Solar Panel & Battery Sizing Calculator

Panel wattage and battery capacity for an off-grid or IoT node, given daily load and worst-month sun hours.

Example: 100 Wh a day at 2 peak sun hours needs about 62.5 W of panel and 29.24 Ah at 12 V.

Inputs

Daily energy use(Wh)

Add up every load: watts × hours per day.

Peak sun hours (worst month)(h)

Not daylight hours — the equivalent hours at full 1000 W/m². Northern Europe in December is around 0.5–1.

System voltage(V)

Days of autonomy

How long the system must run with no sun at all.

Battery type

System efficiency

Charge controller, wiring and any DC-DC conversion combined.

Result

Panel rating

62.5 W

Battery capacity: 29.24 Ah
Battery energy: 350.9 Wh
Usable energy needed: 300 Wh
Peak charge current: 5.208 A
Average load current: 347.2 mA
Charge C-rate: 0.178C

This sizes against 2 peak sun hours — use your location's worst month, not the annual average. A system that balances on average will run flat every winter, which is when it is hardest to fix.
Only 90% of lifepo₄ capacity is usable, and round-trip efficiency costs another 5%. That is why 300 Wh of demand needs 350.9 Wh of nameplate battery.
Panels rarely produce their nameplate rating: temperature, dust, orientation and partial shade all take their share. The efficiency figure here should account for that — 80% is realistic for a well-oriented fixed array, less for anything compromised.

Panel size against available sun

Peak sun hours	Panel	Charge current
0.5	250 W	20.83 A
1	125 W	10.42 A
2	62.5 W	5.208 A
3	41.67 W	3.472 A
4	31.25 W	2.604 A
6	20.83 W	1.736 A

Formula

panel W = daily Wh / (peak sun hours × efficiency)

Worked example

100 Wh a day at 2 peak sun hours needs about 62.5 W of panel and 29.24 Ah at 12 V.

panel = daily Wh / (sun hours × efficiency)
100 / (2 × 0.8)
62.5 W
usable = daily Wh × autonomy days
100 × 3
300 Wh
nameplate = usable / (DoD × round-trip efficiency)
300 Wh / (0.9 × 0.95)
350.9 Wh
capacity = nameplate Wh / system voltage
350.9 Wh / 12 V
29.24 Ah

Frequently asked questions

What are peak sun hours?

The equivalent hours of full 1000 W/m² irradiance in a day, not hours of daylight. Northern Europe in December is often under one, while a desert summer reaches six or more. It is the single most important input, and it must come from data for your actual location.

Why is the battery so much larger than my daily use?

Three multipliers. Autonomy days, then depth of discharge — you cannot use a lead-acid battery's full capacity without destroying it — then round-trip efficiency. Three days of 100 Wh becomes over 400 Wh of nameplate lead-acid.

Should I size for average or worst-case sun?

Worst month, always. A system sized on the annual average balances on paper and runs flat every winter — exactly when access is worst and load is often highest. If the worst month makes the system unaffordable, reduce the load rather than the margin.

Why does my panel never produce its rated wattage?

Ratings are at 25 °C and perfect perpendicular irradiance. Real panels run hot, which costs several percent, and lose more to dust, orientation and partial shade. A single shaded cell can disproportionately affect a whole string.

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