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How is Amount Per Watt Shown on Your Electricity Bill?

By: Chris Hogan MSc - Updated: 2 Sep 2020 | comments*Discuss
Electricity Supplier Units Kwh Watt


On an electricity bill - how do you read the charges for the amount per watt used or how is it shown on the bill? Then how do you compare your usage to the reading etc?

In other words is the cheapest company the one that charges least per unit? What else should I look out for?

(T. Archer, 19 February 2009)


We can't be completely sure here as we don’t know which supplier you're with, nor do we have examples of all the electricity suppliers' bills to hand. But we can make some general assumptions that should work out. The first is that the units should be the same across all suppliers so yes, the cheapest company is the one that charges the least per unit.

Watt Units are Used?

Of the bills that we do have examples of, some show a total in kilowatts per hour (kWh) and others show it in 'units'. Fortunately a unit is one thousand watts used for an hour, so they can be treated as exactly the same thing. If you suspect that your units are not the same as a kWh, or there are some other units being used, phone your supplier. Ask them what they are using to charge and what the relationship of their unit is to a kWh.

To work out what your usage should be you need to look at the power rating of your appliances and then multiply that by the amount of time you use them. This will give you a total in kWh to compare with your bill.

Example Calculations

So if, for example, you have four 60 watt light bulbs that are usually on for most of the evening in the winter, from 4pm until, say 11pm, then that would be four bulbs times 60 watts (240). Then multiply that by seven hours (1680) and then by the number of days in a quarter (roughly 92) which equals 154,560. This divided by 1000 gives you the amount in units or kWh, which is 154.56.

To take another example lets look at an appliance. You can find out the electricity rating by looking on the panel on the back. Assuming you have a tumble dryer that's rated at 2.5 kilowatts (kW) that you use three times a week for about one and a half hours, you can calculate it in the same way:

appliance rating X run time X times used each quarter = kWh per quarter

2.5 X 1.5 X (3 x 12) = 146.25

(The three times twelve is to work out the number of times a quarter the tumble dryer is used, three times a week times the number of weeks in a quarter.)

Try a Consumption Meter

It has to be said that this is a rough estimate and it's time-consuming and painstaking to work it all out. You might be better off getting a consumption meter that tells you what's being used at any given time. You can then go round turning things off and on to see which ones are hogging it all. There is a variety on the market but if you don’t want to buy one you might find that local community groups or green organisations have them to lend out.

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