Solar Panel Sizing Calculator

Size a solar array from daily kWh use and peak sun hours, and size an off-grid battery bank by autonomy days and depth of discharge.

Inputs

Average energy consumed per day. Read it off a utility bill (monthly kWh ÷ 30).

Equivalent hours of full-strength (1000 W/m²) sun for your location. US average ~4–5.

Combined losses: inverter, wiring, soiling, temperature, mismatch. NREL PVWatts default ≈ 86%; 75–80% is conservative.

Rated power of one panel (STC watts).

How many days the battery must run loads with no sun. 0 = grid-tied, no battery.

Nominal DC voltage of the battery bank.

Fraction of battery capacity you draw before recharging. Lead-acid ~50%, lithium ~80–90%.

Result

Solar array size needed
5.56 kW
14 × 400 W panels
  • Array size needed5.56 kW (5,556 W)
  • Panels required @ 400 W14 panels
  • Installed (rounded up)5.60 kW
  • Battery bank energy40.00 kWh
  • Battery capacity @ 48 V833 Ah
Source: Array energy method: array W = daily Wh ÷ (peak sun hours × derate). Loss/derate factor per NREL PVWatts.
Note — A planning estimate. Actual production depends on local irradiance, shading, panel orientation/tilt, and temperature. Confirm with NREL PVWatts for your exact location before purchasing.

Step-by-step

  1. Convert daily use to watt-hours: 20 kWh × 1000 = 20,000 Wh.
  2. Divide by (sun hours × derate): 20,000 ÷ (4.5 × 80%) = 5,556 W.
  3. Number of panels = ceil(5,556 ÷ 400) = 14.
  4. Battery energy = daily Wh × autonomy ÷ usable DoD: 20,000 × 1 ÷ 50% = 40,000 Wh.
  5. Battery capacity = 40,000 Wh ÷ 48 V = 833 Ah.

How to use this calculator

  • Enter your average daily energy use in kWh (monthly utility kWh ÷ 30, or sum of appliance loads).
  • Enter peak sun hours for your location (US average ~4–5; check NREL or PVWatts).
  • Set the derate factor (80% conservative, 86% PVWatts default) and your chosen panel wattage.
  • For off-grid, set days of autonomy and battery depth of discharge to size the battery bank.

About this calculator

This calculator sizes a solar photovoltaic system using the standard energy-balance method: the array must generate, over an average day, at least as much energy as you consume. It divides your daily energy use (in watt-hours) by the product of peak sun hours and a system derate factor, which together convert nameplate panel watts into realistic daily production. "Peak sun hours" is the number of equivalent hours of 1000 W/m² sunlight your location receives. The derate factor (NREL PVWatts uses ~14% total losses, i.e. ~86%) accounts for inverter inefficiency, wiring and soiling losses, temperature, and module mismatch. For off-grid systems it also sizes a battery bank from your required days of autonomy and the battery's usable depth of discharge.

How it works — the formula

Array (W) = Daily use (Wh) ÷ (Peak sun hours × Derate) Panels = ceil( Array W ÷ Panel W ) Battery (Wh) = Daily Wh × Autonomy days ÷ DoD Battery (Ah) = Battery Wh ÷ Bank voltage

The array must replace each day's energy during the limited window of peak sun hours, after derating for real-world losses. The battery bank stores enough usable energy to cover the loads through the specified days without sun, sized up by depth of discharge so it is never over-drained.

Sources: NREL — PVWatts Calculator (loss/derate model) · NREL — PVWatts default system losses documentation

Worked examples

Example 1
20 kWh/day, 4.5 sun hours, 80% derate, 400 W panels
Inputs:
dailyKwh=20, sunHours=4.5, derate=80, panelWatt=400
Output:
20000 ÷ (4.5 × 0.80) = 5556 W → 14 panels
Example 2
10 kWh/day, 5 sun hours, 75% derate
Inputs:
dailyKwh=10, sunHours=5, derate=75
Output:
10000 ÷ (5 × 0.75) = 2667 W
Example 3
Battery: 20 kWh/day, 1 day autonomy, 50% DoD, 48 V
Inputs:
autonomyDays=1, dod=50, batteryVoltage=48
Output:
20000 ÷ 0.50 = 40000 Wh → 833 Ah @ 48 V

Limitations

  • Uses an average daily energy balance — does not model seasonal variation or worst-month sizing.
  • Peak sun hours must already incorporate your array tilt and orientation.
  • Battery sizing ignores temperature derating and inverter surge requirements.

Planning estimate only. Validate with NREL PVWatts and a qualified installer before purchase.

Frequently asked

Peak sun hours are the equivalent number of hours per day during which solar irradiance averages 1000 W/m² (full-strength sun). A location receiving 5 peak sun hours gets the same total energy as 5 hours of perfect midday sun, spread across the whole day. US averages range from about 3 (Pacific Northwest winter) to 6+ (desert Southwest).

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