Energy/Power conversion: Calculator & self-test Quiz

Get comfortable with conversions!

When first working with energy and power numbers, it can be annoyingly easy to miss errors in scale – using the wrong number of zeros, because you’ve mixed kW with MW or MWh with GWh for example. It’s also helpful to be confident working in timescales other than hours, despite the fact that the very unit megawatt-hour seems to lead you down that route.

So the best way to make sure you’re confident moving between power, energy and time – including the different scales and unit multiples – is to play around with some very simple calculations.

You could do this with a pen & paper, a calculator or in Excel.

Or you can access a simple online power/energy/time calculator; like this one (click below and it’ll open in a new window).:

CALCULATOR: Energy, Power & Time

It may be easier to use it on a laptop or tablet rather than on a small-screen phone.

As with all the calculators in this course there are two tabs at the top: “PLEASE READ”, which is self-explanatory, and “CALCULATE”, where the action takes place.

A quick quiz below is designed to make use of the calculator (or your pen and paper, if you prefer!) to illustrate the kind of simple conversions that it’s essential to be confident of doing.

Test Your Understanding…


A rooftop solar PV system operates at an average power output of 1 kW for a year. How many MWh of energy does it produce?

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1 year is 365 days x 24 hours = 8760 hours

1kW x 8760 h = 8760 kWh = 8.760 MWh


The UK consumed 303 TWh of electricity in 2014. What was the average power consumption in GW?

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303 TWh = 303,000 GWh

303,000 GWh / 8760 hours (1 year) = 34.6 GW


A 5 MW power plant operates at full output for a year, while a 50 MW power plant operates at full power for only 7 weeks. Which has the most energy to sell?

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The 50 MW plant

5 MW x 8760 hours = 43,800 MWh

50 MW x 1176 hours (24 hours x 7 days x 7 weeks) = 58,800 MWh


If a country can cut its power consumption by an average of 1GW for a 45 minute period every day (say at a time of peak demand), how many MWh of energy are saved over a year?

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45 minutes = 0.75 hours x 1GW = 0.75 GWh = 750 MWh

750 MWh x 365 days = 273,750 MWh