Solar Panel Size Calculator

Q: How do I calculate solar panel size?

Panel watts = (Daily kWh × 1000) ÷ (Peak sun hours × System efficiency). For example, 30 kWh/day, 5 sun hours, 80% efficiency: (30 × 1000) ÷ (5 × 0.8) = 7,500W required.

Q: What are peak sun hours?

Peak sun hours (PSH) is the number of hours per day when sunlight intensity averages 1,000 W/m². It varies by location: Arizona gets ~6 PSH, the UK gets ~2.5–3.5 PSH. Use a solar map or tool to find your location's PSH.

Q: What system efficiency should I use?

A typical grid-tied system is 75–80% efficient. Off-grid systems with batteries are 70–75%. Losses come from inverters (~5%), wiring (~2%), temperature (~5-10%), and soiling (~2-5%).

Find out how much solar panel capacity you need to cover your daily energy usage. Results in watts, kW, and estimated panel count.

Calculate Solar Panel Size

Daily Energy Usage (kWh/day)

Peak Sun Hours (hours/day)

System Efficiency (%)

Result

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Required Panel Capacity (W)

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Required Panel Capacity (kW)

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Est. 400W Panels Needed

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Est. 300W Panels Needed

Formula

Panel Watts = (Daily kWh × 1000) ÷ (Peak Sun Hours × System Efficiency)

Where system efficiency is expressed as a decimal (e.g. 80% = 0.80). This accounts for inverter losses, wiring losses, shading, and temperature derating.

Peak Sun Hours by Region

Southwest US (AZ, NV, NM): 5.5–7.0 hours/day
Southeast US (FL, TX): 4.5–6.0 hours/day
Midwest / Northeast US: 3.5–5.0 hours/day
UK and Northern Europe: 2.5–3.5 hours/day
Southern Europe (Spain, Italy): 4.5–6.0 hours/day
Australia: 4.5–6.5 hours/day

Example Calculation

A home uses 30 kWh/day, located in Phoenix (6 peak sun hours), grid-tied system at 80% efficiency:

Panel Watts = (30 × 1000) ÷ (6 × 0.80)
Panel Watts = 30,000 ÷ 4.8 = 6,250W (6.25 kW)
Number of 400W panels = 6,250 ÷ 400 = 16 panels
Number of 300W panels = 6,250 ÷ 300 = 21 panels

Frequently Asked Questions

How do I calculate solar panel size?

Use: Panel Watts = (Daily kWh × 1000) ÷ (Peak Sun Hours × System Efficiency). The system efficiency accounts for losses from your inverter, wiring, temperature, and any shading. Typical residential efficiency is 75–80%.

What are peak sun hours?

Peak sun hours (PSH) is the equivalent number of hours per day at 1,000 W/m² solar irradiance. It is not total daylight hours — a cloudy day might deliver only 1–2 PSH even if it is light for 12 hours. Check a solar resource map like NREL's PVWatts for your specific location.

What system efficiency should I use?

Grid-tied systems: 75–80%. Off-grid with battery storage: 70–75%. Losses include inverter (~5%), wiring (~2%), temperature derating (~5-10%), and soiling (~2-5%). Use 75% as a conservative estimate if you are unsure.