And here's the initial post (which seems a bit less reasonable, since I'd spent less time learning about what was going on):

Given current trends in technology and policy, solar panels seem like the easiest way to make clean electricity (and soon the easiest way to make energy at all). I’m interested in thinking/learning about what a 100% solar grid would look like.

Here are my own guesses.

(I could easily imagine this being totally wrong because I’m a layperson who has only spent a little while looking into this. I’m not going to have “I think caveats” in front of *every* sentence but you should imagine them there.)

Overall I was surprised by how economical all-solar seems. Given 10-20 years and moderate progress on solar+storage I think it probably makes sense to use solar power for everything other than space heating, for which it seems like we should probably just continue to use natural gas. I was surprised by how serious and isolated a problem space heating seemed to be.

Other forms of power like nuclear or fusion might be even better, but it feels like all-solar will still be cheaper and easier than the status quo and won’t require any fossil fuels at all. Issues with storage would create big fluctuations in the price of electricity, which would change the way we use and think about electricity but would not change the basic cost-benefit analysis.

ETA: feels like the main problem is if there's variability in how dark winters are and some of them are quite dark. This is related to the heating issue, but might be a big problem even for non-heating needs.

1. Night vs day

It looks like the costs of overnight energy storage will eventually dominate the costs of solar, but still be low enough to easily beat out other power sources.

For example, the cost of a Tesla powerwall like $400/kWH; they can be cycled 5000 times under warranty. If you did that every night for 15 years it’s a total cost of $0.08/kWH stored. The cost of battery storage has fallen by a factor of 3 over the last 5 years and seems likely to continue to fall, and I expect utility-scale prices to also fall to keep up roughly with batteries.

Here are some cost projections of $400/kWH in 2020 falling to $200/kWH in 2030: https://www.nrel.gov/docs/fy20osti/75385.pdf. Here is a description of historical costs that finds them falling from $2150/kWH to $625/kWH in 2018: https://www.eia.gov/todayinenergy/detail.php?id=45596. Overall it looks to me like the $200/kWH looks pretty realistic

(ETA: I now think that forecast is probably pretty conservative and $100/kWH or less is more likely. But the rest of the post doesn't depend on such aggressive estimates, except for the part where I talk about heating.)

The efficiency of storage is 90%+ which is high enough to not matter much compared to the cost of storage, especially as solar prices fall.

Current electricity prices are around $0.10/kWH. So at $0.08/kWH solar couldn’t be quite competitive, but another factor of 2-4 could easily do it (especially if other costs of solar continue to fall to negligible levels at their current very rapid clip). I haven’t seen anyone projecting batter prices to plateau before hitting those levels.

Overall the trends on storage are worse than on panels themselves; it’s already the biggest cost of an all-solar grid and I think it would just become totally dominant. But they still seem low enough to make it work.

Storage is a lot cheaper if you are using some of your electricity directly from panels (as under the status quo) and need to store <100% of your power. You’d only need 100% in the worst case where all solar power arrives in a burst at noon, and the real world isn’t going to be quite that bad.

I could easily imagine cutting this down to only needing to store 50-75% of electricity, which cuts the cost with current technologies to $0.04-0.06/kWH. I think cutting costs in this way would be important in practice, but given that we’re only talking about a factor of 2 it’s not going to make a big difference unless battery costs plateau in the next few years.

Meaningful amounts of solar are only available for ~1/3 of a day (depending on latitude) so if you just used energy constantly and wasted nearly half of the solar power you’d need like 66% storage (depending a lot on latitude and season). Today we have a *lot* of appliances that use electricity a small fraction of their life and that you can run when electricity is cheap; and also most of our needs are when humans are awake and the sun is up. But if you switch to using electricity for more applications and if we industrialize further (as I’d expect) then this will get less possible. Amongst the things that can be shifted, you might as well put them at the peak hours around noon, which helps cut peak losses. (For things like cars and computers with batteries, you can either think of these as flexible appliances or as further reasons.)

That’s a complicated mess of considerations. 50-75% is a rough guess for how much you’d have to store but I’m not at all confident.

Eventually it will be unrealistic to amortize battery costs over 15 years. That said, 30 year interest rates are currently 1% and I think time horizons are changing pretty slowly, so I expect this trend to be much slower than changes in storage costs.

2. Winter vs summer

My impression is that solar panels give like 33% less power in winter than summer (obviously depending a ton on latitude, but that’s a typical value for populated places). Storing energy across seasons seems completely impractical.

That sounds like a lot but even in the worst case it only increases the cost of solar power by 50%, since you can just build 50% more panels. That doesn’t seem like enough overhead to make a fundamental difference.

Most importantly, this doesn’t increase the number of batteries you need. You will have enough batteries to store power in the winter, and then in the summer you will have a ton of extra production that you can't store and so use in some low-value way. So if batteries are the dominant cost, you don’t even care.

I think this is the main answer, and the rest of this section is gravy. But as in the last section, the “gravy” could still cut costs by 10% or more, so I think people will care a lot about it and it will change the way we relate to electricity. So also interesting to talk about.

