One of the interesting aspects of electrification is that fact that we have to become familiar with a whole new set of acronyms and metrics.

Until you own either an electric car, a home storage battery or a solar photovoltaic array, it is most likely that the terms kW, kWh and kWp are unknown to you.

Let us revisit our O-level or GCSE physics.

Power

Power is the rate at which work is done.

Power is measured in Watts.

You could think of watts as a measure of electrical flow. For instance, the old style incandescent bulb in your living room pendant used 100 Watts. It used energy at a higher rate than a modern LED bulb, which may only use 10 Watts.

A Kilo Watt is 1000 Watts.

Energy

Energy is the amount of work done.

Electrical energy is measured in kWh – Kilo Watt Hours.

Now if you recall your GCSE physics, you’ll remember that in physics, they love a good equation. The relevant one here is:

Energy = Power x Time

So Energy is the amount of power used in a given period of time. The period of time that we use here is hours (rather than minutes or any other time period).

In our pre-electrification world, outside of the GCSE Physics classroom, these definitions were almost irrelevant to how we conducted our lives.

Nowadays, it is important to know a few basics.

Car batteries and charging rates

With your car battery, the important thing to know is the Capacity of your Battery. The capacity for doing work is measured in kWh. My Nissan Leaf has a 40 kWh battery, which is fine for day to day life, commuting, & taking kids to rugby or football matches.

At home, I can charge it at 6.8 kW, which means that on my Octopus Go tariff, I can put in just under 28 kWh of Energy during the 4 hours of cheap overnight electricity provided on my tariff. At a rapid charger, the maximum rate of charge is 46 kW, but this slows down to 20 kW or less when the battery is 80% full.

Note that both these rates of charge (the 6.8 kWh at home and the 46 kWh at a public rapid charger are set by the car, rather than the charging machine). Each car is different.

Solar PV peak outputs

With your solar panels, the rate at which electrical energy flows into your house will change from one minute to the next – for instance as a passing cloud obscures the sun. We use the term kWp (Kilo Watt Peak) to define the maximum rate of power that the solar panels could theoretically produce (in the ideal circumstances).

Your solar array might be generating 2.5 kW of Power, and if they keep that up for 1 hour, they will have generated 2.5 kWh of Energy.

If you turn on your Washing machine, it will probably use 0.5 kWh of Energy on a 40 degree Celsius cycle lasting 1 hour. However, that Energy is not consumed at a constant rate. Drawing cold water from the mains supply, it will heat the water during the early part of the cycle. At this point it will probably be using 2 kW of Power, but when the washing machine is simply turning mid-cycle, it will only use 100 Watts. The rate at which work is done will vary during the cycle – principally depending on whether the machine is heating water or not.

So although the rate at which work was done varied from 2 kW down to 0.1 kW, overall during the whole cycle, the total amount of work done added up to 0.5 kWh.

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