Solar PV Sizing Calculator

Enter your daily energy use, the peak sun hours at your location, your expected system losses, and your chosen panel wattage. The calculator works out the array size in kilowatts and the number of panels required.

Typical value: 0.23 (23%). Accounts for wiring, inverter and temperature losses.

How It Works

The sizing formula

A solar PV system must generate enough energy to cover your daily load despite real-world losses. The required array size in kilowatts is:

Array (kW) = Daily kWh ÷ (Peak sun hours × (1 − Loss factor))

Dividing the array kilowatts by the individual panel rating (converted to kW) and rounding up gives the panel count:

Panel count = ⌈Array (kW) × 1000 ÷ Panel (W)⌉

Worked example

A household uses 10 kWh per day, has 5 peak sun hours, and expects 23% system losses (loss factor 0.23). Panels are rated at 400 W.

  • Effective sun hours = 5 × (1 − 0.23) = 5 × 0.77 = 3.85 h
  • Array size = 10 ÷ 3.85 ≈ 2.597 kW
  • Panel count = ⌈2597 ÷ 400⌉ = ⌈6.49⌉ = 7 panels

Loss factor explained

The loss factor (default 0.23) captures the real-world difference between a panel's rated output and what actually reaches your load. Typical contributors: inverter efficiency (~4%), wiring losses (~2%), temperature derating (~5%), soiling (~2%), shading (~2%), and module mismatch (~2%). The PVWatts default of 14-25% covers most residential systems; commercial installs with optimisers can sit closer to 14%.

Peak Sun Hours by Region (Reference)

Peak sun hours (PSH) is not the number of daylight hours; it is the equivalent number of hours per day at full rated irradiance (1000 W/m²). A site with 5 kWh/m²/day of solar resource has 5 PSH.

Region Typical PSH (h/day)
US Southwest (Arizona, Nevada) 5.5 – 7.5
US Pacific Coast / Pacific Northwest 3.5 – 5.5
US Northeast / Midwest 3.5 – 5.0
UK / Northern Europe 2.5 – 3.5
Australia (most areas) 4.5 – 6.5
Canada (south) 3.0 – 5.0

Values are approximate annual averages. Use a site-specific tool such as PVWatts or PVGIS for accurate resource data at your location.

Frequently Asked Questions

How many solar panels do I need?

The number of panels depends on your daily energy use, your local peak sun hours, system losses, and the wattage of each panel. Divide your daily kWh requirement by the effective daily output per panel (panel kW times peak sun hours times efficiency factor) and round up. This calculator does that arithmetic for you.

What are system losses in a solar installation?

System losses cover every source of energy that is generated by the panels but does not reach your load: inverter conversion losses (typically 4-6%), DC and AC wiring resistance, temperature derating (panels produce less power as they heat up), soiling and dust on the glass, shading from nearby objects, and module mismatch. The widely used default of 23% (loss factor 0.23) covers a typical residential system; high-quality installs with string optimisers or micro-inverters can achieve 14-18%.

What are peak sun hours?

Peak sun hours (PSH) is not the total number of daylight hours. It is the number of hours per day at which the sun delivers exactly 1000 W/m² (the standard test condition for panel ratings). A site receiving 5 kWh/m² per day of solar irradiation has 5 PSH regardless of whether that energy arrives over 10 hours of lower-intensity daylight. Use a resource map such as NASA POWER, PVGIS (Europe/global), or NREL PVWatts (US) to find the PSH for your postcode.

How do I size a solar system for my daily kWh use?

First, find your average daily consumption from your electricity bill (total monthly kWh divided by the number of days in the billing period). Enter that figure along with your local PSH, your expected loss factor, and your chosen panel wattage. The calculator returns the minimum array size in kW and the number of panels. Add a 10-20% design margin if you want headroom for future load growth or for months with below-average sun.

Should I size for battery storage or grid-tie?

This calculator sizes the PV array for your daily energy production target, which applies to both grid-tie and battery systems. For a grid-tie system the array only needs to match your net daily consumption. For an off-grid or battery-backup system you also need to size a battery bank to cover night use and low-sun days; use the Battery Runtime Calculator to size the storage side. In practice, grid-tied systems are often slightly undersized (offset by grid imports) while off-grid systems carry a larger safety margin.

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