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This solar panel calculator estimates how many solar panels you need to cover your electricity use. Enter your monthly usage from your utility bill, the peak sun hours where you live and the panel wattage, and it calculates the system size in kW, the number of panels, the expected yearly production and the roof space required.
How to Use the Solar Panel Calculator
- Find your average monthly electricity use in kWh. Your utility bill usually shows the last 12 months; use the average.
- Enter the peak sun hours for your location (see the table below or a solar resource map).
- Enter the wattage of the panels you are considering. Modern residential panels are typically 350–450 W.
- Keep system losses at about 14–20% unless an installer gives you a different figure.
- Choose how much of your usage you want to cover, and optionally the installed cost per watt.
Solar Panel Formula
Daily use (kWh) = Monthly use ÷ 30.4 × Target offset
System size (kW) = Daily use ÷ (Peak sun hours × (1 − Losses))
Number of panels = System size × 1,000 ÷ Panel wattage, rounded up
Peak sun hours are the equivalent number of hours per day when sunlight reaches 1,000 watts per square meter. They are lower than daylight hours and vary by location and season. Losses account for the inverter, wiring, heat, dust and minor shading.
Worked Example: 900 kWh per Month
- Daily use: 900 ÷ 30.4 = 29.6 kWh
- With 4.5 peak sun hours and 20% losses: 29.6 ÷ (4.5 × 0.8) = 8.22 kW
- With 400 W panels: 8,220 ÷ 400 = 20.6, so 21 panels (8.4 kW)
- Expected yearly production: about 11,000 kWh
- Roof space: roughly 440 sq ft (41 m²)
UK example: a household using 300 kWh per month with about 2.7 peak sun hours needs a system of roughly 4.6 kW, or 12 panels of 400 W.
Approximate Peak Sun Hours
| Location | Typical annual average |
|---|---|
| US Southwest (Arizona, Nevada, New Mexico) | 6–7 hours |
| California, Texas, Florida | 5–6 hours |
| US Midwest and Northeast | 4–4.5 hours |
| US Pacific Northwest | 3.5–4 hours |
| Canada (southern cities) | 3.5–4.5 hours |
| United Kingdom | 2.5–3 hours |
| Australia (most capital cities) | 4.5–6 hours |
These are broad averages for planning only. Use a local solar resource map or an installer’s site survey for an accurate figure.
What Affects How Many Panels You Need
- Roof direction and tilt: south-facing roofs (north-facing in the Southern Hemisphere) produce the most energy.
- Shading from trees, chimneys and nearby buildings can reduce output significantly.
- Panel efficiency: higher-wattage panels need fewer panels and less roof space.
- Future use: plan ahead for an electric vehicle, heat pump or home battery.
- Roof size and condition: check your roof area with our roofing calculator and consider re-roofing before installing panels.
Costs and Incentives
Installed costs are usually quoted per watt and vary by country, region, equipment and roof complexity. Tax credits, rebates and export tariffs change frequently and differ by location, so check the current programs available to you and get several quotes from certified installers. Solar installations involve electrical work and usually require permits and utility approval.
Frequently Asked Questions
How many solar panels do I need for 1,000 kWh per month?
With 4.5 peak sun hours, 20% losses and 400 W panels, 1,000 kWh per month needs a system of about 9.1 kW, or 23 panels.
How much electricity does one solar panel produce?
A 400 W panel in a location with 4.5 peak sun hours and 20% losses produces about 1.44 kWh per day, or roughly 525 kWh per year.
What are peak sun hours?
Peak sun hours are the number of hours per day when the solar energy reaching a surface averages 1,000 watts per square meter. They are a standard way to compare how sunny different locations are.
How much roof space do I need for solar panels?
A typical 400 W residential panel takes about 2 m² (21 sq ft). A 20-panel system needs about 40 m² (430 sq ft) of suitable roof, plus space for setbacks.
Can solar panels cover 100% of my electricity?
Often yes, if you have enough unshaded roof space. Many people size systems for 80–100% of their use, depending on budget, roof area and local export rates.
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