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How Many Solar Panels Do I Need?

A household using 900 kWh a month in a region with 5 peak sun hours needs about 16 standard 400W panels.

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Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.

Try a scenario

Click to load — tweak from there.

Inputs

Result

Solar panels needed

19

Total system size (kW)

7.60

Daily energy needed (kWh)

30.0

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How to use this

  1. 1Enter monthly electricity usage (kWh).
  2. 2Enter peak sun hours per day (hrs).
  3. 3Enter panel wattage (W).
  4. 4Enter system efficiency (%).
  5. 5Read your solar panels needed on the right — it updates as you type.
  6. 6Hit Share to keep the scenario or send it to someone.

About this calculator

Solar panel sizing starts from your monthly energy usage, converts it to a daily average, and divides by how much energy one panel produces per day given your region's peak sun hours and the panel's wattage, then adds a system inefficiency buffer for inverter and wiring losses.

FormulaDaily kWh Needed = Monthly kWh ÷ 30. Panel Output (kWh/day) = Panel Watts × Peak Sun Hours ÷ 1000 × System Efficiency. Panels Needed = Daily kWh Needed ÷ Panel Output.

Worked example

Using the values the calculator loads with:

Inputs

  • Monthly electricity usage: 900 kWh
  • Peak sun hours per day: 5 hrs
  • Panel wattage: 400 W
  • System efficiency: 80 %

Results

  • Solar panels needed: 19
  • Total system size (kW): 7.6
  • Daily energy needed (kWh): 30

What each field means

Inputs

Monthly electricity usage (kWh)
The monthly electricity usage used in the calculation, measured in kWh. Starts at 900 kWh so you have a working example on load.
Peak sun hours per day (hrs)
The peak sun hours per day used in the calculation, measured in hrs. Starts at 5 hrs so you have a working example on load. Accepted range: 2–8 hrs.
Panel wattage (W)
The panel wattage used in the calculation, measured in W. Starts at 400 W so you have a working example on load. Accepted range: 100–600 W.
System efficiency (%)
The system efficiency used in the calculation, measured in %. Starts at 80 % so you have a working example on load. Accepted range: 60–95 %.

Results

Solar panels needed
Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Total system size (kW)
Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Daily energy needed (kWh)
Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.

FAQ

How many solar panels do I need?

For 900 kWh/month usage with 5 peak sun hours a day, 400W panels, and 80% system efficiency, you need about 19 panels, forming a roughly 7.5 kW system.

What are 'peak sun hours'?

It's the equivalent number of hours per day sunlight is intense enough to produce a panel's rated output — sunny states like Arizona average 6-7 hours, while cloudier northern regions average 3-4.

Why include a system efficiency factor?

Real-world solar systems lose 15-25% of rated output to inverter conversion, wiring resistance, panel soiling, and temperature effects, so ignoring this understates the panels you actually need.

Does roof space limit how many panels I can install?

Yes — a standard 400W panel is about 18 sq ft, so a 19-panel system needs roughly 340 sq ft of usable, unshaded roof space, which can be a constraint on smaller roofs.

Accuracy and limitations

  • Estimates assume standard, average conditions — local rules, pricing, and materials vary.
  • Results are rounded for readability; add a buffer before ordering, booking, or committing.
  • Double-check anything with a real cost attached against a local quote.

Related tools

How Much Will My Electric Bill Be?

A household using 900 kWh a month at 16¢/kWh plus a $10 base fee has a bill of about $154.

How Much Does It Cost to Run an Air Conditioner?

A 3,500-watt central AC run 8 hours a day at 16¢/kWh costs about $4.48 a day, or roughly $134 a month.

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APA
RevenueLab. (2026). How Many Solar Panels Do I Need?. Retrieved from https://www.revenuelab.fyi/toolbox/how-many-solar-panels-do-i-need
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