5 Calculations to Do Before Buying Solar Panels

1. Where the value comes from

Electricity you generate and use yourself avoids buying it at the retail rate, which is the largest saving.

Electricity exported earns a payment that is typically well below the retail rate.

So the return depends heavily on how much you consume during daylight, which depends on your household pattern.

A household out all day exports most of what it generates and saves considerably less than one that is home.

2. What affects generation

Roof orientation and pitch, with south-facing generally best in the northern hemisphere.

Shading, which affects output disproportionately, since shading part of an array can reduce the whole string.

Panel area available, and the local climate.

A proper quote models this for your specific roof rather than quoting a generic figure.

3. Batteries

Storage raises self-consumption by holding daytime generation for evening use.

It adds substantial cost and has its own lifespan, typically shorter than the panels.

Whether it improves the return depends on the price paid, the tariff and the consumption pattern; it is not automatic.

4. Doing the calculation

Estimate annual generation from the quote.

Estimate the proportion you would use directly, honestly, based on when you are home.

Value that portion at your electricity rate and the remainder at the export rate.

Compare the annual figure against the installed cost to get a simple payback period, then consider it against the system's expected life.

5. Practical checks

Roof condition and remaining life, since removing and refitting panels to repair a roof is expensive.

Structural capacity, and whether scaffolding is included in the quote.

Installer certification under the relevant scheme, which is usually required for export payments.

Warranty terms for panels, inverter and workmanship, which differ from each other. Inverters typically need replacing before panels do.

Any grants or incentives available locally, which change the arithmetic materially.

Final Thoughts

Domestic solar is a long-term decision presented with optimistic assumptions, and the arithmetic is checkable. Be honest about how much you would use directly, since that is where most of the return comes from.