Energy Saving Trust solar calculator: guide

What the Energy Saving Trust calculator does

The official Energy Saving Trust solar panel calculator starts with your address, household size and whether someone is generally home during the day. It uses those details to give an initial view of a suitable system, installation cost, potential bill savings and carbon impact, with options to personalise the result further.

It is designed as an early planning tool before speaking to an installer. It does not replace a structural check, detailed roof survey, electrical design or confirmed quote.

The inputs that matter most

Address and roof details help determine the available solar resource and usable roof area. Household occupancy and electricity use influence how much generation may be used at home rather than exported.

When you personalise the estimate, pay close attention to annual generation, assumed system size, installed cost and the split between self-used and exported electricity. Those are the figures most worth comparing with an installer quote.

• Property and usable roof space.
• Roof direction, angle and shading where requested.
• Household size, daytime occupancy and electricity demand.
• Suggested system size and annual generation.
• Installation cost, bill saving and export value assumptions.

How to read the results

Treat the suggested panel count and system size as a starting point for a roof-specific design. Treat the installation cost as a benchmark, not a quote. Scaffolding, roof condition, cable routes, electrical work, equipment and battery storage can all change the price.

For savings, distinguish electricity you avoid buying from export payments. A self-used kWh is valued at the import rate you avoid; an exported kWh is valued at the export tariff. Ask what rates and self-consumption assumptions sit behind any headline total.

Why another calculator can give a different answer

Calculators can use different roof data, generation models, electricity prices, export rates, installed-cost benchmarks and household-use assumptions. They may also model battery behaviour or future changes differently. A difference is not automatically an error; it is a prompt to compare the inputs.

This site's calculator exposes the quote cost, annual generation, import rate, export rate and self-consumption directly. It uses simple payback and does not assume future tariff inflation, finance, maintenance, inverter replacement or degradation. That makes it useful for testing why a broader property-led estimate changes when one assumption moves.

A practical two-calculator workflow

Start with the official EST calculator for a property-led estimate. Save the system size, annual generation, cost and savings figures it gives you. Then enter the comparable assumptions into this site's calculator so you can see the self-use, export and payback arithmetic separately.

Repeat the comparison when you receive installer quotes. Use the quote's actual installed price and annual kWh estimate, then run a lower-generation case. If a battery is included, compare solar-only before adding the battery cost and a cautious self-use uplift.

• Compare your quote with the EST cost estimate.
• Replace the default import rate with your actual tariff.
• Use the export tariff you can actually access.
• Use a roof-specific annual kWh estimate where possible.
• Test no battery before adding storage.

Independent status

Solar Savings Calculator UK is not affiliated with, endorsed by or operated by Energy Saving Trust. Energy Saving Trust owns and operates its official calculator. This page is an independent companion explaining how to compare estimates without implying that either tool is a final quote.

Compare the estimates