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Comment by opwieurposiu | Hacker News Reader

I put some used PV panels on my shed's roof and wired them up to the bottom element of my electric water heater. This cut my household electric use in half. Not connected to the grid so no expensive permits needed.

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Yeah I wish there was more of this happening. BTW what happens when your water is fully hot and the sun is shining - does that panel detect there is no load so doesn't generate?

Presumably over-shooting to some degree in the middle of the day is good, since it provides a cheap form of energy storage.

Obviously you'd want a cut off at some temp though, for safety concerns.

Yeah exactly 60 degree water will take your skin off.

There is no thermostat for the PV panels. The summer power output of the panels comes into equilibrium with the heat loss of the tank at 80-85C. I did add a 93C thermal fuse to augment the safety of the PTV value.

I have a mixing valve on the output of the water heater to mix with cold down to about 50C. This lets you get around 2x the nameplate capacity of hot water out of the tank so there is enough to bridge a cloudy day.

To get the full capacity of the panels, it necessary to choose a water heater element that matches the source impedance(Vmpp/Immp) of the solar panel array.

Sounds neat! I'd like to read a writeup on your build if one exists.

I dunno if this the O.P. but it's very similar.

https://www.youtube.com/watch?v=r7g-27AXFEg

Sorta related if you don't want to hack that sort of thing yourself -

Hybrid Hot Water Heater Saves 69 Percent On Energy Consumption:

https://russell.ballestrini.net/hybrid-hot-water-heater-save...

> Instead of using 4,500 watts for 45 minutes (.75 hours), a hybrid hot water heat pump uses 350 watts for 3 hours.

I saw video of a guy installing one in his off grid house. He was mostly stoked with it because it only drew about 300W. But had a problem because it would do a check of the heating elements on startup which would crowbar his inverter. He fixed that by installing lower wattage elements.

I watched the same video to inform myself prior to the purchase, if I remember correctly he was off grid in Hawaii.

If your into small scale Food Production checkout my channel: https://youtube.com/c/RussellBallestrini

My house inverter is grid tied so I have a backup for the start up spike in energy.

I was thinking to do this but I had some concern about electrolysis, as electric water tanks are designed for AC, not DC.

But I never learned if this was a real concern or not.

In north america both terminals of the heating element are galvanically isolated from the tank, so this is not an issue if the wiring is installed with due care.

In countries like brazil and germany that allow "bare wire" heating elements you might have trouble with the tank lining plating onto the element or visa versa.

Thank you! I will check the regs for my country.

There's a slew of DC powered electric water heaters. Relatively common on RVs and travel trailers.

Interesting, mind if I ask the total cost for the system you made?

The materials cost was about $1400, half of that is the panels the rest is wire, conduit, brackets, etc. A solar permit in my area costs $1200+ . The high permit costs would make a system this small uneconomic. Local codes vary but in my jurisdiction I could avoid need for permit by putting the panels on my shed instead of my house and using the power for the water heater instead of connecting to the grid.

The payback is 3-6 years depending on what ratio of peak/off-peak power pricing you assume.

very cool, thank you. Payback is also hot water when the power goes out :)

This is would be fairly inexpensive to do. DC heating elements are < $40 and used panels can be had for ~0.20$ a watt or less.

You'd wire the PV panels directly to the heating element. (Of course you'd have to size your element to match the volts/amps of your panels)

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