Third Year Results

Monthly Solar Production 2020

Hard to believe it’s been over three years since the system went online. Let’s have a quick look at the numbers for 2020. We cranked out 7,125 kWh last year, our best year yet and a 5% increase over 2019. The summer drought, which lasted until late September, probably had something to with that—definitely fewer gray days than normal.

We consumed 3,490 kWh on premises, a big jump from 2019 that can be attributed to the electric car we acquired last year. 3,635 kWh went to the grid, a significant drop from the previous two years again owing to charging the car battery. We saved $1,122.29 on electricity costs in 2020, bringing the lifetime cost savings to $3,306.46, or 20% of our investment. Depending on the price of electricity and other factors, we should recoup our investment around 2030.

Just like in 2019, May was the biggest month of production: 861.9 kWh.  As usual, December brought the least production at 295.1 kWh. Interestingly, production was exceptionally high in October, matching June and just behind August. October can often bring gray skies and rain, especially later in the month, but not so much in 2020.

As in past years the system required almost no maintenance—just monitoring and occasional battery checks. Very pleased about that.

Looking forward to another great year in 2021.

Lightning Protection for Rooftop Solar

lightning strike

Put a bunch of metal-framed objects on the roof, then connect them to the home’s electrical system, and the hazard of lightning becomes potentially quite serious.  In doing research I found  a wide range of opinions on this matter. Lightning is extraordinarily powerful and to some extent unpredictable–even experts disagree about the most effective mitigation measures. But after studying it I came away with two basic principles that guided my approach:

  1. When lightning strikes an object it will usually take the easiest path to the ground. If the path of least resistance to ground happens to be your array’s equipment ground conductor–or any conductors attached to your panels for that matter–then a massive surge of current could very easily damage or destroy expensive equipment and appliances. Worst case scenario, it could ignite a fire in your house. On the other hand, if you provide an easier path with less resistance directly to ground, bypassing the interior of your house, it’s much more likely the lightning current will take that route instead.
  2. Protect your equipment from current spikes with surge arrestors. Surge protective devices, or SPDs, make a good line of defense for valuable components like inverters or charge controllers. Lightning is potentially so powerful–up to hundreds of millions of volts–that no surge arrestor is 100% guaranteed to protect what’s behind it. But it’s better than having no protection and in most cases will do the job of absorbing/diverting the surge.

How big a risk is lightning? It depends. How prevalent is lightning in your area? And how exposed is your house?

You can answer the question of frequency with your own experience plus data. The map below shows the annual density of lightning by area throughout the U.S. Obviously if you live in Florida or Louisiana the risk is much greater than if you live in Oregon.

lightning density in the U.S.A.
This map shows the density of lightning in the United States from 2009-2019, based on the number of flashes per square mile each year.

Where we live in Maine, lightning frequency is moderate but we do get the occasional thunderstorm particularly during the summer. Our house isn’t especially exposed, and there are tall trees near the house (not too near) that make a likelier target than the house itself. But a strike to the house is certainly possible and a nearby strike could still inflict damage.

Guided by the principles above, I decided to take two mitigation measures. First, I installed a 4-gauge solid copper conductor from the rooftop array directly to ground. The conductor clamps tightly onto one of the metal rails supporting the panels, runs down the eave, then down to a copper ground rod driven into the earth several feet from one corner of the house. This ground rod is connected or “bonded” to the rods installed by our electrician at the service entrance–it’s important to bond all the ground rods together (in our case that includes another pair of rods at the location of the inverter and battery bank). This setup provides a direct low-resistance path for the lightning current to reach ground without entering the house/destroying equipment/starting a fire.

lightning conductor
A 4-gauge solid conductor runs from the rooftop array rack down the exterior of the house to a ground rod, providing a path of least resistance for lightning.

Second, I installed an SPD at the main service panel to protect against lightning strikes (or other surges) coming from the grid, and will install SPDs at both combiner boxes and at the inverter.  For the main panel I used a Siemens product called FirstSurge, which attaches to a knockout on the side of the panel and connects to a dedicated 20A circuit breaker on the main bus. For the other locations I’ll be using SPDs offered by MidNite Solar.

