Solar Panel System Sizing Calculator
Calculate exactly how many solar panels your home needs based on your electricity bill, local sun exposure, panel wattage, and desired offset. Ideal for homeowners planning a rooftop PV installation.
Last updated: June 2026
Solar panels needed
21 panels
9.33: 7-10 W per monthly kWh is the normal range for a 100% offset at 4.5-5.5 sun hours. The example (9.3 W/kWh: 21x400W panels for 900 kWh) sits here.
This is installed watts per kWh of monthly usage; the formula's own math ties panel count to your offset target, sun hours, and losses, with ~7-13 W per monthly kWh being typical.
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About this calculator
This calculator converts your monthly electricity consumption into a daily figure (monthlyUsage / 30.4), then scales it by your desired offset percentage. That daily energy target is divided by the effective daily output of one panel, which accounts for real-world system losses such as inverter inefficiency, wiring resistance, soiling, and heat degradation. The full formula is: panels = ⌈(monthlyUsage / 30.4) × (offsetTarget / 100) / (sunHours × (1 − systemLosses / 100)) × 1000 / panelWattage⌉. Peak sun hours represent the equivalent hours per day at 1,000 W/m² irradiance — a value that varies significantly by location (e.g., ~4 h in the UK vs ~6 h in Arizona). The ceiling function ensures you always round up to a whole panel count, so your system meets or exceeds the target output.
How to use
Suppose your monthly bill shows 900 kWh, your location gets 5 peak sun hours/day, you want to offset 80% of usage, system losses are 14%, and your panels are 400 W each. Daily need = 900 / 30.4 = 29.6 kWh. Offset portion = 29.6 × 0.80 = 23.7 kWh/day. Effective sun hours = 5 × (1 − 0.14) = 4.30 h/day. Energy per panel per day = 400 W × 4.30 h / 1000 = 1.72 kWh. Panels needed = ⌈23.7 / 1.72⌉ = ⌈13.8⌉ = 14 panels.
Frequently asked questions
How many solar panels do I need for a 1,000 kWh monthly electricity bill?
It depends on your location's peak sun hours, panel wattage, and how much of your bill you want to offset. As a rough guide, a home using 1,000 kWh/month in a region with 5 peak sun hours, targeting 100% offset with 400 W panels and 14% losses, needs about 16 panels. Use the calculator with your specific inputs for a precise answer. Always round up to the next whole panel to ensure you meet your target.
What are peak sun hours and how do they affect solar panel sizing?
Peak sun hours are the number of hours per day during which sunlight intensity averages 1,000 W/m², the standard test condition for panel ratings. A location with 4 peak sun hours generates significantly less electricity per panel than one with 6, so you need more panels to produce the same annual energy. You can find your local value from solar irradiance maps or tools like NASA's POWER database or the NREL PVWatts tool. Entering an accurate figure is one of the most impactful inputs in this calculator.
Why do system losses reduce the output of my solar panels?
Real-world solar systems lose energy at several stages: the inverter converting DC to AC (typically 4–6%), heat reducing panel efficiency above 25 °C (up to 10%), wiring resistance, dust and soiling on the glass, and mismatch between panels. Combined, these losses commonly total 10–20%, meaning a panel rated at 400 W may only contribute around 330–360 W of usable AC electricity. The calculator applies your stated loss percentage to the effective daily sun hours, so the panel count it returns already accounts for these inefficiencies.