Solar Panel Calculator
Work out how many solar panels your daily energy use needs.
Grid-tied solar requires utility interconnection approval, permits and NEC-compliant rapid shutdown. Array sizing must also account for roof structure and shading. Use a licensed installer.
How this calculator works
panels = daily kWh ÷ (panel watts × sun hours × efficiency ÷ 1000)
Peak sun hours is not daylight hours — it is the equivalent hours at full 1,000 W/m² irradiance. A location with 12 hours of daylight may only have 4.5 peak sun hours.
Worked example
With daily energy use 30 , panel wattage 400 , peak sun hours 4.5 , system efficiency 0.8 , this calculator returns 20.83 .
Panels needed by daily consumption
Figures below assume panel wattage 400 × peak sun hours 4.5 × system efficiency 0.8 . Change any measurement in the calculator to get your own numbers.
| Daily energy use | Panels needed | System size |
|---|---|---|
| 10 | 6.94 | 2.78 |
| 15 | 10.42 | 4.17 |
| 20 | 13.89 | 5.56 |
| 30 | 20.83 | 8.33 |
| 40 | 27.78 | 11.11 |
| 50 | 34.72 | 13.89 |
| 70 | 48.61 | 19.44 |
400 W panels, 4.5 peak sun hours, 80% system efficiency. Peak sun hours vary from about 3.5 in the Pacific Northwest to 6 in the desert south-west.
Solar system size by household
| Job | Size | Panels needed |
|---|---|---|
| Small apartment | 10 × 400 × 4.5 × 0.8 | 6.94 |
| Two-bed house | 20 × 400 × 4.5 × 0.8 | 13.89 |
| Average US home | 30 × 400 × 4.5 × 0.8 | 20.83 |
| Large home with AC | 45 × 400 × 4.5 × 0.8 | 31.25 |
| Home with an EV | 55 × 400 × 4.5 × 0.8 | 38.19 |
| Average home, sunny climate | 30 × 400 × 6 × 0.8 | 15.63 |
The average US household uses about 29 kWh per day. Your own figure is on the utility bill — use twelve months of it, not one, because summer and winter differ sharply.
What does a solar system cost?
Material typically runs $3 to $4 per watt installed. Before incentives. A 7 kW system therefore runs roughly $17,500 to $26,600 gross, and the 30% federal tax credit brings that down substantially.
What moves the price:
- Roof complexity — multiple planes, steep pitch or a shaded array all raise labour
- Whether the main panel needs upgrading, often $2,000 to $4,000 on its own
- Battery storage, which typically adds $10,000 to $15,000
- Microinverters versus a string inverter
- State and utility incentives, which vary enormously
Prices are US averages and move with fuel, season and haul distance. Always get two local quotes before budgeting.
Peak sun hours by region
| Type | Peak sun hours per day | Notes |
|---|---|---|
| Pacific Northwest | 3.50 | Seattle, Portland |
| North-east and Midwest | 4.00 | Boston, Chicago, New York |
| Mid-Atlantic and South | 4.50 | The figure used above |
| Texas and the Plains | 5.00 | Dallas, Denver |
| South-west desert | 6.00 | Phoenix, Las Vegas |
| Hawaii | 5.50 | High year-round consistency |
Peak sun hours are not daylight hours. They express total daily solar energy as the equivalent number of hours at full rated intensity, which is why the figure is much lower than the hours of daylight.
Frequently asked questions
How many solar panels to power a house?
A typical US home using 30 kWh a day needs about 21 panels of 400 W at 4.5 peak sun hours. In a sunnier climate the same house needs around 16.
How much roof space do solar panels need?
About 18 square feet per 400 W panel. A 7 kW system of 18 panels needs roughly 320 sq ft of unshaded roof, which most single-family homes have on a south-facing plane.
How long do solar panels last?
Twenty-five to thirty years. Most carry a warranty guaranteeing 80 to 85% of rated output at year 25 — panels degrade slowly rather than fail outright.
Do solar panels work in winter?
Yes, and cold actually improves panel efficiency. The limit is daylight hours and sun angle, not temperature — northern winter output typically runs 40 to 60% of the summer figure.
How many solar panels do I need?
Divide your daily kWh use by the daily output of one panel. A home using 30 kWh a day with 400 W panels and 4.5 peak sun hours needs about 21 panels, an 8.3 kW system.
What are peak sun hours?
The equivalent number of hours at full 1,000 W per square metre irradiance. It is far fewer than daylight hours — most of the US sits between 3.5 and 6 peak sun hours as a yearly average.
Why apply an 80% efficiency factor?
Real systems lose energy to inverter conversion, wiring resistance, panel temperature, soiling and mismatch. Eighty percent is the standard derate for a well-designed system; shading pushes it lower.
Should I size for annual or winter use?
Grid-tied systems size to annual consumption since net metering carries the summer surplus into winter. Off-grid systems must size to the worst month or the batteries run flat in December.