Solar System Size Calculator

Sizing a solar system isn't one calculation — it's a chain of them. It starts with how much electricity you use, factors in how much usable sunlight your location gets, accounts for real-world losses in wiring and equipment, and ends with a practical combination of panels, an inverter, and — if you want backup power — a battery bank. Skip a step, and the final number can be badly wrong in either direction: undersized for your needs, or oversized for your budget and roof.

What information you need before you start

Your electricity bill reports usage in kilowatt-hours (kWh), usually over a month. Convert that to a daily figure first:

A bill showing 600 kWh over 30 days works out to 20 kWh per day — the same default used in the calculator above. If you're designing from scratch instead of working from a bill, you can build this figure appliance by appliance:

A worked appliance breakdown

Note that a refrigerator's compressor cycles on and off rather than running continuously — its effective daily runtime is far less than 24 hours, which is why "hours/day" above reflects actual run time, not time plugged in.

Peak sun hours, explained properly

A peak sun hour is one hour during which sunlight arrives at an intensity of 1,000 watts per square meter. It does not mean "one clock-hour of visible sunshine." A location averaging 5 peak sun hours doesn't get 5 hours of maximum-intensity sun — the total energy received across the whole day adds up to the equivalent of 5 hours at that peak intensity. This figure changes with latitude, season, cloud cover, panel tilt, and shading, which is exactly why using an accurate local figure matters more than most other inputs in the calculation.

The core sizing formula

The performance factor stands in for real-world losses: wiring resistance, inverter conversion loss, panel temperature effects, dust, minor shading, and degradation over time. A reasonable starting assumption is 75–85%.

Two homes using identical electricity can still need different solar capacities — a home with only 4 peak sun hours needs roughly 25% more PV capacity than one with 5, to produce the same daily energy.

Turning kW into a panel count

Real installations rarely land on a clean number — installers round based on available stock, roof layout, and inverter string voltage limits. A calculated 12.5-panel requirement becoming a 12 or 13-panel real system is normal, not an error.

Inverter sizing is a separate calculation

Inverter capacity does not need to equal panel capacity. Sizing depends on continuous load, peak simultaneous load, and the starting surge from motor-driven equipment like pumps, air conditioners, and refrigerator compressors — which can briefly draw two to six times their running wattage. In practice, a 5 kW array is frequently paired with an inverter rated somewhat below that figure, commonly where the array sits at 110–130% of the inverter's rated AC output, since panels rarely produce their full rated output simultaneously.

Battery sizing

For 5 kWh of critical daily backup load: a lithium-ion battery (80% depth of discharge, 90% efficiency) needs about 6.9 kWh of nominal capacity. A lead-acid battery (50% depth of discharge) needs about 11.1 kWh for the same usable energy — nominal capacity alone is a misleading way to compare battery products.

On-grid, off-grid, or hybrid

Roof space

A typical panel needs roughly 2 square meters of usable roof area once spacing and mounting clearance are included. Ten panels need about 20 m² (215 sq ft), twenty need about 40 m² (430 sq ft), and thirty need about 60 m² (645 sq ft). Orientation, shading, and structural condition still need a site visit to confirm.