Best water heater for efficient small home with low usage?
I’m designing a small energy efficient guest home in Phoenix AZ and now I’m trying to find the “best” water heater for it. “Best” means as energy efficient as possible, but with some limitations and factors due to the house size and usage amounts.
It’s not as obvious as I’d like it to be.
Some more assumptions:
* Ground water temp is 60 F or so
* Only two people in the guest house
* Relatively low hot water usage — maybe 30 gallons per day at the very high end. Could be half that, most days.
If this was a full size house with a family then I’d choose a hybrid electric in a heartbeat. But the 50 gallon tank is massive overkill and the space requirements for the heat pump are completely fatal to a small home without notable ducting.
Maybe solar heat, since we are in AZ? Much too expensive! Solar water heaters are ungodly expensive and can take up a lot of space, to boot.
Electric tankless? I love the ultra small size and the fact that it can adjust to actual use instead of estimated use. But I’d likely need a 13 kW or even an 18 kW unit, which would be pulling on the order of 50 – 70 amps. I was planning on 50 amps total for the house! Plus we might be going 100% off the grid with solar in the future and a spike like that could be problematic. Some back-of-the-envelope calculations assuming $0.13 kWh costs on average suggests maybe $150 a year.
Propane tankless? Natural gas isn’t an option. I like it for the same reasons as the electric tankless, but now with no spike in electricity (more PV friendly). But… now we’re dealing with propane tanks and the wildly fluctuating costs of propane. My experience with propane is severely limited, too — essentially just the 20lb tanks for the grill. Is it worth the hassle of dealing with the tanks? My calculations with the same assumptions as above but with propane running $2.40 / gallon suggest $175 a year — similar to but slightly higher than the electric.
I’m unable to find propane water heaters with 20 gallon or similar tanks, so it’s difficult to include them in the mix.
And so now I’m down to a standard electric water heater with a 20 gallon tank. Trivial to setup; no electricity spikes; would definitely work. But I’d never even consider a standard electric heater in a normal case, so it’s weird to be considering it now. Since this is so small, though, maybe it wouldn’t cost very much to run. But…I’m also having a very hard time finding any reliable numbers on what it would cost per year to run one in this case. Online calculators give me numbers ranging from $150/year to $300/year — essentially making those numbers useless.
What do y’all think? Is there an obvious choice here that I’m just not seeing? Are some of my assumptions just flat out wrong? This choice can’t be this hard!
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