Solar panel calculator: how many panels, and what size of system, from your own bill
Put in the kilowatt-hours from your latest bill and how much roof you could give to panels. The calculator returns an indicative system size in kilowatts-peak, a panel count, the glass area that takes, and the arithmetic it used with your figures in it.
It is an estimate of size. Every assumption is on the page and you can change the ones that matter.
Solar panel calculator
Two figures from you: the kilowatt-hours on your latest bill, and roughly how much roof you could give to panels. The result is an estimate of size. It is not a quotation and it carries no price.
Your figures
Use the kilowatt-hours line on the bill, not the amount due. For the roof, pace out the one face that gets sun through the middle of the day. Where the kWh figure sits on the bill.
Assumptions, yours to change
Daytime use is set at 45 percent of the month, a working assumption for a house that is half empty in the day. Peak sun hours is the Global Solar Atlas figure at one named point in the area you pick. The panel is a 600 W module of about 2.6 m², and the performance ratio is 0.8, for inverter losses, heat, wiring and dust.
Estimate
- System size
- 3.2 kWp
- Panels, at 600 W each
- 6
- Glass area
- 15.5 m²
- Against your roof
The formula, with your numbers in it
- Daily need: 400 kWh × 100% ÷ 30 days = 13.3 kWh a day.
- Array: 13.3 ÷ (5.23 peak sun hours × 0.8) = 3.2 kWp.
- Panels: 3.2 kWp × 1,000 ÷ 600 W = 5.3, rounded up to 6.
- Glass: 6 × 2.6 m² = 15.5 m².
An estimate, from a yearly average of sun. The rainy months make less and the dry months more, a panel needs room around it beyond its own glass, and the roof itself decides the mounting. We measure the roof before we quote anything.
The formula, written out
Four steps, and the calculator prints each one with your numbers so nothing happens out of sight.
- 1. Daily need, in kWh
- Monthly kilowatt-hours, times the share the system should cover, divided by thirty days.
- 2. Array size, in kWp
- Daily need divided by the peak sun hours at your place, then divided by a performance ratio of 0.8. The ratio covers inverter losses, heat on the glass, wiring and dust, which together take about a fifth before anything reaches the meter.
- 3. Panel count
- Array size in watts divided by one panel’s rating, rounded up. The calculator uses a 600 W module. A smaller panel means more of them for the same array.
- 4. Glass area, in m²
- Panel count times one panel’s face, about 2.6 m² for a 2278 by 1134 mm module. The roof has to be larger than this figure, because panels need room around them.
Where each input comes from
The kilowatt-hours are on your bill as a line of their own, separate from the amount due, and reading the meter yourself gives you the same figure between bills. The peak sun hours are the Global Solar Atlas value at a named point in each area, published by the World Bank Group and ESMAP, on Solargis data, and the five figures and their coordinates are on a page of their own. The default is San Fernando, Pampanga, because that is where this business is.
The panel size and face come from a current datasheet, the JinkoSolar JKM580-605N-72HL4-(V), the same module the 3kW, 5kW and 10kW page works from, so the two pages agree. Type over any of them if you have better figures for your own roof.
What the estimate leaves out
- Which way the roof faces, and what shades it
- The atlas figure is for a flat surface in open sky. A roof facing away from the sun or shaded by a water tank or a neighbour’s wall makes less, and only a survey measures how much.
- The season
- Peak sun hours is a yearly average. Rainy months fall below it and dry months rise above, so no single bill will match the estimate exactly.
- The roof structure
- GI sheet, standing seam, concrete deck and tile each take a different mounting, and a tired roof is repaired before anything is bolted to it. What a roof survey checks.
- Nights and brownouts
- A system sized to the whole bill still makes nothing after dark. Net metering credits the surplus and a battery stores it, and neither is in the panel count. How net metering works, and what a battery changes.
- The price, and how it is paid
- The calculator sizes a system and does not price one; the price comes from a quotation after a survey. A GSIS member can borrow toward it under the GSEL, which asks for that quotation. The GSIS solar loan, its terms and who qualifies.
Questions about sizing
How many solar panels do I need?
Take the kilowatt-hours on your bill, divide by thirty for a daily figure, divide that by the peak sun hours at your place times a performance ratio of 0.8, and you have the array in kilowatts-peak. Divide by one panel’s rating in watts and round up. The calculator above does it with your numbers and prints each step.
What is a peak sun hour?
The day’s sunlight expressed as hours of full sun at one kilowatt per square metre. The calculator uses the Global Solar Atlas figure at one named point in each area, between 5.09 at Malolos and 5.31 at Tarlac City, rather than an average for a province.
Why does the calculator ask what the system should cover?
Because a system that exports its surplus under net metering, or stores it in a battery, can be sized to the whole bill. A system with neither only helps while the sun is up, so it is sized to daytime use, which the calculator takes as 45 percent of the month unless you change it.
Is the roof area the same as the glass area?
No. The glass figure is the panels alone. A roof needs room around them for the mounting, for walking access and for clearance at the edges, and how much depends on the roof. If the glass already exceeds the roof you typed, the calculator says how many panels would fit and what size that is.
How much will it cost?
This page does not say, and it is deliberate. Solar is priced per roof after a survey, and the published guides disagree with each other by about a factor of two for the same size. The size is the half of the arithmetic a web page can do honestly.
Send us the two numbers
Your monthly kilowatt-hours and your town are enough for a written first answer. The panel count becomes a quotation only after somebody has stood on the roof with a tape measure.
Tell us the property and what you are trying to cut, and you will get a written answer with a size and a next step. No visit is booked and nothing is charged for asking.
Nothing on this page is a quotation. The calculator is an illustration built from figures you supply and assumptions you can change, and its output is not a promise about what any roof will generate or save. Peak sun hours: Global Solar Atlas, data version 2.2.68, read 23 September 2026. Panel dimensions: JinkoSolar datasheet JKM580-605N-72HL4-(V), F8 EN edition, read 1 October 2026.