What does the National Electric Code have to say about protecting solar systems from lightning? This is addressed specifically in NFPA 780 “Standard for the Installation of Lightning Protection Systems.” Definitely have a good look at this. Rather than try to summarize everything here, I’ll simply highlight a few key recommendations:

  • install SPDs close to the array and electrical systems
  • install an SPD at the AC output of the inverter
  • ensure all equipment is properly grounded
  • enclose the PV output circuit in braided wire sheath, wire mesh screen, or bonded metal conduit
  • ensure all lightning conductors run separately and outside the cable path of PV output circuit

The latest NEC (2020 edition) talks quite a bit about “zones of protection”–although there seems to be some disagreement about this concept in a home solar application. I decided to stick with these two basic measures for now knowing that my system will be much more resilient if a lightning strike every occurs (knock on wood!).

Useful Resources:

https://www.solarinsure.com/protect-your-solar-power-system-from-lightning

https://www.solar-electric.com/learning-center/solar-system-lightning-protection.html/

 

How to protect your solar power system from lightning

Second Year Performance Review

Monthly Solar Production 2019

Time to run the numbers again: let’s check out the performance of our solar system during year two. In 2019 we produced 6,764 kWh of power, about 7% less than 2018. Spring and Fall were noticeably grayer in 2019, which likely accounts for the drop. We consumed 2,300 kWh of the power we generated on premises and fed the other 4,464 kWh of excess power production to the grid for others to use.

At the 2019 utility rate of 15.33 cents per kWh, we saved $1,036.92 on our utility bills in 2019. Combine that with 2018 savings and after two years we’ve recouped 14% of our investment.  At the current rate in 12 more years we’ll have saved as much as we spent on the system. But of course the utility rates will rise during that time so we’re probably looking at 10 years or possibly less, depending on the rates.

July was the biggest month in terms of power generated in 2019, compared to May in 2019 (as I said earlier, gray wet spring). March was also a big month last year, which makes sense given the bare trees (one of our strings is partially shaded by oak trees in the morning) and the cold weather, which noticeably boosts panel efficiency. March is also the month when the sun’s angle is optimal for our 45 degree panel orientation.

So far, so good. Here’s to a sunny spring this year.

Electric Cars and Home Solar

Bud Wheeler inspecting the front battery bank in his electric car.

When I was 10 my grandfather built an electric car out of an old 1970 Volkswagen Fastback, a modest little two-door that was a bit bigger than the Beetle. This was 1977, shortly after the Arab Oil Embargo, when many Americans were first starting to think seriously about just how valuable alternative energy could be. It could make the nation less vulnerable to oil price fluctuations or supply shortages, relieve us of the expensive and hazardous burden of “policing” the Middle East to protect our oil supply, and cleaning up our smog-filled air (climate change was not quite on the public’s radar yet).

Rear battery bank in Bud Wheeler's Electric Car.
The trunk was filled with another battery bank and electric relays. This is the area where the explosions occurred: sparks from the relays ignited explosive hydrogen gas produced by the batteries.

Grandpa loved working on cars and took it upon himself to build an electric. He used 6V golf cart batteries for power and built the whole thing out of various commercially-available parts. At night he plugged it into an outlet in the garage, then drove it to work in the morning. It had a range of 45 miles. The thing that always struck me was how quiet it was.

Bud Wheeler's Electric Car
Bud Wheeler made his electric car in 1977 from a shiny black 1970 Volkswagen Fastback.

It had some issues, including the occasional explosion in the trunk when hydrogen gas produced by the batteries would accumulate in the confined space and  ignite when one of the relays sparked. But it worked well and was a real testament to Grandpa’s ingenuity and resourcefulness.

Plain Dealer Profile of Bud Wheeler's Electric Car
Published in the Cleveland Plain Dealer on October 27, 1977.

And now, 42 years later, my wife and I bought a production electric car–the Nissan Leaf. It’s remarkable how far electric car technology has come since 1977, although at the same time it’s also kind of remarkable that it’s taken that long for electric vehicles to begin going mainstream. But that certainly seems to be where things are going. Certainly any serious climate change solution will require a significant shift away from gasoline to electric vehicles.

