Estimate how long a battery bank can run a tank heater, and optionally a ballpark wattage from tank gallons and a watts-per-gallon freeze-protection guide.
Use this for remote tanks on DC heaters or inverter setups when you know heater watts and usable battery watt-hours. The gallons guide is a rough sizing check — not a heat-loss model for every wind chill.
Insulation, lids, and thermostats cut runtime demand sharply versus a heater that runs continuously.
Runtime hours = usable Wh ÷ heater watts. Usable Wh from a battery ≈ Ah × volts × (DoD% ÷ 100).
usable_Wh = Ah × V × (DoD% ÷ 100)
hours = usable_Wh ÷ heater_W
guide_W = gallons × W_per_gallon
Lead-acid often plans ~50% DoD; LiFePO4 often ~80–90% — follow your battery specs.
100 Ah 12 V lead-acid at 50% DoD → 600 Wh usable. 250 W heater:
Runtime = 600 ÷ 250 = 2.4 hours continuous. A 100 gal tank at 2.5 W/gal suggests ~250 W guide size.
Assumptions: continuous full-power draw; no inverter inefficiency (add 10–15% loss if inverting).
Limits: watts/gallon guides ignore wind, insulation, and ambient extremes. Thermostatic heaters cycle and last longer.
Responsibility: use GFCI/electrical codes for AC heaters; size wire and fusing for DC loads.
Enter Ah, volts, and DoD — the page updates usable Wh. Or type Wh directly and ignore the helper.
Divide runtime by inverter efficiency (e.g. ×0.85) or reduce usable Wh by ~15% as a quick allowance.
Only if average power × hours fits your battery and solar recharge. Insulation and a thermostat matter more than peak watts alone.