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Quick Answer: To calculate how many solar panels you need, divide your daily energy consumption (kWh) by your peak sun hours, then adjust for system losses. For a typical U.S. home using 900 kWh/month, you need a 7.6 kW system, which requires roughly 19 panels of 400W each.
This tool estimates the solar system you'd need based on your monthly electricity usage, average daily sunlight hours, panel wattage, and a real-world efficiency loss factor (accounting for inverter losses, wiring, temperature, and dust). It returns the system size in kW, the number of panels required, the roof area needed, a rough annual generation estimate, and the estimated system cost and payback period.
If your home uses 900 kWh a month (10,800 kWh a year), your array needs to produce at least that much. Panels don't run at full rated output all day, so the calculation uses "peak sun hours" — in most temperate regions that's 4-5 hours per day. Here is the exact formula used by our calculator:
Use this quick reference table to see how much solar you might need based on typical home sizes and energy usage.
| Home Size | Avg. Monthly Usage | System Size Needed | Est. Panels (400W) | Roof Space Needed |
|---|---|---|---|---|
| 1,000 sq ft | 600 kWh | 5.0 kW | 13 Panels | ~280 sq ft |
| 2,000 sq ft | 900 kWh | 7.6 kW | 19 Panels | ~400 sq ft |
| 3,000 sq ft | 1,200 kWh | 10.0 kW | 25 Panels | ~525 sq ft |
| 4,000+ sq ft | 1,800 kWh | 15.0 kW | 38 Panels | ~800 sq ft |
Most residential installs are grid-tied, meaning the system stays connected to the utility and any excess power can often be sold back through net metering. An off-grid system instead relies on battery storage to keep the lights on at night or during outages, and is typically sized 20-30% larger than a grid-tied system to keep batteries charged through cloudy stretches.
Not every roof performs equally. In the northern hemisphere, south-facing roofs get the most direct sun; east- or west-facing roofs may need 15-20% more panels to make up for lower output. Roof pitch matters too — a tilt close to your latitude generally gives the best year-round production. The "System Efficiency Loss" field in this calculator lets you account for these real-world factors rather than assuming a perfect installation.
Most homeowners weigh system cost (after any available incentives) against their current electricity bill to find a payback period, commonly somewhere between 6 and 10 years depending on local rates and system cost. As of 2026, the average cost of solar installation ranges from $2.50 to $3.50 per watt.
Don't forget to factor in the Federal Investment Tax Credit (ITC), which allows you to deduct 30% of the installation cost from your federal taxes. Many states also offer additional rebates and net metering policies that drastically reduce the payback period. After the payback period, the electricity the system produces is essentially free, and solar has also been shown to support home resale value in many markets.
Solar panels are incredibly durable. Most modern panels come with a 25-year performance warranty, meaning they will still produce at least 80% of their rated capacity after 25 years. Maintenance is minimal—typically just an occasional wash to remove dust and debris, and an annual check of the inverter, which usually needs replacement once every 10-15 years.
Our calculator gives you the technical specs. Now get a personalized quote from certified local installers.
Get Free Solar Quotes →Q: How much roof space does a 5kW system need?
A: A standard 5kW system using 400W panels consists of about 13 panels. At roughly 2 square meters per panel, that's about 26-30 square meters (280-320 sq ft) of unobstructed roof space.
Q: Do solar panels work at night?
A: No. Solar panels need sunlight to generate power. For power at night you need either a grid connection or a battery storage system like a home battery unit.
Q: What is a solar inverter?
A: A device that converts the direct current (DC) electricity your panels generate into the alternating current (AC) your home appliances use. Inverter sizing must match your total panel capacity.
Q: How long do solar panels last?
A: Most modern panels come with a 25-year performance warranty. They rarely fail outright, but their efficiency gradually declines over decades.
Determine exactly how much power you need.
Plan safe cable gauges for solar arrays.
Calculate roof area for panel mounting.