Here are some guesses about what you could scale up in the summer:

* You can run some machines only during the summer, e.g. if the main cost of your computer was the cost of running it (rather than capital costs) then you might as well scale down your datacenters in the winter. Of course, you probably wanted to move that kind of machine towards the equator anyway where electricity prices would be lower.

* You could imagine literally migrating your machines to wherever the power is cheap (e.g. moving your datacenter to the other hemisphere for the winter). This sounds a bit crazy but I wouldn’t be at all surprised if it works for a non-negligible % of energy. Perhaps the simplest case would be moving vehicles and having people travel more during their summer.

* There are lots of small tweaks on the margin, e.g. using 10% more energy during the summer than you would if electricity was constant-price and using 10% less energy during the winter. You can do everything a bit slower and more efficient when it gets colder.

These things are interesting to think about—I like the image of a civilization that hums to life during the summer days—but it doesn’t seem like it changes the calculation for feasibility of solar at all. You could just totally ignore all of this and pay a small premium, from 0% (if batteries dominate anyway) to +50%. Some of these changes would happen from the most cost-conscious customers.

3. Other fluctuation and peaking power

In addition to winter/summer and night/day there is also some variability from weather day-to-day. My impression is that this is a smaller deal than the other two factors and doesn’t change the calculus much for a few reasons:

* As discussed in the last section, you probably want to have too many panels anyway and be bottlenecked by storage. In that case, variability in weather doesn’t matter much since you have too much capacity most days.

* It only really potentially matters in the winter months with already-low light, where you may fall short on total production (and not even be able to charge batteries).

* But once you are talking about a small fraction of the year, it’s pretty cheap for some people to just turn off the power (e.g. scaling down my datacenter) from time to time. If I’m doing that for 5% of the year it effectively increases capital costs by 5%, which is only really a problem if electricity costs are a tiny fraction of my net expenses. And there only have to be a few industries that can do that. So we only have a problem if the all-solar grid is serving every industry exceptionally well (in which case the absolute size of my problem is quite small).

There are also fluctuations in demand. In general having variable demand seems like it’s probably good, since by passing on costs appropriately you can shift demand to times when power is available. But big exogenous changes could cause trouble. This seems to be by far most troubling at the scale of days rather than hours (since you have a ton of batteries to handle within-day variability).

I think most people can avoid huge fluctuations in demand most of the time---there just aren’t that many things that bounce around from day to day where I have very little control over when they happen. The big exception I know about is climate control—people want to use a lot more power for AC during hot days and for heating during cold days (if we move away from natural gas for heating).

AC isn’t a problem, because it happens during hot summer days when you have tons of extra power anyway. So that brings us to...

4. Heating

Heating during cold periods seems like a big problem. As far as I can see it's the biggest single problem with an all-solar grid (with all-electric heating)

Unfortunately, I think heating is a giant use of energy (at least in the US). Right now I think it’s almost half of home energy use, mostly natural gas, and I’d guess something like 10-20% of total energy use in the US.

It's also the worst possible case for solar. It’s seasonal, which is already bad, and then there is huge variation in how cold it actually gets. And it’s really bad if you aren’t able to keep things heated during a cold snap. In practice you should just stop other uses of electricity when it gets cold. But with an all-solar grid you aren’t going to be putting many energy-intensive activities in places with cold winters, so you may have less cheap slop than you wanted and blackouts from cold could be quite expensive (even if you literally aren't having people freeze in their homes).

Here are some options:

* Use peaking power plants basically just for heating. It’s crazy to me to imagine the world where this is the *only* reason you actually need peaking power plants. I suspect you don’t want to do this.

* Use natural gas to heat homes. This is appealing because it’s what we currently do so doesn’t require big changes, it’s pretty clean, and you don’t need to maintain a bunch of peaking power plants with significant efficiency losses in transit. I think the main cost is maintaining infrastructure for delivering natural gas.

* Do something creative or develop new technologies. In some sense heating is an incredibly “easy” problem, since anything you do with electricity will generate heat. The problem is just getting it where you want to go. You could move more electricity-consuming appliances into homes/offices you want to heat, or do cogeneration with data centers, or something else crazy.

Here are some reasons the heating cost may not be so bad, so that you may be able to just eat the costs (for any of the above proposals).

* If we are doing a lot of electricity-intensive industry then space heating may be a much smaller fraction of costs. Honestly, I think this whole discussion is mostly relevant if we want to scale up electricity use quite a lot, but I don’t expect to scale up space heating in the same way. So I think it would be reasonable to keep meeting our heating needs in a primitive way while scaling up an all-solar grid for our increasing energy needs.

* You could massively improve insulation over the status quo if heating costs were actually a big deal. Right now heating is a huge fraction of energy but a much smaller fraction of costs. Under an all-solar grid the energy for heating would be by far the most expensive energy, and so incentives to save on heat would be much larger.

* We could generally move to hotter places. They get more appealing as AC gets cheaper / heating is more expensive, and I’m told global warming will make everywhere a bit hotter.

* We could probably modestly improve the energy efficiency of heating by using heat pumps Unfortunately it’s kind of hard to improve efficiency in any other way. And heat pumps are pretty scary since they don’t work when it gets really cold.

Overall my guess is that you should just leave natural gas infrastructure in place, especially in cold places, and use solar for everything else.