Size a power station from your real load
A portable power station is just a battery in a box with an inverter. The number that matters is its capacity in watt-hours (Wh) — think of it as the fuel tank. To size one, you only need two things: what you will run, and for how long.
The formula
- Device energy
- watts × hours per day = watt-hours per day for that device.
- Daily total
- Sum every device’s watt-hours. This is your baseline daily draw.
- Trip total
- Daily total × days off-grid (when there is no recharge).
- Real-world capacity
- Trip total × (1 + inverter loss) × (1 + headroom) — losses and a reserve so you never fully drain the bank.
Written as an equation:
Capacity (Wh) = Σ (device watts × hours/day) × days × (1 + loss) × (1 + headroom)
Worked example
A phone charger (10 W × 3 h = 30 Wh), LED camp lights (12 W × 4 h = 48 Wh), a laptop (60 W × 3 h = 180 Wh), and a 45 W cooler running 10 h (450 Wh) total 708 Wh/day. For a 2-day trip with no recharge, 15% loss and 20% headroom: 708 × 2 × 1.15 × 1.20 ≈ 1,955 Wh. Round up to a 2,000 Wh (2 kWh) station, or smaller if you can solar-recharge daily.
With vs without recharge
- No recharge: size for the entire trip total. A 2-day 1,955 Wh need rounds to a ~2,000 Wh bank.
- Solar or vehicle daily: size for one day’s draw plus buffer (~850–900 Wh here), and let panels or the alternator refill it. A ~1,000 Wh station covers it.
- Matching panels: to refill 708 Wh in a 5-hour sunny day you need roughly 708 ÷ 5 ÷ 0.75 ≈ 190 W of panel — see our solar sizing for the exact math.
Data sources & retrieved: 2026-08-15. Battery math follows standard watt-hour accounting (Wh = W × h) with typical 10–20% inverter losses as cited by portable-power-station manufacturers (e.g., Goal Zero, EcoFlow, Jackery help centers). Pair this with our sister solar sizing tool at pvfig.com to size panels that refill the bank off-grid.