The Gas Battery

I’m really hoping to install solar at my house this year.

I’m old enough to remember when “installing solar” might mean putting in a Trombe wall, or some glass boxes on the roof for domestic hot water. Still valid ideas, but in 2020, “solar” means just one thing: photovoltaic (“PV”). Panels covered with silicon that make electricity.


In rapidly descending order of awesomeness, the reasons I want this are:

Reason 1: I really want to do some research-y things with a solar array, to test out some ideas I have (and will be writing about at some point) for energy storage at household scale. I should clarify, the awesomeness is that I want to do those things. Whether I’ll get my act together and scrape up the motivation, energy and time to do so…well, let’s say worst-case, at least I wanted to. “Keep up the good intentions,” as they say. Anything beyond that is awesomeness gravy, in my book.

Most home solar arrays are “grid-tied”. This means they are always attached to the utility company, which supplies power as needed to make up any difference between what the solar array is producing at a given point in time, and what the house is consuming. A pure grid-tied system doesn’t have any form of energy storage, such as batteries. And a pure grid-tied system also has the disappointing property that, if the grid goes down due to a power outage at the utility level, your solar array shuts down too. That’s a mandatory safety feature, so utility company employees working on the system don’t find out the hard way that your solar array is pumping dangerous voltage into the lines.

You can install a manual disconnect switch of an approved type, that will let you physically disconnect your system from the grid so your array can safely power your house when the grid’s down (assuming it’s sunny, of course). I’ll want to have that disconnect available on my system so that I can do experiments part of the time, then go back to normal grid-tied mode.


The other main type of PV system is called “off-grid,” and as the name implies, it doesn’t ever connect to a utility grid at all. This is used for homes that don’t have electrical service and are electrically self-sufficient. An off-grid system uses batteries to store electricity for when the sun isn’t shining. Batteries are currently pretty expensive, but if you’re out in the boonies, running electric service to your house could be expensive too (if not impossible). There are also “hybrid” systems, which can be grid-tied part of the time, but also have a disconnect and battery backup in case of an extended utility outage.

Reason 2: PV systems have never been cheaper. First of all, prices of the panels themselves have been plummeting for years. (China and manufacturing improvements, mostly.) Prices of the ancillary components (panel mounts, inverters, wiring, etc.) haven’t fallen as far, but they’ve moved some.

Another reason 2020 is a great year for solar: rebates. There’s been a federal income tax credit for home solar since way back in 2005. From 2016-2019, the rebate was 30% of the system cost. It’s being phased out, so this year the credit is a still-respectable 26%; in 2021 in will be 22%; and after that, it’s gone.

So this year we’ve lost 4% of the federal rebate, BUT, I live in New Mexico, and our legislature passed its own solar tax credit this year, for 10%. So the net rebate in 2020 is 36%. That’s 36% of your system cost taken directly off your tax bill, making this possibly the best year of all time to buy a PV system1. (We can certainly hope that the panels will continue to drop in price and/or the federal tax break will be reinstated, but who knows?)

Reason 3: I hope to get grandfathered in on net metering. This is the last reason on my list, and maybe the least awesome, as I’ll now explain.

What is net metering?

Net metering applies to grid-tied systems, and is one answer to the question, “what if my solar array is producing more power than my house is using?”

The simplest solution might be to shut off part of your array so it doesn’t generate power. A panel in the sun that’s not part of a closed circuit will just sit there with a lot of energetic, very frustrated electrons, like a kid whose teacher says “he has so much potential but he’s not using it!” (terrible pun, sorry). This could be an actual switch, or more likely, a function of the control electronics for the array.

More commonly, the excess power is fed back into the grid, and bought by the utility, which will deploy it to other customers if they can. Originally, how this worked was that the power flowing into the grid from your array literally ran your electric meter backwards, directly reducing your meter reading and thus your bill. In modern times, the effect is similar but the meter might not actually spin in reverse. In fact, there might be metering in both directions, so the utility knows how much power you drew and how much you put back. Most utilities won’t pay you to be a net producer–you will never get a check, just maybe a zero bill. Also, some utilities will want to pay you for your juice at a lower rate per kilowatt-hour than they charge you for theirs.

