You can’t go very far into the electric future without encountering kilowatt hours (kWh) – electricity consumption, battery storage, heat output – kilowatt hours are everywhere.

A kWh is a unit of energy – it can be used for any kind of energy – electrical, heat, food, exercise. Yes there’s lots of other units of energy, some current such as Joules and calories, some historic such as British Thermal Units (BTUs), some scary such as tonnes of TNT for explosions. The kWh is favoured for the electric future as it represents a useful amount of energy so we can all avoid lots of zeros.

For example 1 kWH is equivalent to 3,600,000 Joules – way too many zeros.

So what can a kWh of energy actually do?

**Heating Water **1kWh is enough energy to raise the water temperature in our 250 litre hot water tank by about 3.5C or to boil around 10 litres of water from 10C – a mid-range value for incoming water temperature in the UK.

**Keeping the lights on **A modern bright led bulb might be rated at 10W – A kWh of energy will keep the bulb on for 100 hours. If you have 10 downlighters each taking 5W then a kWh will keep the lights on for 20 hours. If you have two old 100W bulbs with glowing filaments you won’t make it through the night – a kWh will only last 5 hours – make the change to led, for the planet and your electricity bill.

**Driving an EV** 1 kWh lets us drive our EV around 4 miles.

**Driving a petrol car** There’s around 8.9 kWh per litre of petrol, work through the numbers and you get: 1 kWh will get a small car doing 50 miles per gallon around 1.2 miles. Interesting comparison with EV above – electric motors can be around 95% efficient, petrol engines in road cars are around 20-30% efficient.

**Cycling uphill** Have to simplify here and am only going to consider the energy used to lift the mass of bike and rider – not all the losses due to air resistance and tyre rolling resistance. Mass of bike and rider is 100kg, force of gravity is 10 m/s/s (I know it’s 9.81 but 10 will do). 1kWh is enough energy to lift the bike and rider 3.6 km!

**Power a human being** An adult food intake might be 2000 kilocalories a day – this equates to about 2.3 kWh – so 1kWh will power a typical human for around 10 hours – we pretty much use as much energy as an old 100W bulb would if turned on all day!

**A note about power and energy **

The kWh is a unit of energy but it’s name includes the Watt which is a unit of power. A simple definition of power is energy divided by time. More power means more work can be done/energy can be used in a given time.

A domestic kettle might have a power rating of 2,200 Watts – this means it uses 2200 joules of energy in a second – a watt is defined as 1 joule per second. If the kettle is on for 3 minutes it will use:

2200 joules x 180 seconds = 396,000 Joules of energy

In kWh that is 396,000/3,600,000 = 0.11 kWh (3,600,000 J in a kWh)

An easier way to get the same answer is:

2,200 W for 1 hour = 2,200 Wh = 2.2 kWh

For 3 mins = 2.2 * 3/60 = 0.11 kWh

We pay for energy used, the power consumption determines how quickly the bill mounts up. A low power device that is on all the time can cost more than a high power device only used for a short time.

For example a 40 W cupboard heater uses 40W x 24h = 0.96 kWh of energy in a day.

If the 2,200W kettle runs for 20 minutes a day it will use 2,200 x 20/60 = 0.73 kWh of energy

**Why does this matter?** When trying to save energy (reduce your bills!) it’s not enough to concentrate on the high power appliances – how long things are turned on also matters.