How Many Solar Panels Do I Need?

How to use this calculator

Enter five numbers and the tool returns the system size, panel count, required roof area, and estimated annual production:

1. Monthly electricity use (kWh) — pull this from your last 12 utility bills and average them. The US Energy Information Administration’s 2025 Residential Energy Consumption Survey reports a national mean of 870 kWh per month.
2. Peak sun hours per day — local irradiance figure from NREL PVWatts. The continental US averages 3.5 (Pacific Northwest) to 6.5+ (Phoenix, Las Vegas).
3. Panel wattage (W) — leave at 400 unless you have specific quotes. Tier-1 residential panels in 2026 commonly span 380–460 W.
4. System efficiency (%) — the derate. Use 78% for a typical pitched-roof install with string inverter or micro-inverters.
5. Offset target (%) — 100% covers your annual use; lower if you only want partial offset; higher if you plan to add EVs or electric heating soon.

The formula

System sizing reduces to a simple energy-balance equation:

required_kW = (annual_kWh × offset / 100) ÷ (peak_sun_hours × 365 × derate)
panel_count = ceil(required_W ÷ panel_wattage)
roof_area   = panel_count × panel_area × 1.08 (mounting clearance)

Worked example for an Atlanta household:

• Monthly use: 1,050 kWh → annual 12,600 kWh
• Peak sun hours: 5.0 (NREL data for Fulton County)
• Derate: 78%
• Required: 12,600 ÷ (5.0 × 365 × 0.78) = 8.85 kW
• At 400 W per panel: ceil(8,850 ÷ 400) = 23 panels
• Roof area: 23 × 21.5 ft² = 494 ft²
• Estimated annual production at 23 × 400 W = 9.2 kW: 13,103 kWh

Reference table by monthly consumption

Using 4.8 peak sun hours (US average), 78% derate, 400 W panels, 100% offset:

The relationship is linear: doubling consumption doubles panel count under the same conditions.

What changes the panel count

Sun hours by region

The single biggest variable. A household in Phoenix (6.5 PSH) with 900 kWh/month needs only 14 panels for full offset; the same household in Seattle (3.5 PSH) needs 27 panels — almost twice as many. NREL’s National Solar Radiation Database is the reference dataset for any US zip code.

Roof orientation and tilt

The calculator assumes a south-facing array at latitude tilt. East/west orientation costs about 12% production, raising panel count by 12%. A 5/12 pitch on a south roof is near-optimal across most of the contiguous US. The solar panel orientation calculator and tilt angle calculator quantify the impact for your specific roof.

Panel wattage

Going from 400 W to 460 W panels reduces panel count by 13% and roof area by the same margin. For tight roofs in the Northeast, 460 W modules from REC Alpha Pure-R or Q CELLS Q.TRON often pay for themselves through fewer mounting rails and balance-of-system parts.

Future loads — EV, heat pump, induction

Adding a Level 2 EV charger typically lifts annual consumption by 3,500–4,500 kWh. A cold-climate heat pump in a 2,000 ft² home in Massachusetts adds 5,000–6,500 kWh on top of an electric water heater. Sizing for current use only forces a panel addition 2–3 years later, when the new ITC and rate base may be less favorable. EnergySage advises oversizing by 25% if any of these are on the 3-year horizon.

Net metering rules

A handful of US states (California’s NEM 3.0 the most prominent example) cap or devalue export. In those markets, oversize beyond consumption only makes sense if paired with battery storage. Consult your utility’s interconnection agreement before sizing.

Common mistakes

• Using a single high-bill month for sizing. Summer-peak homes oversize by 20%+ if the calculation runs off a July bill alone. Always average 12 months.
• Not derating. Sizing on rated wattage (no derate) underbuilds by 22–28%. The result: a system that hits 80% offset instead of the targeted 100%.
• Ignoring degradation. Tier-1 panels degrade about 0.5%/year. Year-25 production is ~88% of year-1, so a system sized exactly to 100% offset hits 88% by warranty endpoint. Add 5–8% headroom for long-term offset.
• Overlooking shading. A single shaded panel on a string inverter can drag the entire string down 50%+. Use module-level optimisers (SolarEdge) or micro-inverters (Enphase) to limit losses to the affected panel.

How this calculator differs from a designer’s quote

A real solar designer uses NREL PVWatts (or Aurora, Helioscope) tied to your exact roof model — measured pitch, azimuth, shading from trees and chimneys, and panel-level production simulations across an 8,760-hour weather year. This calculator uses national averages and PVWatts default losses, so expect ±10% variance versus a site-specific quote. Use the result as a budgeting and roof-fit sanity check before requesting installer proposals.

Sources

• NREL PVWatts v6 — system loss conventions and irradiance by zip
• NREL National Solar Radiation Database — TMY3 peak sun hours
• EIA Residential Energy Consumption Survey 2025 — average household consumption
• EnergySage 2026 Residential Solar Marketplace Report — system-size distribution by state
• SEIA Annual Industry Report 2026 — installed capacity and average residential system size