Sizing & runtime
Electrical Consumption Calculator
Add up to six devices to get daily energy (Wh) and daily amp-hours at your system voltage.
Formula
Wh/day = Σ (W × h × qty)
Ah/day = Wh/day ÷ V
Peak W = Σ (W × qty)Worked example
Fridge 45 W × 24 h (duty cycle ignored) + 4 LEDs 10 W × 5 h + laptop 60 W × 2 h = 1,400 Wh/day → 116.7 Ah/day at 12 V.
Limitations and warnings
- Compressor fridges run only part of the time; enter effective hours (duty cycle) for accuracy.
Four values that are often confused
- Nominal capacity (Ah)
- Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
- Usable capacity
- The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
- Cranking current (CCA)
- Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
- Stored energy (Wh)
- Ah × voltage. The right unit to compare batteries of different voltages.