How to use this calculator
- Enter Monthly usage in kWh.
- Enter Peak sun hours.
- Enter Panel wattage in W.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
Monthly kWh ÷ 30 ÷ peak sun hours ÷ 0.85 derate = system kW; ÷ 0.4 kW per panel = panel count. A 900 kWh/month home with 5 sun hours needs about a 7 kW array — 18 panels. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Where this fits in the project
Efficiency upgrades interact: better insulation and correct R-values shrink the heating and cooling load, which shrinks the solar array needed to offset it. Size loads first, generation second.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the solar panel calculator:
- Solar ROI Calculator — Calculate payback period from system size.
- Electrical Load Calculator — Solar panels offset electrical consumption.
- Roof Square Footage Calculator — Available roof area limits solar panel quantity.
- Roofing Calculator — Roof condition affects solar installation feasibility.
- Wire Size Calculator — Solar requires specific wire sizing for DC and AC runs.
- HVAC Size Calculator — Solar can offset HVAC electrical consumption.
Frequently asked questions
How many panels power a house?
15–22 for a typical US home (10,800 kWh/yr) with 400 W panels — sun hours drive the spread.
What is the 0.85 derate?
Real-world losses: inverter efficiency, wiring, temperature and soiling shave ~15% off nameplate output.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.