How we work out the numbers

Short version: we show every step, we say which numbers are facts and which are examples, and we say what we left out.

Three kinds of number

✓ Documented A fact with a source, a date and a place, for example the Ofgem price cap for 1 October to 31 December 2026 in England and Wales.
◆ Chosen An example we picked for the model, such as 90% round-trip efficiency or a £6,800 quote. We say so wherever it appears. Change it to yours.
? Unknown Something nobody can know for your home, such as the standby power of your battery. We say what it means for the answer.

Three different ideas, never mixed

Gross running saving: what you save on electricity before the battery's price is counted.
Net result: what is left after the price is taken off.
Simple payback: the price divided by the average yearly saving. It is a rule of thumb.

A figure such as a price "per night" is the battery's price shared out over its life. It is a way of comparing, not a change on your electricity bill.

What the calculators leave out

Standby power (except in the plug-in battery comparison, where it is counted).
Interest if you pay monthly.
Replacing parts, such as the inverter.
Standing charges and future changes to tariffs.

Standby, interest and replacing parts would lower the result, so leaving them out makes a battery look slightly kinder than it is. Tariff changes could move the result either way.

How the maths is checked

Before each video we work the numbers out in a spreadsheet. The calculation code behind each calculator is tested against the formulas in that spreadsheet, using the video's numbers and several other sets of numbers (different prices, different efficiency, no saving at all, too much VAT). We run the tests again after every change. The wording and charts around the calculations are checked by eye.

What this does not mean. The tests show that the calculators match our own spreadsheets. They do not show that the sums describe your home. The formulas have not been reviewed by an engineer or a financial adviser, and have not yet been read line by line against every source. Treat the answers as a starting point for your own checks and for questions to ask an installer.

Households and images

The households in the videos are models, not real families. Their numbers are examples. Images of the model house are AI-generated and the narration uses a synthetic voice.

Reusing our work

You are welcome to use the calculators, link to them and tell others about them. Please link to the page rather than copying it. The code, text and images may not be reused elsewhere without permission. We show the steps so you can check the maths, not so the page can be copied. Anything a browser shows can be copied technically, so this rests on our request, not on a lock.

Not advice

This is general information, not financial, tax or electrical advice. Always use a qualified electrician, and get written quotes.