LWV) called a meeting to promote the idea of creating a solar purchasing co-op program in Miami-Dade County through a national non-profit called Solar United Neighbors, www.SolarUnitedNeighbors.org. The LWV asked for volunteers to make it happen.  

Coconut Grove resident Jody Finver stuck up her hand because she wanted to devote her every waking hour to promoting solar in Miami-Dade County.  

I stuck up my hand because I knew where to get the money to hire someone like Jody.  I sit on the board of the Green Corridor, an interlocal group that includes Cutler Bay, Palmetto Bay, Pinecrest, South Miami, Coral Gables, Miami, and Miami Shores.  We run a financing program for energy efficient home or business improvements, and we had a little money in our do-something-good account.  We agreed to fund the solar co-op program.

After interviewing applicants, Solar United Neighbors hired Jody as the first ever coordinator for the Miami-Dade Solar Co-op.  They had to - she's amazing.  Her taller half, Simon Rose, is pretty wonderful too (photo credit Chevrolet).  That's their house.

Jody sets up solar purchasing co-ops in Miami-Dade County several times a year.  Look for these at www.SolarUnitedNeighbors.org/Miami.  Dozens of local organizations have officially endorsed our solar co-op program, including local governments and local environmental organizations.  [If you live outside greater Miami, look at Solar United Neighbors main website for co-ops elsewhere].

The solar co-op costs nothing to join and there's no obligation to purchase.  What you get is the experience and hard work of Jody and the organization.  They put out RFPs (requests for proposals) to regional solar installers.  They know how to write the bid specifications so you don't have to.  The bids might include various options such as high-efficiency panels or batteries.  At some point the co-op closes to new members and the process continues forward.  The entire co-op membership decides democratically what criteria are most important (e.g., duration of roof-warranty, price, experience, etc.).  A group of members volunteers to be on the selection committee that reviews the bids.  The committee evaluates the bids based on those criteria and selects ONE installer.  At that point, any co-op member has the option of signing a contract with the chosen installer.  

If you already own a house and you want to add solar or solar+batteries, I always recommend to GO WITH THE CO-OP.  That gets you a good price and a professional consumer advocate (e.g., Jody or local equiv.)  to assist you with the process.  If you are building a house, and can't work around the schedule of the co-op, I can recommend some reputable installers for you to interview yourself, some of which offer "South Miami pricing".

Jody says to tell you...  
"Last information session for the current Miami co-op is March 19th - sign up online.  The current co-op is open to new participants through April 19th.  Also, 2019 is the last year for the 30% Federal Tax Credit.  In 2020, the Tax Credit steps down to 26%. In 2021, it steps down to 22%, and in 2022, it’s gone."  [...unless we can convince the Congress to extend it]. Any tax credit obtained but unused can roll forward to the succeeding tax year.

What about the fabled Tesla solar roof tiles?  As of today, Tesla's solar tiles still have not been certified for the Florida market, which requires a crazy-high wind-rating.  The tiles super-strong, but certification has not happened yet and production is still scaling up.  Tesla recommends people in Florida get conventional solar panels.  I don' t expect that the serial connection of solar roof tiles can finesse the problem of partial shading, whereas several solar inverter systems for conventional panels can accommodate partial shade without compromising productivity of the whole array.  If you have trees nearby, partial shading is an issue to consider.

Batteries are not needed unless you want to use solar power in lieu of a whole house generator.  
In the power outage that followed Hurricane Irma, approximately half of the whole house generators in South Miami failed. Some were repaired fairly quickly, but others needed back-ordered parts.  One family would up in the hospital with carbon-monoxide poisoning because they'd installed their generator too close to the house, without pulling a permit.  The one in this photo stayed functional because the owner had a good service contract that tested it every month.  A neighbor's generator developed bad bearings that squealed terribly.  For two nights we prayed that it would fail, and on the third night our prayers were answered and we were able to sleep with the windows open.

