A year of sun.

01Why it changes

Two things happen as the year turns. The days get longer and shorter, and the sun climbs higher or sits lower in the northern sky. In summer it's nearly overhead at midday and up for more than fourteen hours. In winter it's low, its light spread thin across the panels, and gone by five.

Put together, a clear midsummer day delivers several times the energy of a clear midwinter one. Over a whole month, with winter cloud included, Melbourne's record shows about four times as much sun in January as in June.

02What it does to a roof

A system that makes forty kilowatt-hours on a January day might make ten on a dull day in June. Nothing is wrong with it. It's doing what the sky allows.

This is why a single day's output, or a single month's, is a poor guide to anything. The figure that describes a system is what it makes across a year.

A system that looks oversized in January can look modest in June. Both are true.

03What your home does meanwhile

Your use changes with the seasons too, and not always in step with the sun. A home cooled by air conditioning uses most on exactly the days the roof makes most, which is a happy match. A home heated by a heat pump uses most in July, when the roof makes least.

Homes differ so much here that we don't guess. A house on gas heating barely changes its electricity use in winter. An all-electric one can double it. So we work from the season your own bill covers and tell you which one it was.

04Sizing for the year

You could size a system to cover a winter day. It would be very large, and for eight months of the year it would export most of what it made for a small credit. You could size it for summer instead and buy most of your winter power from the grid.

Good sizing sits between the two: enough to carry the shoulder months well, accepting some export in summer and some import in winter. Where exactly depends on your bill.

05Batteries through the seasons

A battery behaves differently across the year as well. In summer it's often full before lunch and the rest of the day's surplus is exported. In the depths of winter there may not be enough spare sun to fill it at all.

That isn't a reason not to have one. It's a reason to size it for the nights you actually have, across the whole year, and not for the best day in January.

06How we show it

Our simulation has three days in it: an average day, a summer day and a winter day. Yearly savings are worked out month by month from the sunshine record, not from one average.

It's more arithmetic, and it gives a smaller, truer number than multiplying a good day by three hundred and sixty-five.