How does this solar panel sizing calculator work?
The calculator reduces system sizing to an energy-balance equation — how much energy you need, divided by how much one kW of panels produces at your location — then rounds up to a buildable number of whole panels.
Enter your use & state
Monthly electricity use drives everything; picking your state fills in local peak sun hours for you.
Set panel and offset assumptions
Panel wattage, system efficiency, and how much of your bill you want solar to cover.
Get your panel count
See system size, panel count, roof area, and estimated annual production.
The formula
System sizing reduces to a simple energy-balance equation: how much annual energy you want covered, divided by how much energy one kW of panels produces per year at your location.
panel count = ceil(required watts ÷ panel wattage). Roof area = panel count × panel area × (1 + mounting clearance). Annual production uses the actual installed capacity — panel count × panel wattage — not the bare-minimum requirement, since you can only buy whole panels.
Worked example: an Atlanta household
1,050 kWh/month, 5.0 peak sun hours (Fulton County), 400 W panels, 78% efficiency, 100% offset
- Annual use: 1,050 kWh × 12 = 12,600 kWh.
- Required: 12,600 ÷ (5.0 × 365 × 0.78) ≈ 8.85 kW.
- Panels: ceil(8,850 W ÷ 400 W) = 23 panels → 9.2 kW installed.
- Roof area: 23 panels × 21.5 ft² × 1.08 clearance ≈ 534 ft².
- Annual production: 9.2 kW × 5.0 h × 365 × 0.78 ≈ 13,100 kWh.
Result: 23 panels, a 9.2 kW installed system, about 534 ft² of roof area, and roughly 13,100 kWh of annual production — comfortably above the 12,600 kWh target, since whole-panel rounding always builds in a small surplus.
Reference table by monthly consumption
Using 4.5 peak sun hours (the U.S. average in our state resource table), 78% efficiency, 400 W panels, and a 100% offset target — select your own state in the calculator to replace this with a local figure:
| Monthly kWh | Annual kWh | System size | Panels | Roof area |
|---|---|---|---|---|
| 500 | 6,000 | 4.8 kW | 12 | 279 ft² |
| 700 | 8,400 | 6.8 kW | 17 | 395 ft² |
| 900 | 10,800 | 8.8 kW | 22 | 511 ft² |
| 1,100 | 13,200 | 10.4 kW | 26 | 604 ft² |
| 1,300 | 15,600 | 12.4 kW | 31 | 720 ft² |
| 1,500 | 18,000 | 14.4 kW | 36 | 836 ft² |
| 2,000 | 24,000 | 18.8 kW | 47 | 1,091 ft² |
The relationship is close to linear: doubling your monthly consumption roughly doubles your panel count under the same sun hours and efficiency assumptions.
What changes the panel count
Sun hours by region — the single biggest variable. The same 900 kWh/month home needs about 15 panels in a sunny 6.5-peak-sun-hour location like Phoenix, but roughly 28 panels at 3.5 peak sun hours in a cloudier region like Seattle — nearly double, for identical electricity use. NREL's National Solar Radiation Database is the reference dataset for any U.S. zip code.
Roof orientation and tilt — this calculator assumes a south-facing array at a near-optimal tilt. East/west-facing arrays typically lose about 12% of production, which raises the panel count by roughly the same margin to hit the same offset target.
Panel wattage — going from 400 W to 460 W panels cuts panel count and roof area by about 13% for the same target, since each panel is doing more work. For tight roofs, higher-wattage modules often pay for themselves through fewer mounting rails and less balance-of-system hardware.
Future loads — EV, heat pump, induction — a Level 2 EV charger typically adds 3,500–4,500 kWh a year; a cold-climate heat pump can add 5,000–6,500 kWh on top of that. Sizing for today's usage only often forces a costly panel addition two or three years later. EnergySage advises oversizing by roughly 25% if any of these are on a 3-year horizon.
Net metering rules — some states cap or devalue exported solar power (California's NEM 3.0 is the most prominent example). In those markets, oversizing beyond your own consumption usually only pays off if it's paired with battery storage — check your utility's interconnection agreement before sizing above 100% offset.
Common mistakes
Summer-peak homes that size off a single July bill can overbuild by 20% or more. Always average 12 months.
Sizing on rated wattage with no efficiency loss underbuilds by roughly 22–28% — the system lands closer to 80% offset than the 100% you targeted.
Panels degrade about 0.5%/year, so a system sized exactly to 100% offset today is closer to 88% by year 25. Add 5–8% headroom for long-term offset.
One shaded panel on a string inverter can drag the whole string's output down 50%+. Module-level optimizers or micro-inverters limit the loss to just that panel.
How this calculator differs from a designer's quote
A real solar designer uses PVWatts-style modeling (or tools like Aurora or Helioscope) tied to your exact roof — measured pitch, azimuth, shading from trees and chimneys, and panel-level production simulated across a full 8,760-hour weather year. This calculator uses national averages and PVWatts-style default losses, so expect roughly ±10% variance versus a site-specific proposal. Treat the result as a budgeting and roof-fit sanity check before requesting installer quotes — our solar panel estimate calculator takes the same sizing math further into installed cost, payback, and 25-year savings.
Sources
- NREL PVWatts v8 — system loss conventions and location-specific irradiance.
- NREL National Solar Radiation Database — typical-meteorological-year peak sun hours.
- U.S. EIA Residential Energy Consumption Survey 2025 — average household electricity consumption.
- EnergySage 2026 Residential Solar Marketplace Report — system-size and oversizing guidance.
Privacy and appropriate use
Your inputs are processed directly in your browser and are not sent to a database. This is a planning-stage sizing estimate using national averages, not an engineering, permitting, or utility-interconnection document — have a licensed solar installer verify final panel count, layout, and roof structural capacity for your specific site.