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I get that it makes less efficient resistive heating more economical, but the tank size seems like a red herring. If you go for a heat pump you can have a larger tank anyway, and heat it during the solar peak too. A larger tank will allow you to store water at a lower temperature (instead of hotter water to be diluted). Smaller delta makes heat pump more efficient and heat loss slower.

by penteract10 hours ago|

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To check I understand you: the smaller tank with heatpump would consume less energy outside the time window in which energy is free than the large tank with resistive heating, but has a higher capital cost which would outweigh the amount saved on energy?

If that's right, it's not obvious to me that building a suitably sized solar panel is environmentally worse than building a heat pump.

by stubish9 hours ago|

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The smaller tank with heatpump will consume a lot less energy than the larger tank with resistive heat.

Economically to me, the larger tank is cheaper, because the appliance is cheaper, and I never pay for the power it uses.

Environmentally, yes, it is not obvious. The large tank requires many more solar panels to power it but no battery. The small tank and heatpump needs much less solar but battery for nighttime use.

But it is weird, because for decades heat pump tech has been pushed as the environmental choice and there are still a number of government subsidies to invest in heat pump hot water systems. And maybe that no longer makes sense, with the money saved buying cheaper and less efficient devices spent on more solar deployments.

by hnaccount_rng8 hours ago|

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But it’s also environmentally better for you to take the resistive heating thing. As long as you never need to heat it up outside of the noon-window that’s strictly positive. Because the “additional” solar panels will be necessary anyways to cover the night/late evening usage. The optimal buildout will always have superfluous energy at noon. That’s fine. We just need to get over the whole “energy costs anything” thing. That’s only true if you need to spend fuel to generate it

by Panzer046 hours ago|

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The magic of market pricing means people will figure out the best solution and optimise towards that.

Hot water heater tanks are easily one of the most obviously good applications of noon excess energy, and resistive heating elements might as well be free.