Should I pull out solar hot water in favor of solar electric

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New Member
May 9, 2024
Northern New Hampshire
I put in SHW about eight years ago and it's been okay (water heater failed and even on warranty it cost me about $2k for labor) but I am thinking about putting in solar electric and I need to decide if pulling the SHW out so I can get more solar electric panels is worth it. It's just my wife and I and my guess is that SWH is saving about 100 gal of oil a year (oil this year averaged $4.25/gal).

I can get seven more solar panels on the roof if I pull the SWH out but then I'll have to pay an HVAC guy to do it and then get an electric HW heater. My guess is $3k for that.

Seven panels will generate about 3,300 more kwh. I'm not sure how to do the calculation but it doesn't seem that 3,300 kwh would come close to the 100 gallons of oil.

One way to look at it is, how many kwh would I have to generate (at $0.21/kwh) to equal $425 (the cost of the oil). Each panel produces about 500 kwh/yr, or 500*.21=$105 so four panels is about the right number and since I can get 7 up there, it might be worth it (ignoring the cost of pulling SWH out)

Can anyone shed some light on this for me?
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I have a 20 plus year old SHW system and get free hot water for 5 months a year and preheat well water the other months. If its working SHW system I would be hard pressed to swap it out for something else. The only reason they are no longer in favor is the initial capital cost. If you do take them down, they are worth a few bucks for scrap copper.

One thing to factor in is NH (and all the other states) will be offering big rebates on Heat Pump Hot Water Heaters the COP for a HPHWH is in the 3 to 4 range so it is going to require far less power to heat hot water than with a standard hot water heater.
Don’t spend another dime on the HW system. It’s not worth spending that kind of money if it’s still working. I’d have the inverter sized like I was going replace the HW panels. (Don’t know if this is a good idea but it seems reasonable.)

You will want a heat pump water heater eventually or even now. How may panels can you fit keeping the HW panels? Have you run the numbers assuming you could make say 5% annual rate of return on what your actual out of pocket is on the installation? Down here I its just hard to justify that kind of upfront cost with a breakeven roi that about 15 years out.
I would agree with EbS-P - unless doing solar now gets incentives that might be absent once the solar HW is past its lifetime (given that the presence of the HW decreases the amount of PV panels that can be mounted).

In my case I had good net metering when I installed my PV with many years no resetting of the kWh bank. If I would like to add something (panels or battery) now, that would cancel my previous netmetering agreement and revert to whatever is the current one, which is less good for me.

So, I would look at the net metering you get now with your PV panels, and if that is really good, then I would consider taking the HW off and fill the roof up with PV - IF you indeed need as much PV that it warrants making space for it.
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Thanks for the quick answers. Seven panels if I don't remove the SHW and 15 if I do. That's the calculation. At its simplest, the question is, should I remove the SHW (at an expense of about $3K) just so I can get another eight solar panels on the roof? (Yes, I will put in a Heat Pump Water Heater). Will the eight panels make up for the loss of hot water?

I have room on an adjacent roof for 16 panels so keeping the SHW I can get just about 100% of my annual usage so I am leaning to keeping it but I wanted to check in with the experts for their two cents.
If you can get about 100% of annual usage while keeping the SHW, I'd keep it until it dies. It's the essence of free...

If it dies you can always do a heat pump water heater then, and consider adding panels at that time to make more kWhs for that HPWH (though see my remark about changing net metering agreements over time).
I havent done the calculations, but I have seen several individuals say that 2 standard sized PV panels on the roof will cover the additional electric demand for a heat pump hot water heater. Unless someone has a good handle on their hot water demand, coming up with an estimate is the equivalent of throwing darts. A rule of thumb is an oil boiler uses 1 gallon of oil per day during non heating periods to supply hot water. I think a lot of this is wasted heat but its s start. The best way is install a temporary water meter on the hot water supply of the house for a few months.

You also need the temperature rise but if you are pulling from a deep well you can assume 45 F for the supply and 140 F for the demand.

The other unknown is what utility do you have in NH?. We used to have great net metering, but the state PUC diluted the benefits. You now need to build up a lot more credit in the summer to ride through the winter. They basically buy power during the summer from the solar owner at low summer rates, charge some taxes and build up a dollar credit. Then in the winter when you need it, they sell you back the power at high winter rates with some extra taxes thrown in.