Path-Q Solar
How should a heat pump's backup heating source affect solar sizing?
Answer
Solar sizing should reflect the electricity used by the heat pump and the expected operation of its backup source. If the backup is electric, cold-weather or emergency operation can materially increase electricity use; if it is fuel-based, the home's electric load may be lower but total heating energy still needs to be considered separately.
Key points
- Identify whether backup heat is electric resistance, fuel-based, or another type.
- Estimate heat-pump and backup operation under representative cold-weather conditions.
- Do not size only from mild-weather heat-pump performance if backup operation is likely.
What it depends on
- Heat-pump capacity and efficiency
- Climate and design heating conditions
- Backup source and control setpoints
- Building insulation and heating demand
- Whether backup heat is used routinely or only during unusual conditions
Risks and trade-offs
- Ignoring electric backup can underestimate winter electricity use.
- Sizing for rare extreme events may create excess solar capacity relative to normal consumption.
- Changing backup settings can affect comfort, equipment operation, and peak electrical demand.
Information you may need
- Heat-pump model and operating data
- Backup heating type and capacity
- Heating bills or fuel-use history
- Expected indoor temperature and weather conditions
- Planned building-efficiency improvements
Your next step
What to do next
Obtain a seasonal heating-load estimate that separates heat-pump electricity from backup-source use, and add backup consumption to the solar load forecast only to the extent that the backup source uses electricity.
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