Warm-up time and cost
- Warm-up cost
- Heat delivered
- Cost to hold temperature
- Warm-up cost
- Heat delivered
- Cost to hold temperature
Indoor temperature during warm-up
Each line shows how the home warms, minute by minute.
Worth knowing
Faster isn't always better
Heat pumps are designed for long, steady operation. An oversized system recovers faster but can reduce comfort and efficiency. Good sizing uses a room-by-room Manual J load calculation.
Setbacks work differently
With a heat pump, small setbacks of 2–3°F or a smart thermostat that starts early usually beat deep overnight setbacks, which can trigger costly backup heat.
Cold weather matters
A heat pump's heating power drops as it gets colder. Cold-climate models hold up much better, and the North Bay's mild winters work in their favor.
How these numbers are calculated
The home loses heat to the outdoors in proportion to the indoor–outdoor temperature difference, based on its size and insulation. Warming it up takes heat to raise the temperature of the air, furnishings and inside surfaces, estimated at 3 BTU per °F per sq ft. The calculator steps forward one minute at a time, adding the equipment's heat and subtracting heat loss, until the target is reached.
A furnace's useful heat is its input times its AFUE efficiency, and doesn't change with outdoor temperature. A heat pump's heating power and efficiency (COP) both drop as it gets colder, and between 20°F and 40°F it loses about 10% to defrost cycles.
Gas cost is the furnace's input over the run time at 100,000 BTU per therm. Heat pump cost is its heat output divided by its COP, converted to kWh. This leaves out duct losses, fixed utility charges, backup resistance heat, fan power and equipment warm-up.
About this planning tool
Is this a sizing tool?
No. It's an educational estimate of warm-up behavior. Equipment should be sized by a qualified professional using a Manual J load calculation and the manufacturer's performance data.
Why does my heat pump show it can't reach the target?
At that outdoor temperature, the house loses heat about as fast as the heat pump can add it. Real systems use backup heat or a larger, cold-climate unit for those conditions.
Why email my results?
It gives you a copy to review later, share with family, or bring to a conversation with an HVAC contractor.
About NRG Geek
NRG Geek is Robert Kelly's independent educational platform for practical Northern California guidance on solar, battery backup, EV charging, heat pumps and home electrification.
Privacy, legal and terms
How is my information used?
The Email My Results button opens your email app with your inputs and results. Nothing is sent until you send it. If you ask for personal guidance, Robert may follow up. NRG Geek does not sell your personal information.
Is this a proposal or guarantee?
No. Results are estimates for educational planning and are not a load calculation, equipment selection or guarantee.
Accuracy
Real warm-up depends on sizing, ducts, controls, sun, wind, air leakage, thermal mass, defrost and actual equipment performance. Verify important information before making decisions.
Copyright
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