Solar panel calculator

What is a solar panel calculator?

A solar panel calculator is a planning tool that turns your electricity demand into a concrete photovoltaic (PV) system: the total array size in kilowatts, the number of individual panels you would need to install, and roughly how much roof area they occupy. Instead of guessing, you enter how much energy you use, how sunny your location is, and the specifications of the panels you are considering, and the calculator sizes the array for you.

This matters because an undersized array leaves part of your bill uncovered, while an oversized one wastes money on panels and inverter capacity you never use. Getting the size right is the first step in any rooftop solar project.

How does the calculator work?

The calculation rests on a simple idea: each kilowatt of installed panels produces a predictable amount of energy per day, and you need enough kilowatts to cover your daily demand.

First, your consumption is reduced to an average daily energy demand. If you enter a monthly figure, it is divided by the average number of days in a month:

$$E_{\text{day}} = \frac{E_{\text{month}}}{365.25 / 12}$$

The energy a single kilowatt of panels delivers per day depends on two things: the peak sun hours at your location (the equivalent number of hours per day of full 1000 W/m² sunlight) and the performance ratio (a derate factor that captures real-world losses from heat, wiring, the inverter and dust). The required system size in kilowatts is therefore:

$$P_{\text{system}} = \frac{E_{\text{day}}}{H_{\text{sun}} \times r}$$

where $E_{\text{day}}$ is the daily demand in kWh, $H_{\text{sun}}$ is the peak sun hours per day, and $r$ is the performance ratio.

Knowing the rated power of one panel, $W_{\text{panel}}$ (in watts), the number of panels is the system size in watts divided by the panel rating, rounded up to a whole panel:

$$N = \left\lceil \frac{P_{\text{system}} \times 1000}{W_{\text{panel}}} \right\rceil$$

Finally, if you supply the physical area of one panel, $A_{\text{panel}}$, the calculator estimates the roof area the array occupies:

$$A_{\text{roof}} = N \times A_{\text{panel}}$$

Understanding the inputs

• Peak sun hours condense a full day of changing sunlight into the equivalent number of hours at standard 1000 W/m² irradiance. Sunny, low-latitude regions may average 5–6 hours, while cloudy northern climates may see 3 or fewer.

• Performance ratio is the fraction of the panels’ nameplate output that actually reaches your meter. Typical real installations land around 0.7–0.8; this calculator defaults to 0.75. A lower ratio means more panels are needed for the same demand.

• Panel wattage is the rated power of one module under standard test conditions. Modern residential panels commonly fall between 300 W and 450 W.

Worked example 1: a daily demand

Suppose a household uses 30 kWh per day, the location averages 5 peak sun hours, and you plan to use 400 W panels with a 0.75 performance ratio. Each panel measures 1.95 m².

The system size is:

$$P_{\text{system}} = \frac{30}{5 \times 0.75} = \frac{30}{3.75} = 8 \text{ kW}$$

The number of panels is:

$$N = \left\lceil \frac{8 \times 1000}{400} \right\rceil = \lceil 20 \rceil = 20$$

And the roof area is:

$$A_{\text{roof}} = 20 \times 1.95 = 39 \text{ m}^2$$

So an 8 kW array of twenty 400 W panels, covering about 39 m² of roof, meets the demand.

Worked example 2: a monthly bill

Now suppose you only know your monthly usage of 900 kWh, in a location with 4 peak sun hours, using 350 W panels at a 0.75 performance ratio.

First convert to a daily demand:

$$E_{\text{day}} = \frac{900}{365.25 / 12} \approx 29.57 \text{ kWh}$$

The system size becomes:

$$P_{\text{system}} = \frac{29.57}{4 \times 0.75} \approx 9.86 \text{ kW}$$

And the panel count rounds up:

$$N = \left\lceil \frac{9.86 \times 1000}{350} \right\rceil = \lceil 28.16 \rceil = 29$$

Fewer peak sun hours and smaller panels push the array up to 29 modules for a similar yearly consumption.

Practical notes

• The result is a planning estimate, not an engineering design. Roof orientation, tilt, shading, and seasonal variation all shift real output, so treat the panel count as a starting point.

• Rounding the panel count up guarantees the array meets (rather than just approaches) the target demand; the last panel typically adds a small surplus.

• If you want to cover only part of your bill, scale the consumption you enter by that fraction before calculating.

• A more conservative performance ratio is wise in hot climates or installations with long cable runs, since both reduce delivered energy.