acalculator

How many solar panels do I need?

Type your monthly electricity use from your bills, the peak sun hours where you live, and the rating of the panels. The solar panel calculator shows the system size, how many panels to buy and how much energy they make.

Your numbers

Solar panels
20

To make 100% of 900 kWh a month at 4.5 peak sun hours you need 7.65 kW; 400 W panels to buy: 20, making 941.7 kWh a month.

System size needed (kW)
7.65
System size with whole panels (kW)
8
Energy made per month (kWh)
941.7
Energy made per year (kWh)
11,300
Share of use covered
104.6%
Electricity use per day (kWh)
29.59

Solar panels: 20. To make 100% of 900 kWh a month at 4.5 peak sun hours you need 7.65 kW; 400 W panels to buy: 20, making 941.7 kWh a month.

System size needed (kW) by peak sun hours per day

How to calculate

Computes the solar system size in kilowatts and the number of panels that cover a share of a home’s electricity use, from the monthly use, the peak sun hours, the panel rating and the system losses, and the energy they make.

Example with the default inputs (Electricity use per month (kWh) 900, Peak sun hours per day 4.5, Panel rating (W) 400, System losses 14%, Share of use to cover 100%): To make 100% of 900 kWh a month at 4.5 peak sun hours you need 7.65 kW; 400 W panels to buy: 20, making 941.7 kWh a month.

Method: System kW = monthly kWh ÷ (365 ÷ 12) × share ÷ (sun hours × (1 − losses)); panels = system W ÷ panel W, rounded up; energy per month = panels × panel kW × sun hours × (1 − losses) × 365 ÷ 12.

  • Peak sun hours are the average daily solar energy on the panels, in kWh per m² per day, typed for your site and tilt; the calculator uses no location data.
  • Panels make their rated power at 1,000 W/m² of sun, less the system losses (14% by default, the PVWatts default).
  • An average month is 365 ÷ 12 days, and every month gets the average sun; real output is higher in summer and lower in winter.
  • No prices, incentives or battery storage: the calculator sizes the panels only.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Electricity use per month (kWh) 900, Peak sun hours per day 4.5, Panel rating (W) 400, System losses 14%, Share of use to cover 100% gives Electricity use per day (kWh) 29.589041, System size needed (kW) 7.645747, Solar panels 20, System size with whole panels (kW) 8, Energy made per month (kWh) 941.7, Energy made per year (kWh) 11,300.4, Share of use covered 104.633333%.Source: NREL PVWatts Version 5 Manual (Dobos 2014), 14% default system losses
  2. Electricity use per month (kWh) 816.14, Peak sun hours per day 4, Panel rating (W) 300, System losses 14%, Share of use to cover 100% gives Electricity use per day (kWh) 26.832, System size needed (kW) 7.8, Solar panels 26, System size with whole panels (kW) 7.8, Energy made per month (kWh) 816.14, Share of use covered 100%.Source: NREL PVWatts Version 5 Manual (Dobos 2014)
  3. Electricity use per month (kWh) 1,200, Peak sun hours per day 6, Panel rating (W) 500, System losses 20%, Share of use to cover 50% gives System size needed (kW) 4.109589, Solar panels 9, System size with whole panels (kW) 4.5, Energy made per month (kWh) 657.Source: NREL PVWatts Version 5 Manual (Dobos 2014)

How it works

One kilowatt of panels makes (peak sun hours × (1 − losses)) kWh a day. The calculator sizes the system to make the share of your use you ask for:

  • Daily use = monthly use ÷ (365 ÷ 12)
  • System size (kW) = daily use × share ÷ 100 ÷ (peak sun hours × (1 − losses ÷ 100))
  • Panels = system size × 1,000 ÷ panel rating in watts, rounded up to a whole panel
  • System with whole panels (kW) = panels × panel rating ÷ 1,000
  • Energy per month = whole-panel kW × peak sun hours × (1 − losses ÷ 100) × 365 ÷ 12; per year the same × 365
  • Share of use covered = energy per month ÷ monthly use × 100

The chart shows the system size needed at 2 to 8 peak sun hours, with your other numbers held.

