8 years ago, I proposed that pool pumps really should be designed to make use of solar (or wind) power. We have now seen used solar panels show up at the ridiculous price of $50 for 250 peak watts.

What this means is a solar powered pool pump could do most of the pumping with solar for what it costs for just a few months of electricity. These are not high quality panels. They are so cheap that it would reasonable to hand them on the north fence of the pool area even though vertical hanging is less efficient, though the roof would be great. (Hanging on a south facing fence means getting only 60-70% of what you get from optimal tilt on a roof, but at this cost it could be a win, though sadly performance is worst in summer.)

(Panels could actually be designed for vertical mounting, with the cells mounted on a sort of staircase platform which tilts the cells close to latitude while making the hanging structure just a few inches thick. One could make an adjustable angle frame which eager people could actually adjust a few times a year for even better efficiency.)

With solar this cheap, and getting cheaper, we face the problem that the other costs of solar (mounting, labour, wiring, permits, inverters, grid-tie) now dwarf the cost of the panels. At 20 cents per peak watt, the other costs are now an order of magnitude greater. The big opportunity comes in finding ways to eliminate these costs with easy self-install or basic contractor install.

This means you want loads you can power without grid-tie. As soon as you grid-tie you immediately need a whole level of special gear and permits. You are playing with dangerous stuff.

You also want loads that can deal with intermittent power. Pool filtering is ideal for that. You don't care when the water is filtered. You don't care if the filter stops when a cloud goes over. You just care that you get enough filtering to keep the water clean, on average. It's OK to filter more one day and less the next. There are few loads that are large and meet that criterion, particularly in the home. Charging EVs would meet the criterion except you want to disconnect the EV often during the day to drive it places.

All you need is a pool pump that can take the power directly from the panels by DC, and filters at whatever the sunlight can provide. When it's sunny it filters at a good speed. When it's night or raining it doesn't. It also turns out that you need more filtering in summer and less in winter, which corresponds well to how solar panels provide output, though less so for vertical ones on a fence.

The pump would also be able to switch over to the mains, ideally to add to the solar, but more simply just temporarily disconnecting them. You would use that when running a pool vacuum that needs high flow or other special operations. You would also do that if the system detected the sun just didn't provide enough filtering in recent days (and when forecasts say the sun isn't coming tomorrow.)

Because it would just switch power, or combine them at the DC level, the system would never feed power back to the grid. It would be unable to. Low-level filtering only needs about 350 watts typically. The electrical wiring would be simple and safe, on the order of a basic extension cord.

Panels could be nailed on a roof, hung on a fence, or just put out on spare land for those with large lots, in that case tilted to latitude.

Pool owners are often spending $30-$60/month in electricity for their pool pump. The panels and pump modifications (which should not add very much to the cost of a pump) would pay for themselves in months, not years. Even an add-on box on the power circuit to a modern smart pump could do the job.

The closest thing you can get to this are floating pool filters with solar panels. These aren't very popular, because they need to be big, and even then, can only do about 1/4 of the filtering job. It's a nice idea, self-contained, but ideally you want 20 square feet of panels, which is too much to float in a pool. Also you can't have a pool cover as easily.

(As I noted in the prior article, wind is also a perfect match for pool filtering -- no surprise since using wind to move water is ancient stuff -- but wind prices have not fallen through the floor like solar has.)

A new era of solar -- too cheap to be optimal

We now enter an interesting era of solar, where panels are so cheap that it is wrong not to waste them. What matters with solar is the cost of the whole system over a period divided by the kwh the system will deliver in that period. So even if vertical panels only deliver half the electricity of properly roof mounted ones, that's still a green win if you save more money not going on the roof than it costs to buy extra panels. If you have to throw away 2/3rds of your power to avoid grid tie, but it's 80% cheaper to not grid tie, it's a win.

That's because what matters is how many kwh you unload from the dirty grid per dollar you spend. That's the way to be green, to remove the most fossil from the grid with the money you have.

That may seem at odds with what I have written about solar panels on cars. They are wasteful (horizontal, not always in sun, useless when battery is full) but they are also expensive (need robust environment.) On the other hand, they don't need grid tie. But if you already have grid tie, then more panels on the roof or in a solar farm do much better at kwh taken off the grid per dollar.

Other loads that can use solar

Another load that could use solar is any constant load, like computer servers. With an always on device, you just need a power supply that takes any solar power it gets and supplements any shortage from the grid. When the sun is high, the device will be running entirely on the solar. At night, it will run entirely from the grid. If the panels match the load at peak, you use 100% of their power. Again, no grid tie, there is never surplus. This could also be done for all the always-on parasitic loads of a house, but it's hard to reach them all. Most of the loads in a house are intermittent, and you care about when they are on or off. To use those with solar you need either grid tie or batteries, and these are now the most expensive part of the system.