4 ms·

I have NEM2 solar, so obviously a heat pump is a big win. But I'm convinced the heat pump I installed a couple years ago would save money even without the solar

I have NEM2 solar, so obviously a heat pump is a big win. But I'm convinced the heat pump I installed a couple years ago would save money even without the solar: I pay $0.30-$0.40/KWh for electricity, and $0.08/KWh for natural gas ($2.35/therm) *. My heat pump has a COP of 3.62 when the outdoor temperature is 47F: it uses 1KWh of electricity to move 3.62KWh of heat into my home. The old gas furnace was 80% efficient ("AFUE"): it used 1KWh of gas to dump 0.8KWh of heat into my home. So, at 47F, as long as the ratio between the cost of electricity and natural gas is less than 4.53x (3.62 / 0.8), the heat pump saves me money. In my case, this means I save money when electricity costs less than $0.37/KWh, which it does almost all the time. At $0.30/KWh-electricity, I effectively pay $0.083/KWh-heat with the heat pump, a 17% total heating cost savings over the $0.10/KWh-heat the old gas furnace cost. Heat pumps do become less efficient as it gets colder outside: at an outdoor temperature of 17F, my heat pump COP is only 2.44, which would cost more than my old furnace ($0.123/KWh-heat vs $0.10/KWh-heat). Extrapolating linearly between the 17F and 47F COPs from the manual (it only gives those two points; this isn't strictly correct, but close enough), the temperature below which my heat pump starts to cost me more money than my old gas furnace is roughly 30F (3.0 COP). In the decade I've lived in the bay area, I've never seen it get that cold, which is why this is such a perfect climate for heat pumps. * These numbers are from August 2022

vladgur 3y ago
Is your $0.30-0.40/kwh rate based on your Solar cost? The non-solar rates[1] in Bay Area right now range starting from $0.42/kwh on tiered plan which you will blow through if you elect heat-pump heating and going all the way to higher than 50 cents per KWh if you are on one of the Time of US plan. [1]https://www.pge.com/assets/pge/docs/account/rate-plans/residential-electric-rate-plan-pricing.pdf.html https://www.pge.com/assets/pge/docs/account/rate-plans/resid... As much as i love the efficiencies offered by heat pumps, unless i splurge $10-$20k on a solar system with a battery backup, heat pumps are too expensive to operate with CA electrical rates. My gas(which i use for heating) during same period cost $2.54 per therm, although i dont know how to compare it to kwh for heating purposes. Update: I just checked my electrical bill for January. My total cost for generation and delivery of 478 kwh which is what my household used in 32 days, cost me $209 after fees and taxes which makes my rates around 44 cents per kwh on average.
- jcalvinowens 3y ago
  > Is your $0.30-0.40/kwh rate based on your Solar cost? It's just out of date from the math I did two years ago, I should have mentioned that. > heat pumps are too expensive to operate with CA electrical rates. It depends on your furnace. Typically it's going to be 80AFUE. $2.54/therm is $0.087/KWh-gas. If you pay $0.42/KWh-electricty, a heat pump with a COP >= 3.7 saves you money. Heat pumps with COPs above that at bay area temperatures are widely available. If you have a 96AFUE furnace, the necessary COP is 4.6. That's a lot harder to find: I'm no expert on the heat pump market, but it seems like the standard units are mostly 3.5-4.0. I can find mini splits up to 4.5 (like [1]), but they're more expensive. [1] https://www.homedepot.com/p/GREE-Sapphire-9000-BTU-0-75-Ton-Wi-Fi-Ductless-Mini-Split-Air-Conditioning-with-Heat-Kit-230-Volt-208-Volt-60Hz-SAP09HP230V1AK/303540384 https://www.homedepot.com/p/GREE-Sapphire-9000-BTU-0-75-Ton-...
bagels 3y ago
Converting from a gas furnace to a heat pump costs tens of thousands of dollars, needs to be factored in as well.
- jcalvinowens 3y ago
  It should absolutely be under $10K, unless you live in a big house or you need other stuff done. I don't think it would ever make economical sense to replace a working furnace: my pitch assumes the furnace needs to be replaced, and you're deciding whether to install another furnace or a heat pump.

vladgur 3y ago

Is your $0.30-0.40/kwh rate based on your Solar cost? The non-solar rates[1] in Bay Area right now range starting from $0.42/kwh on tiered plan which you will blow through if you elect heat-pump heating and going all the way to higher than 50 cents per KWh if you are on one of the Time of US plan. [1]https://www.pge.com/assets/pge/docs/account/rate-plans/residential-electric-rate-plan-pricing.pdf.html https://www.pge.com/assets/pge/docs/account/rate-plans/resid... As much as i love the efficiencies offered by heat pumps, unless i splurge $10-$20k on a solar system with a battery backup, heat pumps are too expensive to operate with CA electrical rates. My gas(which i use for heating) during same period cost $2.54 per therm, although i dont know how to compare it to kwh for heating purposes. Update: I just checked my electrical bill for January. My total cost for generation and delivery of 478 kwh which is what my household used in 32 days, cost me $209 after fees and taxes which makes my rates around 44 cents per kwh on average.

jcalvinowens 3y ago
> Is your $0.30-0.40/kwh rate based on your Solar cost? It's just out of date from the math I did two years ago, I should have mentioned that. > heat pumps are too expensive to operate with CA electrical rates. It depends on your furnace. Typically it's going to be 80AFUE. $2.54/therm is $0.087/KWh-gas. If you pay $0.42/KWh-electricty, a heat pump with a COP >= 3.7 saves you money. Heat pumps with COPs above that at bay area temperatures are widely available. If you have a 96AFUE furnace, the necessary COP is 4.6. That's a lot harder to find: I'm no expert on the heat pump market, but it seems like the standard units are mostly 3.5-4.0. I can find mini splits up to 4.5 (like [1]), but they're more expensive. [1] https://www.homedepot.com/p/GREE-Sapphire-9000-BTU-0-75-Ton-Wi-Fi-Ductless-Mini-Split-Air-Conditioning-with-Heat-Kit-230-Volt-208-Volt-60Hz-SAP09HP230V1AK/303540384 https://www.homedepot.com/p/GREE-Sapphire-9000-BTU-0-75-Ton-...

bagels 3y ago

Converting from a gas furnace to a heat pump costs tens of thousands of dollars, needs to be factored in as well.

jcalvinowens 3y ago
It should absolutely be under $10K, unless you live in a big house or you need other stuff done. I don't think it would ever make economical sense to replace a working furnace: my pitch assumes the furnace needs to be replaced, and you're deciding whether to install another furnace or a heat pump.