Off-Grid System Calculator

Enter your daily energy consumption and get the battery bank, solar array and inverter sizes needed — with every sizing factor (DoD, autonomy, derate) explicit.

Last updated: 2026-09-15

How the calculation works

Daily consumption is first inflated by inverter losses — the battery must supply more than the loads actually use.
The battery bank covers the inflated load for your autonomy days, divided by DoD so the bank never crosses its discharge limit.
The array must replace a full day's consumption within the worst-month sun hours, again after derate.
If a peak load is entered, the inverter is sized 25% above it for surge margin.

Formula

Battery Wh = (daily Wh ÷ η_inv) × autonomy days ÷ DoD
Battery Ah = Battery Wh ÷ system V
Array Wp = (daily Wh ÷ η_inv) ÷ (PSH × derate)

Formula variables
Symbol	Meaning	Unit
`Wh`	Energy	Wh
`DoD`	Depth of discharge	—
`PSH`	Peak sun hours	h/day

Worked example

A cabin using 2,000 Wh/day on a 24 V system, LiFePO4 (DoD 0.85), 2 days autonomy, 3.5 PSH worst month: battery = (2222 × 2) / 0.85 = 5,228 Wh = 218 Ah @ 24 V. Array = 2222 / (3.5 × 0.8) = 794 Wp — round to 800 W (2 × 400 W panels).

Interpreting the result

Every input is a design trade-off: more autonomy days mean a bigger bank for cloudy spells; higher DoD (lithium) shrinks the bank; using the worst-month PSH instead of the annual average makes the system work year-round but costs more in panels. Undersizing any factor shows up as dead batteries in winter.

Assumptions

Daily energy use is constant across the year.
Array fully recharges the bank each sunny day; generator/backup not included.

Limitations

Charge-rate limit not checked: panel W ÷ system V must stay within the battery's max charge C-rate.
Temperature effects on battery capacity (lead-acid loses ~1%/°C below 20 °C) are not modeled.
Seasonal load variation (pumps, heating) requires sizing per season.

Frequently asked questions

12V, 24V or 48V system?

Above about 1,000 Ah or 1,500 W of continuous load, go 24 V; above 3 kW, 48 V. Higher voltage halves the current, halving cable cost and losses.

How many days of autonomy do I need?

2–3 days suits most climates. Sunny regions can use 1–2 with a generator backup; critical loads in cloudy climates justify 4–5.