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

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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

What are peak sun hours?+
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).
Why divide by a derate factor?+
A panel rated 400 W at Standard Test Conditions never delivers a full 400 W in service. Inverter losses, DC/AC wiring resistance, dust and soiling, high cell temperatures, and panel-to-panel mismatch all reduce output. NREL's PVWatts model uses about 14% total system losses (an 86% derate); 75–80% is a more conservative planning figure.
How do I find my daily kWh use?+
Easiest: take a monthly electricity bill total in kWh and divide by 30. For off-grid loads, add up each appliance's watts × hours used per day and divide by 1000.
How is the battery bank sized?+
Battery energy = daily watt-hours × days of autonomy ÷ usable depth of discharge. Dividing by depth of discharge ensures you never drain below the safe limit (50% for lead-acid, 80–90% for lithium). The result is divided by the bank voltage (12/24/48 V) to give amp-hours.
Does this account for panel tilt and orientation?+
No — it assumes your peak-sun-hours figure already reflects your tilt and azimuth. For precise output by tilt and orientation, run your design through NREL's free PVWatts calculator, which models hourly irradiance for your exact coordinates.
Is this for grid-tied or off-grid systems?+
Both. Set days of autonomy to 0 for a grid-tied system (no battery sizing). Set it to 1–3+ days for off-grid or backup systems, which then sizes the battery bank for cloudy-day reserve.

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