For these reasons, many people would prefer solar+batteries.  

Solar systems have come a long way since Jimmy Carter put them on the Whitehouse lawn. Prices fell exponentially, but now have leveled off.  By contrast, home and car batteries are at the beginning of a revolution.  Expect big improvements in the technology and falling prices for the next 40 years.  Still, who can wait 40 years?  Let's get on with it!  Several home battery systems are available at this time, and more are coming along.

The solar double-whammy: clouds on laundry day

If it's sunny on laundry day, I can break even with solar, but not if it's cloudy.  No big deal if we're on grid, we just borrow a little power from the surplus credit we get from net-metering. But if we're off-grid and run out of power, I can't run next door and ask the neighbors for a cup of electrons.  Well I could, I suppose, but they'd just say "Maybe you should just turn the power back on."  My neighbors are sensible like that.

I started the day with a 56% battery charge.  Five loads of laundry ran through the clothes washer and dryer while I was in meetings.  How did we manage to generate five loads of laundry?  We've had five people in the house this week, and some of them have spent their days at the beach (extra towels). We could line dry everything if we circled the yard twice round with clothes line.  Then it would rain for sure. 

From time to time, I peeked nervously at the energy flow.  At 11 am, the battery was down to 36%, even as the sun rose higher in the sky.

Then at 12:20 pm it happened - clouds rolled in. 
I'd feared this.

Could the house make it through a cloudy day running the freakin' clothes dryer non-stop?  If we had not fully charged the car the day before, at least we could have started the day with nearly full battery.  Remember that for next time, but too late now.

The sun played peek-a-boo until sunset when it left us for California.  Clouds also shade the rest of the house, so at least the A/C takes a bit of a rest on a cloudy day.  If I were a nail-biter, I would have bitten my nails.  I was steeling myself for the ridicule I'd get if the power ran out because I'd been too ambitious.  I'd tell them "Hey, that's what experiments are for, right?  We could use this evening to express our solidarity with the folks in Venezuela."  (Sure, Dad.  How about you turn the power back on so we can use the internet this evening to read what's happening in Venezuela and you can resume your experiment tomorrow?).

The next morning I woke up (on fresh sheets), groped in the dark for my phone, and saw the battery reserve was down to 13%.

I briefly operated the toaster and the microwave in the course of making breakfast.  Nobody else was ratting around the house using a hair-dryer or an arc-welder.  The three college kids here for spring break prefer not to rise with the neighbors' chickens (surprise).

At 8 am, dawn's rosy fingers tickled the solar panels and the batteries began to recharge.

It was a squeaker, but the solar house had made it through the biggest challenge of the week.  We should have easy coasting from here, even with some clouds.

Solar home economics (+ Day #3 off-grid)

If you have a house with a sunny roof and no solar panels, then you are leaving money on the table.

One can look at home solar economics two ways:

1) If I credit my monthly electricity savings toward paying myself back for solar, how long until my energy is free?

• It takes 7-8 years to pay off the system and get free electricity.

2) If I amortize my purchase and maintenance costs over the life of my solar system, what is my annual rate of return on the investment?

• Internal rate of return on capital invested in rooftop solar is 14%. (wow!) ** 

These figures are at the solar prices available today through the solar purchasing cooperative (more on that in another post), and with the current federal tax credit.

Because I also invest for retirement, I look at it the second way.   Solar on the roof is equivalent to 25-year Treasury bonds with a coupon rate of 14%.  I'd put my entire retirement account there if I could.  That’s what rooftop solar is, except that we are limited in how many such “virtual bonds” we can buy – our maximum is set by our electric bill.  

Of course, if you had an electric car, you could expand your “virtual bond holdings to cover the increased charging demand.  That would give you a 14% annual return for never buying gasoline.

