Solar Panel Calculator
Work out the solar system size, panel count, roof area and annual kWh your home needs from your daily use and peak sun hours, then see the simple payback.
System size = daily kWh ÷ (peak sun hours × derate factor); panel count rounds that up to whole panels; annual production = installed kW × sun hours × 365 × derate. The 0.77 default derate is NREL's classic PVWatts DC-to-AC value covering inverter, wiring, soiling and mismatch losses — modern inverters often justify 0.80–0.86, so edit it. Simple payback ignores finance costs, net-metering rules, escalating tariffs and maintenance. Planning estimates only, not a design or a quote.
What is the Solar Panel Calculator?
The ByteTools Solar Panel Calculator turns two numbers you already have — your daily electricity use and the peak sun hours where you live — into a system size.
- System size, panel count and roof area from daily kWh and peak sun hours
- Editable derate factor, panel wattage and physical panel dimensions
- Annual, monthly and lifetime production with a yearly degradation rate
- Bill saving and simple payback at your own electricity price
- Roof area in square feet or square metres, in seven currencies
- Runs entirely in your browser — no address, no signup, nothing uploaded
How to use the Solar Panel Calculator
- 1
Enter your daily electricity use in kWh (take a monthly bill figure and divide by 30) and the share of it you want solar to cover.
- 2
Set the peak sun hours for your location and adjust the derate factor if you know your losses — 0.77 is the classic PVWatts default.
- 3
Enter the panel wattage and its two edge lengths so the calculator can work out the roof area needed.
- 4
Add your installed cost after incentives and your electricity price per kWh to see the annual saving and simple payback.
- 5
Read the system size, panel count, roof area, annual production and lifetime saving in the result tiles.
About the Solar Panel Calculator
The ByteTools Solar Panel Calculator turns two numbers you already have — your daily electricity use and the peak sun hours where you live — into a system size. It divides your daily kWh by sun hours × a derate factor to get the array size in kW, rounds that up to whole panels at your chosen panel wattage, and reports the roof area those panels occupy.
It then projects what the array actually produces: annual and monthly kWh, the bill saving at your unit rate, a simple payback in years, and lifetime output with a yearly degradation rate applied. Everything is editable, so you can model a partial offset, a shadier roof, or bigger panels without starting over.
All the arithmetic runs locally in your browser — nothing is uploaded, no address is asked for, and no quote request is generated. It is a planning estimate rather than a solar design: a real proposal accounts for roof pitch and azimuth, shading through the day, local net-metering rules and inverter clipping.
Frequently asked questions
How many solar panels do I need for a 2,000 square foot house?
Floor area is the wrong input — electricity use is what matters. A home using 30 kWh a day at 5 peak sun hours needs roughly a 7.8 kW array, which is about 20 panels at 400 W each. Enter your own daily kWh above rather than working from square footage.
What is a derate factor in solar?
It is the fraction of the panels' rated DC output that actually reaches your meter after inverter losses, wiring resistance, soiling, shading, temperature and module mismatch. NREL's original PVWatts default was 0.77; modern equipment often justifies 0.80 to 0.86, and the field here is editable.
What are peak sun hours?
Peak sun hours are the number of hours per day at 1,000 W/m² that would deliver the same energy as the real, varying sunlight. Much of the southern US sees 5 to 6, the northern US and central Europe 3.5 to 4.5, and the UK closer to 2.5 to 3.
How long does it take for solar panels to pay for themselves?
Simple payback is installed cost ÷ annual bill saving, which for many homes lands between 7 and 12 years. That figure ignores financing costs, tariff escalation, net-metering credits and any maintenance, so treat it as a first sanity check rather than a financial model.
How much roof space do solar panels need?
A typical 400 W residential panel is about 74 by 41 inches, roughly 21 square feet or 1.96 square metres. Twenty of them need around 420 square feet of unshaded roof before you allow for setbacks, walkways and obstructions, which the calculator reports as you change the panel size.
Does this calculator account for my roof angle and direction?
Not directly — that is what the peak sun hours and derate figures absorb. A roof facing away from the equator or badly shaded in the afternoon effectively has fewer usable sun hours, so lower those inputs rather than expecting the tool to model geometry.
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