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Battery Runtime Calculator for Boondocking

By Kyle TiceUpdated October 24, 2026Runs in your browser; nothing is sent anywhere.

Loads

Amps are typical 12 V draws from manufacturer specs and common measurements; edit any value to match your gear. Inverter loads already include an 85% inverter efficiency.

OnLoadAmpsHours/dayAh/day
Furnace blower (while running) –
12 V compressor fridge (running avg.) –
Absorption fridge control board (on propane) –
LED lights (per 6 fixtures) –
Water pump (while running) –
Roof vent fan (MaxxAir, medium) –
CPAP, no humidifier (via 12 V adapter) –
CPAP with heated humidifier –
Starlink Standard via inverter (~60 W) –
Laptop via inverter (~50 W) –
12 V TV (24–32 in) –
Phones and tablets (USB) –
Inverter idle draw (if left on) –
Phantom loads: detectors, stereo memory, tank monitor –
Battery bank

Common capacities: Group 24 flooded 70–85 Ah, Group 27 85–105 Ah, Group 31 95–125 Ah, a pair of 6 V golf-cart batteries in series 210–230 Ah (enter as one 12 V battery), a typical lithium drop-in 100 Ah. Temperature derating uses typical published curves for lead-acid; lithium loses less capacity cold but most lithium batteries cannot be charged below 32 °F.

How the math works

  • Daily consumption (Ah) = Σ amps × hours for each checked load.
  • Usable capacity = rated Ah × count × usable fraction × temperature factor.
  • Days of runtime = usable capacity ÷ daily consumption. Hours = days × 24.
  • Solar to break even (W) = daily Ah × 12.8 V ÷ (peak sun hours × 0.7). The 0.7 covers controller loss, panel heat, wiring, and less-than-ideal tilt.

Usable fractions and why they differ

ChemistryUsableWhy
Flooded lead-acid50%Discharging past 50% repeatedly shortens life sharply; voltage under load also sags early, which is what makes the furnace fail at "half full".
AGM50–60%Tolerates deeper cycles a little better and holds voltage better under load. 60% is a working figure, not a rule.
LiFePO480–100%Flat discharge curve, little life penalty for deep cycles. 90% is used here to leave a reserve for the BMS low-voltage cutoff.

Temperature factors used

Overnight lowLead-acid capacityLithium capacity
77 °F100%100%
50 °F~88%~97%
32 °F~78%~92%
20 °F~70%~88%
0 °F~60%~80%

These are typical published derating figures, interpolated between rows. Battery age takes a further bite that no calculator can see: a three-year-old flooded battery that has been run flat twice may have 60 percent of its rated capacity left.

What usually surprises people

The furnace. At a 40 percent duty cycle on a cold night the blower alone takes 20 to 25 Ah, which is most of what a single Group 24 battery can give. The second surprise is the inverter left on: an idle inverter can draw as much over a day as every LED light in the rig. The third is the CPAP humidifier, which triples the CPAP's draw. The boondocking battery guide goes through each load and the cheap fixes, and the converter test covers the case where the battery was never full to begin with.