Energy Terminology

Confusing Energy Terminology – Explained

Watts and Kilowatts

A ‘watt’ is also written in short form as ‘W’. But what is a watt?

Without getting too technical, a watt describes how quickly energy flows. So a device rated at 100W can have energy flowing through it much more quickly than a device rated at 10W.

When the number of watts gets too high, the unit often changes from W to ‘kW’. kW is a ‘kilowatt’, basically 1,000 watts.

You might even come across a ‘megawatt’. That’s a 1,000,000 watts and is written as ‘MW’.

Power

Watts and kilowatts are also associated with the word ‘power’.

An electronic device will have a power rating in either watts or kilowatts.

For example, a kettle might be rated at 3 kW. This means the kettle needs 3 kW or 3,000W of power to boil water.

In other words, it needs a lot of electricity flowing through it to work.

Kilowatt hours

Now comes the tricky bit…

Let’s say you have a 1 kW device. If you turn it on, it will draw 1 kW of power. It will draw this power for every second it is on.

Now imagine you leave the device on for 1 hour. The question is: how much electricity was needed to power the device for that hour?

The answer is: 1 kilowatt x 1 hour = 1 kilowatt hour. You can also write this as 1 kWh.

So, a kWh is a unit of electricity. You will be used to seeing these on your electricity bill.

Really big energy users deal in MWh, or megawatt hours. 1 megawatt hour = 1,000 kilowatt hours.

The best way to get all this straight in your head is to jump to examples:

Example: Electric Cars

Here are the main energy terms usually applied to the world of EVs:

Capacity in kWh

Your electric car has a battery inside.

The manufacturer will often make a big deal of the size, or capacity, of the battery in kWh. Let’s say for your car the battery is 40 kWh.

What does this mean? It means the battery inside your electric car can store a maximum of 40 units, or kWh, of electricity.

In other words, kWh for an electric vehicle is a measure of how much electricity can be stored inside the battery.

If you had an 80 kWh battery in your car, you would be able to store twice as much electricity as a 40 kWh car.

Some battery sizes of popular, new EV models are:

• Kia EV6 – 74 kWh
• Hyundai Ioniq 5 – 74 kWh
• Audi Q8 e-tron SUV – 89 kWh
• Kia Niro EV – 65 kWh
• Volkswagen ID.4 – 77 kWh

Range in miles & Miles per kWh

For electric car owners, the size of the battery in kWh has a major influence on ‘range‘, or how many miles the car will go before it runs out of electricity.

Instead of miles per gallon, or mpg, we all need to start getting used to miles per kWh.

For example, if you can travel 150 miles in a fully-charged 40 kWh car before you run out of stored electricity, then your car can do 3.75 miles per kWh (150 miles divided by 40 kWh).

If your car is capable of 270 miles on an 85 kWh battery, then it can do 3.18 miles per kWh.

Obviously the higher the ‘miles per kWh’ rating the better. It shows the car is more efficient at converting kWh of storage into actual miles driven.

Power Output in kW

Now let’s do an about turn and go from kWh to kW…

Your electric vehicle has a battery, but it also has one or more electric motors. It’s the motors that actually turn the wheels, not the battery. The battery supplies power to the motors. The motors turn the wheels.

So, the electric motors will have a power output in kW. This is basically a measure of how much power can be delivered to the wheels.

The higher the power output in kW, the faster the wheels and hence the car will go.

Your car might, for example, have a motor power output rating of 150 kW. That’s the equivalent of roughly 200 BHP (brake horsepower) in old money.

Electric vehicles are famous for fast, instant acceleration. This comes from the amazingly quick power transfer from battery to motor, and the high efficiency of the motor itself.

Though once you’ve bought an electric car, you probably won’t pay much attention to the power output of the motors.

You will be much more focussed on the capacity of the battery in kWh and how many miles you can go for each of those kWh.

Example: Charging Points

To keep an electric car working, you have to find charging points.

You will almost certainly have one installed at home, assuming you have off-road parking.

Away from home, they can be found at motorway service stations, hotels, in some car parks, at supermarkets, on some streets, etc. You may also have one or more at work.

Charging points have a rating in kW.

Charging Point Ratings

At the moment, for most EVs, the charging point energy flow is one way only: through the charging point into the car. Here are typical charging point ratings:

• 3 kW = fairly slow charging
• 7 kW to 22 kW = pretty quick charging
• 43 kW to 50 kW = rapid charging
• 120-150 kW = really rapid charging

There are even 350 kW chargepoints starting to be installed now. Let’s call that “blindingly quick charging.”

The rating of the charging point determines how quickly the battery charges.

For example, with a 3 kW charger, you can’t get electricity into your car’s battery as fast as with a 7 kW charger. This means it will take longer to charge your car with a 3 kW charging point than with one rated at 7 kW.

Charging Times

A 40 kWh car battery with a 3 kW charging point

Assuming the car battery is completely empty, it will take just over 13 hours to charge it to full capacity.

The calculation is: 40 kWh divided by 3 kW = 13.3 hours.

