Can maths give you the power to cut your leccy bill?

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1. The power of knowledge

The average UK household has an annual electricity bill of over £600 - a big expense. But how many of us actually check our electricity bills, or even understand what we are being charged?

In this Guide you'll find the story of Sandra, aka 'Leccy Girl', and how maths enabled her to spot mistakes in her neighbours' electricity bills, saving them hundreds of pounds. You can also find our interactive tool, designed to help you calculate your own electricity bill, and take part in our online vote - do you check your energy bill?

2. Do you check your electricity bill?

3. When energy suppliers make mistakes...

Sandra, aka ‘Leccy Girl’, has helped dozens of her neighbours to understand mistakes in the calculations on their bills and get refunds of hundreds of pounds from their electricity suppliers. Should you check the calculations on your bill?

4. Doing the maths

This copy of Agnes' bill shows the sums 'Leccy Girl' Sandra jotted on it to calculate the correct amounts. Before you read on, can you spot the mistake Sandra picked up in the bill?

At the top under 'Electricity readings' the bill shows the estimated number of kilowatt-hours used since the previous bill. Agnes' meter actually tracks three separate totals for usage at different times of the day: 1955, 203 and 252 kWh. Each is charged at a Day or Night rate.

Below under 'Electricity charges' you can see that the Day rate is 18.75 p and the Night rate is 7.43 p.

Sandra spotted that the rates for each of the totals had been accidentally switched, so that 1955 kWh had been charged at the higher Day rate of 18.75 p per kWh, when it should have been charged at the cheaper Night rate of 7.43 p per kWh - less than half the cost!

5. Calculate your own bill

You can use our interactive tool to check your own electricity bill. Click or tap the hand to begin, then set the pence per kilowatt-hour (kWh) on the first bar and the total kWh used on the second. The third bar will show the total cost in pounds.

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Electricity use is charged in pence per kilowatt-hour. The price is usually given in pence with two, or even three figures, after the decimal point. These extra figures represent tenths, hundredths and thousandths of a penny respectively. This may seem absurd - there is no physical way to actually pay a fraction of a penny - but multiplied by thousands of kWh they can add several pounds to your bill.

6. How much does a kilowatt-hour cost?

A snapshot of the pence per kWh and daily standing charges on the 'Standard' tariffs of the ‘big six’ UK electricity suppliers, in October 2014. Please note rates may change, and cheaper tariffs may be available, including fixed-price deals.

The government is aiming to replace all 53 million electricity and gas meters with ‘smart meters’ by 2020. These will make it easier to see exactly how much electricity you are using, in real time. They will also send data direct to your supplier.

A kilowatt-hour is about enough energy to run a full load in a washer dryer. It is roughly equivalent to the energy in a small 125 ml wine glass full of petrol, a fish and chips meal of 900 Calories, or the energy used in a day by a small toddler.