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Solar System Size Calculator

Find out roughly how big a solar system your home needs — in kWp, number of panels and roof area — from your monthly electricity use. Add a battery if you want power at night or during brownouts.

  • Free, no sign-up
  • Runs in your browser — nothing is sent to us
  • Updated October 27, 2026
Your electricity use
kWh
From your bill. Use an average of several months if you can.
%
100% aims to generate as much as you use in a month. Without net metering, consider covering only your daytime use.
Sunlight and panels
h/day
Philippine average ≈ 4.7 (World Bank / Solargis). Check your location on the Global Solar Atlas.
%
Output left after heat, wiring, inverter, dust and shading losses.
W
From the panel datasheet or quote (e.g. “550 W”).
%
From the datasheet. Used only for the roof-area estimate.
Battery for brownouts or night use (optional)

Leave night-time use at 0 for a system without a battery.

kWh/day
kWh you want the battery to supply each night.
%
From the battery datasheet.
%
Energy you get back out of the battery per unit stored.

Your estimate

Updates as you type. Estimates only.

Suggested system size

2.75 kWp

5 panels × 550 W

Exact size needed
2.69 kWp
before rounding to whole panels
Generation
307 kWh
per month, on average
Use covered
102%
of your monthly kWh
Panel area
13.1 m²
panels only, no gaps
Estimate savings for this size

How this calculator works

It works backwards from the energy you use to the system that would produce it:

Daily energy to cover   = monthly kWh × share to cover ÷ 30.4 days
System size (kWp)       = daily energy ÷ (peak sun hours × performance ratio)
Number of panels        = system size × 1,000 ÷ panel rating (W), rounded up
Panel area (m²)         = installed watts ÷ (panel efficiency × 1,000 W/m²)
Battery size (kWh)      = night-time kWh ÷ usable share of the battery

When you add night-time use, the system is also made slightly bigger to make up for energy lost charging and discharging the battery (night-time kWh × (1 ÷ round-trip efficiency − 1)).

Worked example

Example (estimate). A home using 300 kWh a month, covering 100%, with 4.7 peak sun hours, a 78% performance ratio and 550 W panels:

  • Daily energy: 300 ÷ 30.4 ≈ 9.9 kWh
  • Size needed: 9.9 ÷ (4.7 × 0.78) ≈ 2.69 kWp
  • Panels: 2,690 W ÷ 550 W → 5 panels = 2.75 kWp, about 307 kWh a month
  • Adding a battery for 3 kWh each night: about 3.3 kWh of battery and 6 panels (3.3 kWp) to cover charging losses

These are planning estimates, not a design. Real output depends on your location, roof and shading.

Choosing your inputs

How much of your use to cover

Covering 100% means the system generates about as much energy in a month as you use. But solar produces during the day, and the value of exported energy depends on your utility and net-metering status. If you won’t have net metering, or you use most electricity at night, consider sizing to your daytime use instead — for example, 40–60% — and see how the savings compare in the solar savings calculator.

Peak sun hours and performance ratio

World Bank / ESMAP country data prepared by Solargis put the Philippines’ average daily global horizontal irradiation at about 4.73 kWh/m² — roughly 4.7 peak sun hours — with most of the country between about 4.25 and 5.17 (10th–90th percentile). The calculator starts at 4.7 hours. Look up your own location on the Global Solar Atlas for a better figure. An IEA PVPS study of rooftop systems in tropical Singapore reports typical performance ratios of about 75–80%, so the calculator starts at 78%. Installers’ simulation reports usually state their own figure — ask for it.

Panel rating and efficiency

Use the figures from the datasheet of the panel in your quote. The panel rating (in watts) decides how many panels you need; the efficiency only affects the area estimate. The area shown is the panels themselves — real layouts need gaps, walkways and setbacks from roof edges, so allow more roof space.

Batteries

Enter how many kWh you want available each night or during a brownout. The calculator divides by the usable share of the battery, because most batteries shouldn’t be fully drained. Batteries also lose some capacity as they age, so installers often add a margin.

Limitations

  • Uses one average month. Output is lower in the rainy season and higher in the dry months, so a system sized to 100% of average use will under-produce some months and over-produce others.
  • Doesn’t check your roof’s direction, tilt, shading or structure, your inverter size, or your utility’s connection limits.
  • Battery sizing ignores very high loads (like aircons) that may need a larger inverter, not just more kWh.
  • It is not a quote or an engineering design. A qualified installer should assess your home.

Read more in how many solar panels a Philippine home needs, then compare installer quotes.

Frequently asked questions

How many kWp do I need for a 300 kWh monthly bill?
With about 4.7 peak sun hours and a 78% performance ratio, each kWp produces roughly 112 kWh a month, so covering 300 kWh takes about 2.7 kWp. Your location, roof direction and shading change this — an installer’s site survey gives the real figure.
Should I size solar to cover 100% of my bill?
Not necessarily. Solar produces mostly around midday. Energy you use as it’s produced saves the most; energy you export is credited at a lower rate under net metering, or not at all without it. Many homes size to their daytime use first. Compare options in the solar savings calculator.
Do I need a battery?
Only if you want power during brownouts or want to use solar energy at night. Grid-tied systems without a battery usually switch off during a brownout for safety. See on-grid vs hybrid vs off-grid solar.
Why does the calculator round up?
You can only install whole panels, so the system is rounded up to the next full panel. That’s why the installed size can be slightly larger than the exact size needed.

Estimates only. Results depend entirely on the values you enter and the simplified formulas shown above. They are not a quote, a bill forecast or financial advice. Real consumption, tariffs and solar output vary. Read our calculation methodology and confirm important figures with your utility or a qualified professional.