How many panels does a home need?

01Start with what you use

Your bill shows your average daily use in kilowatt-hours. That number is the beginning of every sizing conversation. A home using fifteen a day and a home using forty need very different roofs, however similar the houses look from the street.

02What a kilowatt of panels makes

In Melbourne, a kilowatt of north-facing panels makes roughly 3.6 kilowatt-hours a day, averaged across a year. More than that in summer, much less in winter. East and west faces make a little less over the year and deliver it at different times of day.

So a 6.6 kilowatt system makes somewhere around twenty-four kilowatt-hours on an average day. It sounds like enough for a home that uses twenty. It usually isn't, for a reason that has nothing to do with the total.

03When you use it matters more

The roof makes its power in daylight. If your home uses most of its electricity in the evening, a lot of what the panels make leaves for the grid while you're out, and you buy it back at night.

This is the gap a battery fills. Without one, the useful size of a system is tied to how much you use during the day. With one, it's tied to the whole day and night.

It isn't how much your roof makes. It's how much of it you're there to use.

04From kilowatts to panels

Panels today are rated around 440 to 475 watts each. Fourteen of the larger ones make a 6.6 kilowatt system. Twenty-eight make a little over thirteen. Each is roughly 1.1 by 1.8 metres, so you can pace out a roof face and get a fair idea of what fits.

What fits is a ceiling, though, not a target.

05What sets the limit

Three things cap the size of a system. The roof: its usable faces, their direction and their shade. The connection: your distributor sets how much a home may connect and export, and single-phase homes have a lower ceiling than three-phase ones. And usefulness: past a certain point, each extra panel mostly earns a small export credit.

06Leave room for what's coming

The one good reason to go larger than today's bill suggests is that your bill is about to change. An electric car, a heat pump replacing gas hot water, an induction cooktop, a pool. Each adds a real load.

If any of those are on the horizon, tell us. It's far cheaper to allow for them now than to add panels in three years.