How to Size a Solar System from a Customer’s Electricity Bill

Every accurate solar quote starts with one question: how much energy does this customer actually use? The bill answers it. Here is a clean, repeatable way to go from a monthly bill to a system size that fits.

Step 1 — Establish real consumption

Pull the monthly units (kWh) from the bill. One month is a starting point, but consumption swings with the seasons — air-conditioning in summer, less in winter. Where you can, average several months to get a representative annual figure.

Step 2 — Convert usage into generation needed

Translate annual consumption into the kilowatt-peak (kWp) of panels required, using a realistic local yield (kWh produced per kWp per year) for the site. Yield depends on irradiance, tilt, orientation and shading — not a fixed national average.

Step 3 — Cross-check against appliance load

A bill tells you energy; an appliance load calculation tells you peak demand and informs inverter and battery sizing. Run both. When they disagree, size to the larger need so you never undersize the system.

Common mistakes

• Sizing from a single winter bill and undershooting summer demand.
• Using a flat tariff instead of the customer’s actual slab.
• Filling the whole roof regardless of what the customer uses — they end up paying for panels they don’t need.
• Ignoring shading and orientation, which can swing real yield by 20% or more.

Make it instant

SolarStack’s AI bill reader pulls usage straight from the bill, and the assessment sizes the system to the larger of load and billed usage — capped by the roof. What used to take a spreadsheet and a site visit becomes a few minutes at your desk.