How big a solar system do I need?

What a proper sizing uses

1. Twelve months of consumption, ideally interval data from your utility rather than monthly totals, because when you use power matters as much as how much.
2. The usable roof area by orientation and pitch, after fire setbacks and obstructions.
3. A shade model across the year, not a single afternoon.
4. Your utility's interconnection and export rules, which can cap system size or make exports worth much less than consumption.
5. Anything changing soon: an electric vehicle, a heat pump, an extension, a child leaving home.

Why bigger is not automatically better

Where exported energy is worth substantially less than energy you use yourself — which is increasingly the case as net metering rules change — a system sized well beyond your consumption sends its surplus somewhere it earns little. That shifts the calculation towards self-consumption, and sometimes towards a battery, and it is entirely dependent on the rules where you live.

Ask what export rules apply at your address, and ask the installer to show the assumption in the proposal. It is the input that most changes the answer and the one most often left implicit.

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