Reading your electricity bill

Energy bills are laid out to satisfy a regulator, not to be read, which is why most people file them without looking. But three numbers on there tell you nearly everything about whether solar or a heat pump would suit your house, and they take five minutes to find.

1. How many units you used

Look for a figure in kWh or labelled units. On a monthly bill it is that month's; what you want is a year.

The easiest source is the annual statement your supplier has to send you once a year, which gives the whole twelve months in one figure. Most suppliers also show it in the app or on the website under something like "your usage". Failing both, add up twelve months of bills.

An average household is around 2,700 kWh of electricity a year. Under 2,000 is a small or frugal household; over 4,500 usually means electric heating, a hot tub or an electric car.

This one number decides how many solar panels are worth fitting.

2. What you pay per unit

Usually shown in pence per kWh. It might be called the unit rate, the standard rate, or just the rate. If you are on a tariff with different day and night prices there will be two, and it is worth noting both.

This is what a unit of electricity costs you, and it is the price every saving is measured against. A unit of solar electricity you use yourself is worth exactly this much, because it is a unit you did not buy.

3. The standing charge

A daily amount, in pence, that you pay for being connected, whether you use anything or not.

It matters for one reason: it does not go away. Solar panels and a heat pump can both reduce the units you buy, but neither touches the standing charge. Anyone showing you a bill going to zero has forgotten about it.

If you have gas as well

The gas bill carries the same three numbers, with two differences.

Gas meters measure cubic metres, not energy, so the bill converts them into kWh for you. Use the kWh figure, not the cubic metres.

And there is a fourth number worth hunting for: gas used in a single summer month, July or August. In summer your heating is off, so almost everything you burn is hot water. That one figure separates your hot water from your heating exactly, which is otherwise the biggest guess in working out what size heat pump a house needs. It is on a monthly bill, or in the app's usage history.

If you already have solar

Your bill shows what you bought, which is less than what you used, because some of your use came off the roof. Any figure you take from it is net of the panels.

You may also have a separate export payment, either from the same supplier or a different one. That tells you what you are paid per unit for electricity you send back, which is generally a good deal less than you pay to buy it. That gap between the buying price and the selling price is what makes a battery worth considering.

What to write down

On a bit of paper, or in a note on your phone:

• Electricity: units a year, pence per unit, standing charge
• Gas or oil: units or litres a year, price, standing charge
• Gas in one summer month, if you can find it
• Export rate, if you have solar already

That is the whole set. Every calculator on this site asks for some of it, and so should anybody quoting you: a year of your own consumption describes your house better than any survey of it, and an installer who does not ask to see it is working from assumptions.

A note on smart meters

If you have one, your supplier's app usually holds half-hourly data going back months, which is far more useful than a bill. It will show you exactly when you use electricity — the morning peak, the evening peak, the quiet middle of the day.

That shape is what decides how much a solar system is worth to your household specifically, as opposed to households in general. If the app lets you download it, it is worth the ten minutes.