The Residential Renewable Energy Tax Credit currently returns 30% of the installation cost for solar power and for batteries charged exclusively with solar. Details and phase-out schedule are found here:
https://www.energy.gov/savings/residential-renewable-energy-tax-credit

If you were buying a new house and had to pay off the solar system on your mortgage, you’d still pocket money at the end of the year.  For a modest 1500 sf house, you would save an extra $811 every year. **

** calculations by Philip Fairey, Deputy Director of the state of Florida’s Solar Energy Center (FSEC) in at the University of Central Florida.

Energy report from day 3 off-grid

Monday, 11 March was another successful day off-grid.  The air conditioner ran regularly, we charged the car, and various home appliances operated normally (microwave, freezer, toaster, etc.).  

Tomorrow is laundry day, with clouds predicted for the afternoon.  That sounds like a challenge: will the batteries still have enough power left to make it through the night?  The rest of the week looks like intermittent sun & rain.  Stay tuned.

A Solar-powered Car (+ Day #2 off-grid)

You could have a really nice solar-powered electric car.  
If you had solar power.  And an electric car. 

Electricity production accounts for 28.5% of America’s greenhouse gas production.  Transportation accounts for another 28%.  With solar-powered buildings and electric vehicles (EVs)  we could reduce our climate-killing air pollution by 56%.  I'm there. 

Ranking slightly behind saving the planet, is fun.  Modern EVs are way more fun to drive than the 20th Century gas-guzzlers we grew up with.  EV’s can accelerate a lot faster than gas cars, scary-fast in the case of Teslas.  EV’s are almost silent, so your music sounds better and conversation is easier.  No exhaust pipe, no exhaust -- I have driven back from the Home Depot with long objects sticking out the back hatch without sucking fumes into the car.  Maintenance is checking the tires, adding windshield washer fluid, and replacing wiper blades.

If you have a charger at home, you don’t have to contend with lines at the pump before a hurricane.  And, as I’m discovering, with home batteries, you can charge your EV when the grid is down.

An EV's range depends on the size of the battery and how you drive the car.  The power used in one mile of highway driving might cover 2 miles on surface roads.  Drive gently and that can become 3 miles.  If you’re taking a long road trip, today’s EV’s will plan the route with restaurant stops adjacent to fast-chargers.

EV chargers come in 3 flavors: slow, fast, and superfast.

A “Level 1” charger typically comes with the car and plugs into a 120 volt AC outlet.  Level 1 is also called a trickle-charger.  That’s what we’re using during our off-grid test. Yesterday’s trickle-charge took a bit over 5 hours.  Most people just plug-in when they get home and forget about it.

A “Level 2” charger runs on 240 volts AC, like an electric clothes dryer.  Level 2 charging is 8X faster than Level 1, and is 5-10% more efficient in its use of power.

The newest EVs can accept a Level 3 charger.  Cars made by Tesla can use their network of Superchargers.

Here's a visual comparison of my Level 2 charging (on-grid), and Level 1 charging (off-grid):

Our setup

We had an electrician wire two 40 amp circuits into our carport ($800).  One of these powers a Level 2 charger with a 25’ cord (Clippercreek HCS-40, $650 retail or $500 eBay).  The long cord is convenient.  It takes 45 minutes to re-charges the Nissan Leaf after my 17-mile FIU commute.  If we run the battery to near empty, charging takes 2 hours. 


When we’re on-grid, the car is programmed to charge at 1 am (as shown above).  This avoids drawing utility power in the early evening generated by the less-efficient “peaker” generators that service the peak demand time.  It reduces our carbon footprint by allowing us to push solar power onto the grid during peak demand time, while we draw back the most efficiently-produced “base load” grid power as Miami sleeps.  

We also set the car to charge to 80% capacity, which prolongs battery life and is likewise more efficient to charge.  We top off the battery for convenience prior to longer runs.  I’ve only needed to charge away from home once after someone (possibly me) forgot to plug in the car the night before.

Our second car is a 2010 Prius hybrid, which averages 50 mpg.  It’s a fine car, but burning gasoline feels so 20th Century.  We will never buy another gasoline-powered car.  

