How to size a solar system

Start from what you actually use, not from what fits on the roof.

Sizing means deciding how many kilowatts of panel to install. The honest starting point is your own consumption, which is on your bills.

Step-by-step

1. Find your annual kilowatt-hours. On the bill, or add up twelve months.
2. Enter your location's solar resource.
3. Read the array size needed to cover that use.

Use a full year

Sizing from a single bill is the commonest error. Consumption swings enormously between seasons — heating, cooling, lighting, and being at home more in winter. A system sized from a July bill in a heating climate will be badly undersized, and one sized from January in a cooling climate will be oversized.

Covering 100% of use is not the target

A system producing exactly your annual consumption does not make you self-sufficient. Production peaks at midday in summer and collapses in winter; consumption does neither. You will export a surplus in July and buy from the grid in December.

What matters economically is what your surplus is worth. Where exports are paid well, a larger array makes sense. Where they are paid poorly — increasingly common — sizing closer to what you consume during daylight is usually the better return, and the payback tool is where that gets settled.

Before sizing, reduce

Efficiency is almost always cheaper per kilowatt-hour than generation. Insulation, an efficient heat pump, LED lighting and dealing with an old freezer can cut consumption enough to shrink the array meaningfully. Sizing a system to power an inefficient house means paying to generate electricity you did not need.

Frequently asked questions

How many panels do I need?

It follows from your annual consumption, your location and your roof. Start from a full year of bills rather than a single month, since consumption varies enormously by season.

Should I cover all my usage?

Not necessarily. Production and consumption do not line up through the year, so a system matching annual use still exports in summer and imports in winter. What your exports are paid decides whether going larger is worth it.

Is it worth adding panels later?

It is disproportionately expensive, since the scaffolding, labour and possibly the inverter all recur. If a heat pump or electric car is likely, size for it now.