Size sunny.
Solar system size for your electricity usage. Accounts for sun hours by region, panel wattage, and system losses so you size right the first time.
How we calculated this
The calculator starts with your monthly electricity consumption (from your utility bill) and divides by 30 to get daily kWh. System size in kilowatts is daily kWh divided by peak sun hours, divided by system efficiency.
Peak sun hours is the daily equivalent of full-strength (1 kW/m²) sunshine your location receives. Arizona and the desert Southwest get 6+ hours; the UK and Pacific Northwest get 3-4; most of North America and Europe falls in the 4-5 range. Use NREL's PVWatts tool for your exact ZIP code if you want precision.
System efficiency accounts for real-world losses that the pure math doesn't capture: inverter conversion losses (~4%), wiring resistance (~2%), temperature derate on hot days (~8%), soiling from dust and pollen (~3%), and shading (variable). 80% is a reasonable default for a clean, well-installed system in typical conditions.
Panel count rounds up to whole panels. Because panels come in discrete wattages, your actual installed system will be slightly larger than the calculated need — that's normal and provides a small production buffer for cloudy weeks. The calculator shows both the computed need and the actual installed size.
Rough rule for roof area: about 17-20 sq ft per panel depending on panel size. A 10-panel system needs roughly 170-200 sq ft of unobstructed, south-facing roof. Complex roof shapes and shading from trees can reduce usable area significantly.
Sources
- NREL PVWatts Calculator — Official US DOE tool for location-specific sun hours and production
- EnergySage — How Many Solar Panels Do I Need — Reference methodology for residential sizing
Frequently asked
How many solar panels do I need for a 2,000 sq ft house?
Not directly related to square footage — it depends on your electricity usage. A typical 2,000 sq ft home uses about 900-1,200 kWh/month and needs 15-20 panels (400W each) in a sunny region, or 20-25 panels in a cloudy region. Pull your utility bill for actual kWh — the calculator above uses that number.
What are peak sun hours?
Peak sun hours measure how much equivalent full-strength sun your location gets per day. The sun is 'peak' at about 1 kilowatt per square meter — roughly midday on a clear day. Weaker morning/evening sun is added up and converted to the equivalent number of peak hours. 5 peak sun hours doesn't mean 5 hours of sun — it means 5 hours of full-strength equivalent.
Should I size for 100% of my usage?
For most homes, yes — size the system to match annual usage. In net metering states, excess summer production offsets winter deficit. In states without net metering, slightly undersizing (80-90% of usage) can make more financial sense because you avoid producing excess you can't use or sell.
Will the system produce exactly what the calculator shows?
It's an estimate — real production varies seasonally. Summer production is 30-50% above average; winter is 30-50% below. Over a full year, the actual number should be within 10% of the calculated estimate, assuming no shading issues or atypical weather.
Can I install solar myself?
Technically possible in most jurisdictions but rarely recommended. DIY installation voids most panel warranties, requires passing a utility-approved inspection for grid connection, and usually disqualifies you from state rebates (which require licensed installers). The cost savings are typically 20-30%, not enough to justify the warranty and safety tradeoffs for most homeowners.
How long until solar pays for itself?
Payback period depends on electricity rates, installation cost, and incentives. In 2025-2026 US market: 6-10 years is typical. Higher electricity rates (California, Northeast) and better state rebates shorten payback. Cheaper electricity (Midwest, South) lengthens it. Federal 30% tax credit applies through 2032.
Do I need a battery?
Not for grid-tied net metering — you export excess and pull from the grid at night. Batteries add cost for backup during outages or to avoid time-of-use peak rates. Most residential solar without batteries is about $2.50-3.50/watt installed; adding batteries pushes that to $4-5.50/watt.
What roof works best for solar?
South-facing with 15-40 degree pitch is ideal in the Northern Hemisphere. East and west-facing roofs produce about 15% less. North-facing is rarely worth it. Flat roofs work with tilted racks. Most importantly: no significant shading between 10 AM and 2 PM when panels produce 70%+ of their daily total.