Solar Savings Calculator

This calculator estimates how much you'd save each year from a rooftop solar PV system, based on its size, your local sunshine and the price you currently pay for electricity. It then turns that into a payback period and a 25-year net saving — the figures that actually decide whether the install is worth doing.

It matters because solar economics have changed sharply since the pre-2020 era of government feed-in tariffs. Today's return depends almost entirely on how much of the electricity you generate you actually use yourself, the price of grid electricity, and what your supplier pays for any surplus exported. The headline "you'll generate £X" number means very little until those factors are stripped out.

It's most useful for homeowners with south-facing or east/west-facing roofs in good condition, who use a reasonable amount of electricity during daylight hours. It's less useful for north-facing roofs, heavily shaded properties, or households that are out from 8am to 7pm with no battery storage — in those cases the export rate becomes the dominant return, and it's far lower than the import rate you avoid.

A worked example

A 4kW UK system on a south-facing roof typically generates around 3,600–4,000 kWh per year (4kW × 3 peak sun hours × 365 ≈ 4,380 kWh, with weather losses bringing it down). At 27p/kWh of avoided import electricity and ~100% self-consumption, that's roughly £950–£1,100/yr of saving. With a £6,000 install cost the payback lands at about 6 years and the 25-year net saving comfortably exceeds £15,000. If self-consumption is only 40% and the rest exports at 15p, the annual saving drops to around £700 and payback stretches to 8–9 years.

Why this matters

Solar is one of the few home improvements where the financial case is genuinely straightforward — there's an upfront cost, a fairly predictable annual return, and a long lifespan. But the spread between a good and bad install is wide: the same panels on a south-facing roof in a home that does its washing during the day can earn twice what they'd earn on a north-facing roof in an empty-during-the-day house. Modelling the real number before signing rather than the brochure number is what makes the decision sound.

Common mistakes

• Assuming 100% self-consumption — for most households without a battery, the real figure is 30–50%.
• Ignoring roof orientation and shading; a south-facing roof generates roughly 25% more than an east/west split and 60–70% more than a north-facing one.
• Forgetting that electricity prices vary — modelling at today's rate when the price may move significantly over 25 years.
• Comparing quotes only on £/kW without checking inverter quality, warranty length and the installer's MCS accreditation.
• Adding a battery as an afterthought rather than at install time, when it's significantly cheaper to include.

Beyond the numbers

Three levers move the return more than the panel price itself. First, self-consumption — using electricity when the panels are generating (typically 10am–4pm in summer, 11am–2pm in winter) — which can be improved by running washing machines, dishwashers and hot-water immersion heaters on timers. Second, battery storage, which lifts self-consumption from ~30% to 70%+ and dramatically improves the economics if your day-time use is low. Third, the export tariff: SEG (Smart Export Guarantee) rates currently range from around 5p to 15p+/kWh depending on the supplier, and switching to a better one is often the single biggest free improvement to the return. Pair this with the Energy Bill Calculator to see the headline bill alongside the solar saving, and with the Insulation Savings Calculator — insulating first usually outperforms generating more.

Related tools: Energy Bill · Insulation Savings · Mortgage Overpayment · Renovation Budget