A 40 kWh car battery with a 7 kW charging point

Assuming the car battery is completely empty, it will take just under 6 hours to charge it to full capacity.

The calculation is: 40 kWh divided by 7 kW = 5.7 hours.

Example: Solar Panels

Terminology surrounding solar panel systems can be confusing, so let’s set the record straight:

1 x Solar Panel: Power Output in W

A single solar panel has a power output rating in watts or W.

In 2019, commercially available panels range in power output from 250W to 390W. Generally, the higher the output, the higher the price of the panel.

1 x Solar Panel: Power Output in Wp

Now, just to confuse things a little, you will sometimes see a panel rating as say 280Wp, rather than 280W. What does ‘Wp’ mean?

Wp stands for ‘watt peak’. It means the maximum number of watts the panel can supply. So a 280Wp panel cannot supply more than 280W, but it can supply fewer than 280W.

For example, if the sun shines on a solar panel at an angle which is not perpendicular to the panel, then the panel will not produce its rated number of watts.

16 x Solar Panels: Power Output in kW or kWp

Let’s say your solar panel system comprises 16 x 250W panels. This will create a total system size of 4,000W, normally referred to as 4 kW for short.

You will often see the total solar panel array described in terms of ‘kWp’. So this system would be 4 kWp.

So, a 4 kWp solar panel system – the most common size in the UK – can output power at a maximum of 4 kW. Very often during the day the output will be less than 4 kW.

There can be various reasons for this. It might be cloudy. The sun might be low in the sky. Part of the solar panel array might be shaded. The panels might be a bit dirty, and so on.

Inverter Power Output in kW

Get ready for more confusion…

You will know from our page about how a solar panel system works, that the inverter converts DC electricity from the panels into AC electricity you can use in the home.

Inverters have power output ratings in kW. A typical inverter installed in a UK home will have a rating of 3.68 kW.

Inverter Rating v. Solar Panel Array Rating

We can hear your next question already: “Why is the inverter rated at a lower output (3.68 kW) than the panels (4 kW)? There are three main reasons for this:

1. The inverter will work more efficiently if it is under-sized in relation to the peak panel array size. This is because, as we have discussed above, the array will very often not be outputting electricity at its peak rating. You don’t want to have a 4 kW inverter with the brakes on if the solar array is only outputting at 2 kW. With the inverter sized smaller, at 3.68 kW, it will be operating at a higher efficiency more often during the day. Higher efficiency means more kWh produced each day.
2. The National Grid in the UK limits the number of amps you can export to the grid without special permission. That limit is 16 amps, or 16A. A 3.68 kW inverter has a maximum output capability of 16A. To get slightly technical, the formula you need is 16A x 230V = 3,680W or 3.68 kW. The grid voltage in the UK is 230V (V = volts). Now, it just so happens that the physicists have worked out that Power (W) = Volts (V) x Amps (A). Therefore when the 16A coming out of your 3.68 kW inverter meets the 230V from the grid, hey presto, you get 3.68 kW of power.
3. The final reason for this very common 3.68 kW (inverter) to 4 kWp (panels) ratio is that for many years the highest feed-in tariff rate could be achieved by limiting the total array size to 4 kWp. Nobody wanted to go over 4 kWp for panels as it would have meant less income per year. Now the feed-in tariff scheme has ended, you will often see a 3.68 kW inverter paired with solar panel systems larger than 4 kWp.

Example: Battery Storage

Battery storage is renowned for having very complex data-sheets with a myriad of features.

We will focus on the main ones only. A lot of the more complicated terminology can be safely ignored.

The first step is to think of home battery storage as an electric car without wheels…

Storage Capacity in kWh

Battery storage is, well, a battery. As such, it has a storage capacity in kWh, just like the battery inside an electric vehicle.

The size of home battery storage can vary enormously, for example:

• Tesla has a battery called the Powerwall sized at 13.5 kWh.
• Powervault, a UK company, has batteries as small as 3.9 kWh and as large as 20.5 kWh.
• The German manufacturer, Sonnen, sells batteries from 2 kWh to 16 kWh.

Charge Power in kW

A flat battery is no good. You need to get electricity into it to make it useful.

Attached to every home battery is therefore a ‘charger’. Your surplus solar electricity – or cheap, off-peak electricity – flows through the charger and charges the battery.

The rate at which the battery can be charged is measured in kW. You will find this charge power rating on the data-sheet of the battery you are interested. It could be say 3 kW.

Discharge Power in kW

If you have stored electricity inside a battery, you need to be able to get it out again.

This is where the ‘discharger’ comes in. Actually it is normally referred to as a battery inverter.

Like a solar inverter, the battery inverter converts DC electricity in the battery to AC electricity you can use in the home or to charge your electric car. The battery inverter has a discharge power output in kW.

Sometimes this discharge rating is different to the charge rating. For example, you might be able to charge the battery at 3 kW, but discharge at 5 kW.

The higher the discharge rating, the more devices you can power at any given instant.