If the incoming and outgoing rates are the same, it’s called “net metering”, because you pay for the net power you consume–what you draw minus what you put back. 41 states plus the District of Columbia have legally enshrined net metering2. (By the way, net metering applies to wind turbines too, but hardly anyone has one of those in their yard.)

So, let’s see how this works. It’s noon on a sunny Wednesday, and your solar array is cranking out the watts. You happen to be at the office (thinking post-pandemic here) and your house is empty and using hardly any electricity. So you’re selling power to the utility.

You get back home at 5:30 pm, as the sun is getting lower in the sky. You turn on the air conditioning, the TV, start cooking dinner, do some email on your laptop, turn on lights, open the fridge a bunch of times…your power usage goes way up. Meanwhile, your solar panels are shutting down for the day. Instead of selling power, you’re now buying power from the utility. Billing-wise, it’s all good; you got credit for that daytime generation and now you’re spending it.

But where does this power come from? It’s not coming from your neighbor’s solar; it’s just as dark over there. What’s happening, in most cases, is that your utility is ramping up the output of some natural gas-fired generating plants to meet the load. As the evening wears on, and through the night and early morning, they’ll keep burning the right amount of gas to provide the amount of electricity their rate-payers draw.

I call this The Gas Battery.

Your solar array–rockin’ as it is on a sunny day–is only good for about 8 hours out of every 24. Luckily for you, you’re plugged into the biggest battery in the world, which was built by your utility company, and runs on gas. And thanks to net metering, you get to use it for free! (Of course it emits a lot of CO2–it’s a fossil fuel, innit?)

A combined-cycle gas power plant

I promised myself I wouldn’t use the word “scam” in this post, but you’ve got to admit, it’s a pretty sweet deal, to say the least. Net metering won’t be around forever, and I’m hoping that those of us who get in now will be grandfathered in, and have the right to sell power to the utility when it’s easy to make, and buy it back at the same price when it’s hard to make, indefinitely.

And honestly, it would be unfair to call net metering a scam. When it started, solar panels were super-expensive, fragile and rare, the electronics to go with them weren’t ready for prime time, installers were new and fumbling, and only a few hardy souls would even consider solar PV. It’s great that governments stepped in to kick-start growth much faster than pure market forces would have. Net metering in the past is one of the reasons that solar panels are now cheap. But times have changed–what effect is it having now?

First off, putting on my social justice hat for a moment, it’s worth mentioning that the tax money for the rebates comes from federal and state tax funds. Let’s divide all taxpayers into two groups: those affluent enough to own a home and to afford the substantial up-front cost of a solar installation (or to qualify for a loan for one, which means having home equity), and everybody else. We could see the tax rebates as a transfer of money from the second group to the first. The first group is wealthier than the second, so there’s arguably a reverse Robin Hood effect here. Still, we might be OK with that if a greater good is served.

As for the climate impact, household solar is good up to a point. When the market penetration of solar is small–i.e. most of the utility’s generation comes from fossil fuel–and a reasonable fraction of the utility’s capacity is dispatchable (easy to turn off and on), then solar is a win, because when it’s sunny, the utility can distribute all the solar electricity among its customers, and turn down the dispatchable plants (almost always natural gas) accordingly. Less gas burned, less carbon emitted, all good. When it’s not sunny, they would’ve been using fossil fuel anyway.

The situation changes when the fraction of solar gets above a certain point. There might not be enough dispatchable generation to adjust to the extreme swing in solar output from midday to night. (Not all generation is dispatchable. Coal and nuclear are considered “baseload” sources; they don’t like to be turned on and off rapidly.) This could result in grid destabilization and power outages, which utility companies hate. They will forestall that risk, typically by building more dispatchable generation–more natural gas plants. Enlarge the gas battery. Which, aside from the fact that we don’t want to build more gas plants (if we’re trying to switch to renewable energy), means more carbon emitted when they are used. Now the situation has become complicated, and you would have to model the whole system to know whether the latest rooftop solar installation is carbon-beneficial, neutral, or even carbon-harmful.

