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Comment by megaman821 | Hacker News Reader

If you have excess generation during the day, why don't you cool your house to 72 in the late afternoon and then back to 78 when the sun starts setting? It would probably delay you batteries having to power your AC for about 4 hours.

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This. Energy can take many forms. You don't have to store all in a battery.

Another idea: assuming you have an electric water heater, when your battery has been fully charged and you still have excess generation, use the excess to heat your water heater up to a very high temperature like 80 degrees C. You'll then be using very little hot water in the evening for showers and dishwashers, etc, and the water heater basically won't need to turn on at all.

> use the excess to heat your water heater up to a very high temperature like 80 degrees C

Wouldn't that be quite dangerous? That's basically instant scalding temperature if anyone accidentally turns on just the hot.

You can get a mixing valve that's installed at the heater. Very easy, then turn the heater up really high, they are only like $15-20 and will effectively double or triple your hot water capacity.

e.g. https://www.watts.com/resources/references-tools/thermostati...

Interesting, I didn't know those existed. Seems like it makes a lot of sense.

It also seems like if you simply leave it on a higher temperature 24/7 it will use more power due to thermal losses overall. This is a great hack for households with several people who like to take long showers (and thus run out of hot water), though, or if you can do some sort of variable temperature setpoint to store energy as described in the thread, though I think at least in the US electric tank-based hot water heaters are relatively uncommon.

Do not install these if you have hard water, they will clog up extremely fast - and the hotter your hot water is, the faster it will clog up.

Which is especially amusing considering this is PG&E territory. The bay area water was so hard you could make limestone castles by rinsing a glass.

There are new 120v heat pump hot water heaters that operate in this way:

https://cleantechnica.com/2021/11/29/120-volt-heat-pump-wate...

"120 volt heat pump water heaters come with a mixing valve integrated into the water heater itself. This mixing valve allows owners to keep their water at a higher temperature and as the water leaves the tank, the valve will mix “in the exact amount of cold water needed to ensure the water comes out of the water heater at the correct temperature.”

Make sure you adjust your faucets so they won’t go full hot. Don’t want to scald someone.

I've been thinking about this. Do you/anyone happen to know if it's a decently efficient use of the stored energy?

I love the idea of our hot-water boiler acting as battery (potentially along with another actual electric battery).

I have a handle on the hardware, but not on the efficiency of it.

Thanks,

Technology Connections does a similar thing, over-cooling his house at night as a thermal battery.

Video: https://www.youtube.com/watch?v=0f9GpMWdvWI

Reddit discussion: https://reddit.com/r/technologyconnections/comments/oo2b3i/b...

He estimates it at 16 KWh of storage. Same ballpark as a Powerwall, right?

And/or add more thermal mass to the house.

We do this here in Sonora, CA. Our PV array supplies more than our A/C uses. We turn the A/C on around 10am, cool to 70°, turn off at 3pm. House insulation has limited indoor temperature to 76° on recent 101° days.

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