Solarposition.org

Calculator · designed for the darkest month

Off-grid solar calculator

List what you run, pick your place, and get the array, battery and charge controller that carry you through the worst month of the year — not an average one.

1 · Where

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2 · What you run
ApplianceWh/dayQtyRemove

Total: —

A fridge's watts are its average draw — the yearly kWh from its energy label ÷ 8.76 — not the peak on its rating plate. A typical 230 V fridge-freezer uses about 300 kWh a year, so 34 W.

3 · System
70°
Snow on the ground

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How the calculator sizes a system

  1. Daily use. The sum of watts × hours × quantity for every appliance, measured at the plug.
  2. Sunlight for your point. Monthly averages of global and diffuse radiation and air temperature from NASA's POWER climatology (satellite data, 1° grid), turned into an hourly profile for a representative day of each month.
  3. Energy on the panel. The hourly light is projected onto your tilt and orientation with the Hay–Davies sky model, including light reflected from the ground (with snow in freezing months if you leave the switch on).
  4. Energy out of the panel. Module temperature (Faiman model), reflection losses at the glass and 8% for wiring, mismatch and dirt.
  5. The design month. The month with the lowest output per kWp. The array is sized so that even this month covers your use after the charge controller (97%), battery round trip (95% LiFePO4, 92% sodium-ion) and inverter (92%).
  6. The battery. Your use × days of autonomy ÷ inverter efficiency ÷ usable depth of discharge (90% LiFePO4, 95% sodium-ion).

Five things that change the answer most

  • The inverter's idle draw. A 3 kW inverter can use 25–40 W doing nothing — up to a kilowatt-hour a day. Add it as an appliance.
  • The fridge. Usually the largest steady load. A household 230 V fridge-freezer uses 200–350 kWh a year, 25–40 W on average.
  • Tilt. Compare 70° with the year-round optimum in the table: in snowy continental climates the steep array is 20–40% smaller for the same winter.
  • A backup source. If a generator may cover a few winter weeks, size for November or February instead of December and save a large share of the array.
  • Shade. A tree or ridge that blocks the low winter sun can halve winter output. Check the December path on the sun map.

Why the default tilt is 70°: the case for steep off-grid panels. For the battery alone, see the battery bank calculator.

Questions

How many solar panels do I need for an off-grid house?
Divide your daily use by the output of 1 kWp in your darkest month and by the system efficiency (about 85% with LiFePO4). For 5 kWh a day where December gives 1.2 kWh/kWp, that is 5 ÷ 1.2 ÷ 0.85 ≈ 4.9 kWp, or eleven 450 W panels. The calculator does this with climate data for your exact location.
How big a battery does an off-grid system need?
Daily use × days of autonomy ÷ usable depth of discharge ÷ inverter efficiency. Three days at 1.5 kWh with LiFePO4 is about 5.4 kWh, or 226 Ah at 24 V.
Should I design for December or for the yearly average?
For a system with no other source, design for the worst month. A yearly average leaves you weeks short every winter. With a generator or grid backup you can size for a better month.