The gas battery isn’t just for nighttime. I mentioned that utility companies hate power outages. That’s understandable, because the very second your power goes out, you hate the utility company, and curse their ineptitude. Maybe it just got cloudy, and everybody’s rooftop solar shut down at once. Not my problem! My phone needs charging! The power company gets no love. So of course they want to have enough dispatchable power that they can keep their customers happy even when there’s zero solar output and zero wind.

The other thing that happens when solar becomes a bigger part of the generation mix is called curtailment. That means you just start shutting off solar panels during midday, because there’s no customer for that power at that moment. Since due to net metering, the utility has to buy your rooftop power, whether they use it or not, what they’re likely to do is curtail some of the big commercial solar farms instead. This already happens regularly in California, which has built out a huge amount of solar (and now requires all new houses to include rooftop solar).3

So what’s the answer? Storage. The gas battery isn’t rechargeable; it releases energy (and carbon) stored over a 550-million-year period4. We need a way to store cheap, abundant solar energy that we collect on sunny days, so we can use it later that same day, or on cloudy days5. Without storage, all the solar panels in the world won’t help with our evening power binge.

This need for storage has some visibility in the renewables community, and there are a number of options, present and future. One of the better ones right now is lithium-ion batteries. They’re pretty expensive. Since energy storage and the intermittency of renewables is the main reason I have a blog, I’ll reserve further discussion for (I hope) many future posts.

Sadly, while some understand the storage issue, many more do not. Ironically, one of the biggest obstacles to faster adoption of renewable energy is that so many of its most motivated and engaged advocates believe that the technical problems have already been solved. So they think it’s now a purely social and political problem, and yell at the evil politicians and utility companies who they think are blocking the salvation of the world out of pure malice.

Well, a lot of the politicians are pretty evil, and the top managers of utility companies can be a short-sighted and greedy bunch too. But the people who actually run utility companies day-to-day are pragmatic engineers who are committed, above all else, to keeping the lights on. Yelling at them to put up more solar and wind farms–as if nothing else stood in the way of our dreams of carbon-free energy–is a waste of the scarcest of all resources: activist energy, not to mention credibility.

Meanwhile, in the depths of the night shift, when the only things running are refrigerators and traffic lights, the gas battery spins on, sending up its invisible plume, working for all of us.

Further reading:

As Rooftop Solar Grows, What Should the Future of Net Metering Look Like? ( “Amid the noise of competing proposals, a pattern is emerging: States are moving away from ‘net metering’ policies that require utilities to pay solar owners the full retail rate for excess electricity sent to the grid.” As is often the case, this otherwise detailed and helpful article does not mention the issues I’ve raised at all, glossing them over as “an aggressive push by utilities to reduce what many of them see as a form of competition that could harm their bottom line.”

COVID-19 will slow the global shift to renewable energy, but can’t stop it ( This has a nice, up-to-date chart of energy usage by source, which shows that natural gas and renewables are both growing share rapidly, with natural gas continuing to be a much, much bigger slice of the pie. Lots of good discussion on other issues too. Does a bit better than the other article, with at least a vague nod to “solar power plus some form of energy storage”. Some form, indeed–there’s the rub.

  1. I’m hoping these rebates won’t evaporate in the face of the COVID-19 pandemic, and the recent plunge in oil prices (New Mexico’s state budget is suddenly in peril due to lost tax revenues from the Permian Basin).
  2. This is an old reference, I need to find a newer one: How Net Metering Works (
  3. See for example California Renewables Curtailments Surge as Coronavirus Cuts Energy Demand (
  4. How Natural Gas Is Formed (
  5. More specifically, we need a way to store surplus electricity that allows us to get it back as electricity–not as heat or some other less useful form of energy.

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