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Size heat pumps to the load and to your solar system

Inverter heat pumps (including ductless mini-splits) move heat rather than making it, so they deliver several units of heat per unit of electricity. Off grid, the challenge is that the largest heating need falls when solar output is lowest.

How

1. Size each indoor unit from a room or zone load calculation, not a guess. PNNL warns oversized units short cycle, waste energy and control humidity poorly.
2. Read capacity from the manufacturer's expanded performance tables at your design temperature, not the headline rating.
3. For cold climates, look for units meeting the US ENERGY STAR cold climate criteria: a COP of at least 1.75 at minus 15 C (5 F). Heating capacity at that temperature must be at least 70% of capacity at 8.3 C (47 F).
4. Performance below that still varies by model, so read the maker's data.
5. In cold climates, choose units with published low-temperature capacity and mount outdoor units above snow height.
6. Give the power designer the winter daily kWh from the heat pump electricity calculator, plus the starting and running watts, so the battery and inverter can carry overnight heating.
7. Use surplus daytime solar to preheat the house and its thermal mass, then let it coast overnight.

Do these first

• power-1

Sources

1. Ductless Mini-Split Heat Pumps. Pacific Northwest National Laboratory, Building America Solution Center, accessed 2026-11
2. Cold climate heat pump sizing and selection. Pacific Northwest National Laboratory, Building America Solution Center, accessed 2026-11
3. Air source heat pumps: key product criteria (cold climate). ENERGY STAR (US EPA), accessed 2026-11

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