Self-Consumption vs Export

Ask most solar calculators what your panels save you and they will do something like this: estimate annual generation, multiply by your electricity rate, call it your annual saving. It is simple, it is intuitive, and in roughly half the world it is wrong by a factor that pushes payback estimates out by years.

1. A generated unit and a saved unit are not the same thing

Solar panels generate most heavily in the middle of the day. Many households consume least then. What happens to the surplus determines what it is worth, and there are broadly three regimes:

Only in the first regime does “generation × retail rate” give the right answer. In the other two it overstates savings by the share you export multiplied by the gap between the two rates.

2. The double-counting trap

There is a subtler version of this error that is easy to miss. A calculator credits all your generation at the retail rate, then adds your export income — SEG payments, or a feed-in tariff — on top as a bonus. The exported units have now been paid for twice.

The correct treatment is a blend: value the self-consumed share at the retail import rate, value the rest at the export rate, and add nothing further. That is what this calculator does, and it is why our UK payback figures are longer than many sites quote.

3. What share should you assume?

Self-consumption depends on occupancy, appliances and whether you have storage. Without a battery, typical figures are:

• United Kingdom — around 45%. Modest generation peaks, and a reasonable chance of someone at home.
• Australia — around 35%. Large systems, strong midday peaks, frequently empty houses.
• With a home battery — 70% or more in either market, which is the main financial case for storage.
• Full net-metering markets — the number does not matter for your bill, because both sides are valued identically.

4. What you can actually do about it

In an export-tariff market, raising self-consumption is usually the highest-return action available to you, and most of it is free. Run dishwashers, washing machines and dryers on daytime timers. Charge an EV at midday rather than overnight where your tariff allows. Pre-heat or pre-cool the house while the sun is up. Each point of self-consumption you gain is worth roughly the difference between your import and export rates.

In a net-metering market, none of this changes your bill — so do not let an installer sell you a battery on savings grounds unless your state has moved to net billing, or you want backup power for its own sake.