Exact arithmetic. Every number is read as the exact decimal you typed, and every step is a fraction, so a system that needs exactly 26 panels gives 26, not 27. The results round only for display.

Rounding. kW and daily use show with at most 2 decimals, energy per month and the share covered with 1, energy per year as a whole number, each rounded half up from its exact value, with trailing zeros left off. Panels are rounded up.

Assumptions

  • Peak sun hours are typed for your site: the average daily solar energy on the panels, in kWh/m² a day. The calculator uses no location data.
  • Panels make their rated power at 1,000 W/m², less the system losses (14% by default, 0% to 90%).
  • An average month is 365 ÷ 12 days; every month gets the average sun.
  • No prices, incentives or batteries.
  • Monthly use 1 to 1,000,000 kWh; peak sun hours 0.5 to 12; panels 10 to 1,000 W; share of use 1% to 200%.

Worked examples by hand

900 kWh a month, 4.5 peak sun hours, 400 W panels, 14% losses. Daily use 900 ÷ 30.4167 = 29.59 kWh. One kW makes 4.5 × 0.86 = 3.87 kWh a day, so the system is 29.59 ÷ 3.87 = 7.65 kW. Panels: 7,645.7 ÷ 400 = 19.11, so 20 panels, which is 8 kW. They make 8 × 3.87 × 30.4167 = 941.7 kWh a month (11,300 kWh a year), 104.6% of the use.

816.14 kWh a month, 4 peak sun hours, 300 W panels, 14% losses. Daily use 816.14 × 12 ÷ 365 = 26.832 kWh. One kW makes 4 × 0.86 = 3.44 kWh a day, so the system is 26.832 ÷ 3.44 = 7.8 kW = 7,800 ÷ 300 = 26 panels exactly, making 816.14 kWh a month (100%). (The same steps in floating point give 26.000000000000004, which would round up to 27.)

Half of 1,200 kWh a month, 6 peak sun hours, 500 W panels, 20% losses. 1,200 × 0.5 ÷ 30.4167 = 19.73 kWh a day; one kW makes 6 × 0.8 = 4.8 kWh, so 4.11 kW, or 8.22 panels: 9 panels (4.5 kW), making 4.5 × 4.8 × 30.4167 = 657 kWh a month.

Other questions people ask

How many solar panels do I need?

Divide your daily electricity use by the energy one kilowatt of panels makes in a day, then by the panel rating. A home using 900 kWh a month uses 900 ÷ 30.42 = 29.6 kWh a day. At 4.5 peak sun hours with 14% losses, 1 kW of panels makes 4.5 × 0.86 = 3.87 kWh a day, so the home needs 29.6 ÷ 3.87 = 7.65 kW, or 20 panels of 400 W.

What are peak sun hours?

The solar energy a surface gets in a day, in kWh per square metre, written as hours of full sun at 1,000 W/m². A site that gets 4.5 kWh/m² in a day has 4.5 peak sun hours, even if the sun is up for 12 hours. NREL publishes the average for any US location; its PVWatts tool and solar resource maps give the number for your site and panel tilt.

What are system losses?

A panel makes its rated watts only at 1,000 W/m² of sun and 25 °C. In use, heat, wiring, the inverter, shading, dirt and aging all take some energy away. NREL’s PVWatts uses 14% total losses by default, and the calculator starts at the same number.

How much energy does one solar panel make?

Panel watts × peak sun hours × (1 − losses). A 400 W panel at 4.5 sun hours and 14% losses makes 0.4 × 4.5 × 0.86 = 1.548 kWh a day, about 47 kWh in an average month.

Why is the answer rounded up?

You can only buy whole panels, so the calculator rounds the panel count up. The system with whole panels then makes a little more than the share you asked for; the result shows how much, as the share of use covered.

Does the calculator include cost or savings?

No. Prices, incentives and power rates change often and differ by place, so the calculator sizes the panels only. Use the system size in kW to compare quotes.