Nissan Leaf
Our new 2019 Nissan Leaf can manage about 150 miles on a full charge.

We always figured an electric car would be a great complement to the home solar system–after all, who wouldn’t want to stop spending hundreds or thousands of dollars on gasoline every year and instead “tank up” with energy you generate for free at home? While cutting carbon  emissions at the same time?

The reality, though, is that charging an EV takes more electricity than your typical home system can generate. Our solar array is relatively large for a home system at 6.4 kW but even at solar noon when it’s cranking at capacity it will be close to maxxed out satisfying the demand of a Level 2 EV charger (240V/30A). It’s possible to charge with a Level 1 charger (120V/12A) but that takes much longer and is  less efficient (energy wasted as heat during the charging process).

EV charging
The charging port on the Leaf accepts two different styles of charger plugs–shown here is the Level 2 charger that came with the car.

Since we’re grid-tied, however, that’s not really a problem–we can charge the car fully during the day using all the energy we generate and the grid will supply the balance. It still saves quite a bit of carbon–even if we only charged at night, using strictly grid power, the car would produce only 96g of CO2e (carbon dioxide estimated) per mile versus 381g for a gas-powered vehicle (https://www.ucsusa.org/clean-vehicles/electric-vehicles/ev-emissions-tool#z/04843/2018/Nissan/LEAF%20(40%20kWh).

EV charger
It wasn’t too difficult or expensive to permanently mount the Level 2 charger that Nissan provided with the car. You’ll need a dedicated 240 volt 40 amp circuit, which I installed myself in about 2 hours using about $50 worth of supplies. Just to be safe I had a licensed electrician look everything over after I finished the installation. This is mounted in an outbuilding that sits at the end of our driveway (a garage would be ideal but we don’t have one), so we just pull the car forward close to the building and attach the charging cable.

The big EV question, of course, is range…just how far will this thing take you before it poops out? Our Leaf has a 40kWh battery which Nissan says will average 150 miles (for $6K more you can get a 62kWh battery rated at 226 miles). We’re still testing and experimenting, but one thing that’s clear is your driving habits have an effect on your range (as do the conditions: wind, terrain, road conditions, etc). For example, the Leaf can actually regenerate power during deceleration–so rather than using your brakes when you pull up to a stop sign, you can switch “gears” and use the car’s momentum to put a little juice back into the battery. It also helps if you don’t drive like a 16-year-old boy trying to impress the girl sitting next to him!

Heating and air conditioning can also reduce your range–unlike a conventional car, the Leaf’s heat is entirely electric, so if you turn on the heated seats and steering wheel with the heating system on high, you’re going to drain the battery faster.

And the driving “experience?” That’s probably not the first criteria for most EV buyers, but the Leaf is no dog. Not exactly like driving a Porsche 911 (athough the chassis-mounted battery gives the car a nice low center of gravity), on the other hand it will get up and go when you ask it to and doesn’t plod up hills like a rented mule. I haven’t pushed it to its limits, but in everyday driving conditions–city, highway, or unpaved roads–it does just fine. You wouldn’t really know you’re driving an electric vehicle from the performance.

It’ll be interesting to see how it does in the wintertime. Cold temperatures reduce battery capacity, and I’m curious to see how it handles in slick road conditions with all-season radials (we may get snow tires if necessary).

It’s great to see electric cars coming into the mainstream–from everything I hear, they are the future and will be a major part of any climate change solution. Here in Maine EVs make up only 1% of vehicles on the road today, but that figure is growing. Rebates certainly help (between rebates offered by Nissan, the State, and the Federal tax credit for EVs, the price on ours dropped from $33K to $18K) but a lot more could be done to encourage people to buy EVs. Expanding the network of charging stations is key. Certainly as EV ranges increase with better battery technology, they will become more and more attractive to car buyers.

I wish Grandpa was around to see these new cars. I’d love to take him for a spin in the new Leaf! He’d be so impressed with how far things have come.