Phoenix, AZ — 30 kWh/day grid-tied
30 kWh/day at 5.7 PSH and 18% loss → 6.4 kW array → 16 × 420 W panels on ~352 ft².
Solar panel calculator
How many solar panels do I need?
Size a solar PV array from your daily electricity use, local peak sun hours, system losses, and panel wattage. Also estimates roof area and an off-grid battery bank using depth-of-discharge and round-trip efficiency.
Solar panel calculator
Array and storage sizing
Recommended solar array7.42 kW
Panel count18 panels
Roof area401 sq ft
Annual energy10.2K kWh
Battery bank51.9 kWh
How the math works
How the math works
- Required PV size (kW) = Daily kWh ÷ (Peak sun hours × (1 − system loss)). NREL's PVWatts uses ~14% combined loss; 18% is conservative for residential rooftops with shading and soiling.
- Panel count = ⌈Required kW × 1000 ÷ Panel wattage⌉.
- Battery bank (kWh) = Daily kWh × Autonomy days ÷ (Depth of discharge × Round-trip efficiency). LiFePO4 supports 80–95% DoD; lead-acid is safe at 50%.
- Roof area ≈ Panel count × Panel area. A 420 W panel is roughly 22 ft²; the calculator scales this with wattage.
Worked examples
Worked examples
Boston, MA — 22 kWh/day grid-tied
22 kWh/day at 4.0 PSH and 18% loss → 6.7 kW → 17 × 400 W panels on ~360 ft². Winter clipping is acceptable for net metering.
Off-grid cabin — 8 kWh/day, 2 days autonomy
8 kWh × 2 ÷ (0.9 × 0.9) = 19.8 kWh LiFePO4 battery bank. Pair with a 3.5 kW PV array at 4.5 PSH to recover within one sunny day.
Solar panel sizing FAQ
Solar panel sizing FAQ
How do I find my peak sun hours?
Use NREL PVWatts or the NREL National Solar Radiation Database for your ZIP code. Typical US ranges: 6+ in the Southwest, 4–5 in the Midwest and Northeast, 3.5–4 in the Pacific Northwest.
What system loss should I use?
PVWatts defaults to 14% (inverter 4%, soiling 2%, wiring 2%, shading 3%, mismatch 2%, availability 3%). Use 18% for shaded roofs, dusty climates, or string inverters; 12% for microinverters with clean conditions.
Do I need batteries with grid-tied solar?
Only if your utility uses NEM 3.0 export rates (California IOUs) or you want outage backup. Most net-metered markets pay back faster without batteries.
How long do solar panels last?
Tier-1 modules carry 25- or 30-year linear performance warranties to ~85% of original output. Real-world degradation is 0.4–0.5%/year median per NREL.
What's the difference between AC and DC kW?
DC kW is the panel nameplate. AC kW is what reaches the grid after the inverter. The calculator returns DC kW; multiply by 0.95–0.97 for AC.
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