How this calculator works
Heating one pound of water by 1°F takes 1 BTU, and a gallon weighs about 8.34 lb. The yearly heat needed is divided by each heater’s efficiency (UEF), then converted to kWh (3,412 BTU each) or therms (100,000 BTU each) and multiplied by your energy price.
BTU per day = Gallons × 8.34 × (Setpoint − Inlet °F)
Electric $ = BTU/yr ÷ 3412 ÷ UEF × $/kWh
Gas $ = BTU/yr ÷ 100,000 ÷ UEF × $/therm
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Hot water used per day | 50 gal |
| Incoming water temperature | 55 °F |
| Tank setpoint | 120 °F |
| Electric resistance UEF | 0.93 |
| Heat pump UEF | 3.5 |
| Gas UEF | 0.64 |
| Electricity rate | 0.182 $/kWh |
| Natural gas price | 1.5 $/therm |
| Result | Value |
|---|---|
| Heat pump (electric) | $151 / yr |
| Natural gas | $232 / yr |
| Electric resistance | $567 / yr |
| Heat delivered per year | 9.9 million BTU |
Step by step:
- Heat per year = 50 × 8.34 × 65 × 365 = 9,893,325 BTU
- Heat pump = 9,893,325 ÷ 3412 ÷ 3.5 × $0.182 = $151
- Gas = 9,893,325 ÷ 100,000 ÷ 0.64 × $1.5 = $232
Common mistakes to avoid
- Comparing efficiency labels from different tests — use UEF for all units.
Frequently asked questions
Is a heat pump water heater cheaper to run?
Usually — with a UEF around 3–4 it uses roughly a third to a quarter of the electricity of a standard electric tank.
Does lowering the setpoint save money?
Yes, it reduces the temperature rise. Many guides suggest 120°F for most homes.