The relationship between Power and Energy – and what it has to do with your home


When we talk about “low energy homes”, what are we really talking about?

Well, when we’re trying to build low energy homes what we’re figuring out how to use the lowest amount of energy to heat the entire space in your home to a comfortable 20DegC (and how to keep it at 20DegC).

Where does energy come from in your home?

Quite a few places, if you think about it. Some of these cost you money, some are free.

  1. Your radiators, underfloor heating, electric space heaters, log fires, boilers, wood-burning stoves and even candles all give off heat. These use gas, electricity, coal or oil, and so they usually cost money to run.
  2. Solar gains: this is radiation from the sun that comes in through your window and warms your home. It’s free.
  3. Electronic items give off heat (like how your laptop warms up when it’s working too hard). It’s a by-product of use, so it’s free.
  4. Cooking and washing all generate steam and heat. Another by-product, also free.
  5. People are little heat generators, too. Give us food and water and some of it becomes heat.

And how is energy lost from our homes?

  1. Energy escapes through the walls, windows, roof and floor through conduction – energy moving from the warm internal space to the cold exterior space.
  2. Appliances like microwaves and TVs use up energy when they’re turned on and in use.
  3. Even drying clothes indoors uses up energy. The evaporation of 1 litre of water from damp washing requires the same energy as boiling a 1 litre kettle dry. It just happens at a much lower temperature, and so it takes longer than evaporating water in a kettle.

That last example helps explain the relationship between Power and Energy.

An old battered kettle boiling water on a side-street

In layman’s terms, what is power and what is energy?

Power is the rate at which energy is used. It is measured in Kilowatt-hours per hour. Yes, those two “hours”  cancel each other out, so we can write Power as Kilowatts, or kW.

Energy is the “work done”, the total amount of kW used over a period of time, normally given in hours. So in that respect we write Energy as Kilowatt-hours, or kWh.

Put another way, Energy is the distance you travelled in a car (miles), and Power is the speed you went at to get there (miles per hour).

So, we have a 3kW electric kettle. The 3kW is its power. That’s how “fast” it uses energy. A 6kW kettle is faster. If it takes a 3kW kettle just 60 seconds to boil a litre of water, it will take a 6kW kettle thirty seconds, because the 6kW has a higher Power.

A car travelling 30mph will take two hours to travel 60 miles, but a car travelling at 60mph (its “rate”) will get there in an hour.

How does power relate to my energy bills?

Let’s give a real world example and work out how much it costs you to boil water for your tea over the course of a year. You are an average Englishman and therefore you drink 10 cups of tea a day. Let’s say you own a 3kW kettle and it boils one litre of water in 1 minute. You boil the kettle ten times a day. The calculation to work out how much energy you use making tea is simple:

3kW = 3000 watts

3000 watts / 60 minutes = 50 watts per minute. DING! This is our Energy.

50 watts x 10 cups of tea per day = 500wh of Energy used per day.

So, every day you use 500 watts of energy, or 0.5 kilowatt-hours boiling your water for tea. It’s the equivalent of leaving your 3kW kettle switched on for 10 minutes straight, remember.

0.5kWh x 365 days = 182.5kWh

So you use 182.5kWh of Energy making ten cups of tea per day. At 10p per kWh this means it costs you £18 a year to drink tea at home. That’s 5p per day plus 1p for the teabag, 1p for the water and 5p for the milk – cheaper than Starbucks.

Let’s bring it all together with our “wet clothes drying” analogy. What is happening when clothes dry?

The water is evaporating because it is being wicked away by warm air, which likes holding moisture (think of the muggy weather you get in very hot, tropical countries. This humidity is because hot air can hold moisture more than cold air. As the molecules in cold air move closer the water forms droplets, and this becomes rain outside and condensation inside).

To move anything, even moisture, takes Energy. Something is being used up, and in this case it is your radiators slowing warming the air enough that it can hold water, which draws that water from your clothes and in the process dries them.

The radiators evaporate a litre of water just like the kettle, but instead of doing it in 1 minute they take two hours. That means they have very low Power compared to the kettle, but ultimately they use the same amount of energy (heating a litre of water, however fast or slow you do it, takes the same amount of energy).

Work out how much it costs you to use all of your appliances by their power rating

Your 19kW boiler that turns itself on for three hours a day to heat your domestic hot water tank?

19kW x 3h x 365 days =  20,805kWh/yr

20,805kWh x 2p (price of gas) = £416.10 per year

Your 60 watt light bulb?

0.06kWh x 4h x 365 days = 87.6kWh/yr

87.6kWh x 1op = £8.76 per year per light bulb in your house.

See how these things can add up?

Researching a Passive House, installing proper insulation, replacing your boiler and your current appliances with low energy ones is the best way to lower your household bills.




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