Report from day 2 off-grid.

Everybody always asks if the solar+batteries can run the A/C.  Yes.  You can see in the graph below that the A/C happily turned on 11 times yesterday (sharp spikes in the blue graph). We have a 3 ton A/C that covers the indoor space just fine.  Tesla's battery installers wired a "soft start" device into the A/C that eases the surge when the A/C turns on.  It does the job.  

The Powerwall controller had a temporary disagreement with my solar inverters at 1:35 pm, though I did not lose power.  Tesla’s technical staff just emailed me saying they are downloading a software modification to my Powerwalls that should resolve the issue. 

Other than that, everything worked as well as the day before.

First full day off-grid

Saturday, 9 Mar, 2019 was our first full day off the grid. 

I had decided to charge the car from the house batteries the evening before.  That consumed 6.2 kWh of battery charge.  

In the morning when I woke, the batteries were at 46% charge.  Had I made a big mistake charging the car from batteries?  Not this time.  The batteries resumed charging as the sun rose, and were at 75% by noon. 

At 1:20 pm, the batteries reached full charge, and shut off the solar system.  
You read that right, the battery controller shut off the solar system. A good thing too.

Why does off-grid solar turn off when house batteries reach full charge?

Here’s an important difference between being on-grid and off-grid.  When you’re on-grid and your batteries are fully charged, all the extra solar power you generate, beyond what house is using that instant, gets pushed onto the grid where you run a net-metering credit.  But what happens to that solar power when you are off-grid and the batteries are fully charged?  The battery controller system tells the solar inverters to turn off, so the batteries don’t over-charge. At that point, you and the planet miss out on that extra solar power you could have been generating.  You can get back in the game by turning on some appliances and depleting the battery a few percent.  

To capture the solar power we were losing because of full batteries, I ran a load of laundry.  Turns out the clothes washer doesn’t use much power, so the solar system turned on and off, and on and off, etc.  But when the laundry was clean, I threw it in the clothes dryer, started a new load in the washer, and plugged in the car.  That did the trick.  I ran down the batteries some and I got to hang onto the last rays of sunlight that hit my solar panels before the sun dipped behind the trees.

Here's a visual analysis of the day's solar & battery budget:
(It took me a while to make this figure so I hope you enjoy it)

OFF THE GRID

“This is only a test”

We are going off-grid for a week to see if our solar+battery system can provide house power during a simulated 1-week grid failure such as we might experience following a hurricane.  We will try to live like we normally do, and learn by trial and error what adjustments we have to make to live within our battery capacity.  

6:15 pm, Friday, 8 March 2019, main breaker for FPL power is turned… OFF.

Here's a video of me turning off the switch for grid power.

(why do video's always choose the most awkward-looking frame to freeze?)

We are off grid!

That was fun.  Should I plug in the car now as the sun is dipping or wait until the sun is up and the solar array is producing power?

Efficiency says no - it's inefficient to charge batteries with charged batteries.  

Convenience says yes - we will want to use the car tomorrow, so we'd best charge it tonight.

Let's favor convenience (this time, anyway) and plug in the car under the premise that we're going to try to live a normal life and compromise only where we have to.

Car-charging on battery power works OK on the Level 1 charger (regular house current, 120 VAC).

The sun is down, the grid is off, and all the power is coming from the batteries.  The car charger is drawing 1.4 kW and the house is drawing another 0.3 kW for a total draw of 1.7 kW.

We had dinner on the patio this evening: grilled salmon, pasta with cilantro pesto from the garden, and green beans from the garden, salad, ice cream and strawberries with Cointreau, and good company.  Load the dishwasher and push the button.

It all works so far.

What we will cover this week.

• Details of our solar system and how we got here.
• Solar home economics.
• The best way to go solar.
• Solar powered cars.
• Laundry day (not sure how that’s going to work).
• Appliance usage under a microscope.