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PV water heater simulator/design app


New Member
May 7, 2024
A few years ago I hooked some some panels to the bottom element of my water heater. It has worked so well, I wanted to make it easier for more people to try it. So made a simulator app that helps with the design of PV hot water systems. The simulator is basically PVWattsV8 with some water heater stuff bolted on.

You put in your info at the top and on the bottom you can view expected monthly production and graph the daily temperature swing of your water heater.

Any feedback about problems you have with this tool, or things I could improve would be much appreciated.


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I'd like to know how you calculate watts thru the day. Are you using theoretical power that can be obtained? For example, going to 2X the ideal resistance should greatly increase daily production.
The graphed power data comes from NREL PVWatts V8 library. PVWatts takes your lat/lng, azimuth/elevation, panel type, nominal array power at STC, and estimated loses. It returns a year of Insolation data and power estimates at 30min intervals and I process that data to make the graph.

The daily graph assumes you are using some kind of MPPT, so a mismatched heating element will not affect the power graph.
My theory about matching is just to try and get close enough for the MPPT to do it's job.

The MPTT thermostats I have looked at have all looked like buck converters, so perhaps a slightly too low resistance would be better then slightly too high?

EDIT: I found a single diode model that let me estimate off-MPPT power. Not yet sure how accurate it is. The model shows that MPPT is most useful on cloudy days, which tracks with what other sources say. The % and kWh do not line up exactly because I am including standby losses in one but not the other. It does show a surprising number of situations where adding one extra panel is more economical then adding MPPT.

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