How many solar panels do you need?

How many solar panels do I need?

Divide the electricity you use in a year by what 1 kW of panels makes at your site: peak sun hours × 365 × (1 − system losses) × inverter efficiency. Then divide the kilowatts by the panel size. Example: 10,800 kWh a year, 5 peak sun hours, PVWatts' default 14% losses and 96% inverter efficiency — 1 kW makes 5 × 365 × 0.86 × 0.96 = 1,507 kWh, so you need 10,800 ÷ 1,507 = 7.17 kW. With 400 W panels that is 17.9, rounded up to 18 panels (7.2 kW), making about 10,850 kWh a year.

What are peak sun hours, and where do I find mine?

Peak sun hours are the day's sunlight on the panels expressed as hours of full sun at 1 kW per square meter, so they equal the daily solar radiation in kWh/m² (PVEducation). The PVWatts calculator from NREL — now the National Laboratory of the Rockies — gives it for your address, tilt and direction as "Solar Radiation (kWh/m²/day)", from the National Solar Radiation Database. Use the average for the year. The 5 in the calculator is only an example.

Why 14% system losses?

It is the default in PVWatts: soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1% and availability 3%. They combine by multiplying, not adding, to 14%. The inverter is counted separately — PVWatts' default is 96%. Raise the losses if trees or buildings shade your roof.

How much roof space do the panels need?

Enter the panel's length and width from its datasheet: the calculator multiplies them by the number of panels. That is the area of the panels alone — the layout also needs gaps between rows and the roof setbacks your local fire and building codes require, so plan on more roof than that.

How accurate is this estimate?

It is a first sizing. It does not model panel temperature, the exact tilt and direction, or the inverter cutting off peaks, which PVWatts calculates hour by hour. Once you have a system size, run it through PVWatts for your address, and check the result against quotes from installers.

How much the sun hours change the answer

Using the same 10,800 kWh a year, the default 14% losses and 96% inverter efficiency, the system size depends mostly on the peak sun hours of the site:

• 3 peak sun hours: 1 kW makes 904 kWh a year, so you need 11.95 kW, which is 30 panels of 400 W
• 4 peak sun hours: 1 kW makes 1,205 kWh a year, so you need 8.96 kW, which is 23 panels of 400 W
• 5 peak sun hours: 1 kW makes 1,507 kWh a year, so you need 7.17 kW, which is 18 panels of 400 W
• 6 peak sun hours: 1 kW makes 1,808 kWh a year, so you need 5.97 kW, which is 15 panels of 400 W

A location with 3 sun hours needs about 2.0 times the system of one with 6. Losses matter less but still count: with 20% losses instead of 14% (for example for a partly shaded roof) the 5-hour case needs 7.71 kW instead of 7.17 kW.

Worked example: starting from twelve monthly bills

Add up twelve months of use from your utility bills, for example 820, 760, 800, 850, 920, 1,100, 1,250, 1,230, 980, 880, 790 and 820 kWh. The total is 11,200 kWh, or 933 kWh a month on average. Summer use is higher here, but a grid-tied system is sized against the yearly total when the utility credits surplus production against later use; check how your utility handles credits, because the rules differ. With 5 peak sun hours the yearly figure gives 7.43 kW DC, or 19 panels of 400 W. If the panels measure 65 by 40 inches (check your datasheet), each is 18.1 square feet, and the panels alone cover 343 square feet of roof.

DC size, AC size and other things to check

The system size here is the DC rating: the sum of the panels' nameplate watts. The inverter turns it into AC, which is why its efficiency appears in the formula, and installers often quote both numbers. Your estimate also depends on tilt and direction, shade at different times of year, and whether electricity use will change, for example with an electric vehicle or a heat pump. None of that is in a yearly average, so treat the result as a starting point for quotes, not a design.

Sources: PVWatts Version 5 Manual (A. P. Dobos, NREL, 2014) — default losses and inverter efficiency; PVEducation (C. Honsberg, S. Bowden) — peak sun hours. For planning only; an installer sizes the real system. How we check our calculators.