Understanding kW

A lot of people, energy professionals included, don't fully understand the difference between kW and kWh. If you are one of them, fear not, this article should set you straight!

Energy calculations, and energy saving, become much easier when you understand the difference between a kW and a kWh.

First of all kW stands for kilowatt and kWh stands for kilowatt hour. A kilowatt (kW) is a measure of power while a kilowatt hour (kWh) is a unit of energy where:

Energy is a measure of how much electricity is used by something (a building, an appliance) over a specified period of time. The kWh is a unit of electrical energy. It is a similar concept to how many litres of petrol your car uses to travel 100kms; and

Power is the rate at which energy is used. So it is more like the speed your car travels at doing its 100kms – the faster you go or the bigger the engine the more litres will be needed to cover the 100 kms because, either way, more power is needed.

“Load”, “Demand”, Things that "use power"

Any items of electrical equipment like light bulbs, computers, and fans, take energy in the form of electricity and use it to do useful things for us. Really they're converting the energy into other forms (light, heat, motion etc.), but we say that they're "using" it. The rate at which these things use energy is their "load" or their "demand”. 

Light bulbs are a simple example: if you have a 100 W light bulb you know that it will use 100 W of power when it's running (100 W of power being the same as 0.1 kW of power). The watts aren't affected by how long the 100 W light bulb is running for... a second, an hour, a day - no difference - so long as it's switched on the bulb will be using 100 W of power. If it's not switched on it won't be using any power (i.e.0 W).

Some equipment is more complicated. Consider a laptop: at any one instant it might be using 50 W of power, or 30 W of power, or 43 W of power, or any similar such value. It depends on what it's doing - if it's sitting there doing nothing it'll probably use less power than if you're hammering away on an Excel spreadsheet, listening to some music, and burning a DVD, all at the same time.

The same applies to your thermostatically controlled fridge and oven – sometimes they are cooling or heating and sometimes they are idle.

Instantaneous power

The instantaneous power (or instantaneous demand, or instantaneous load) is the power that something is using at any one moment in time. Put your laptop on standby and its instantaneous power will drop immediately. Bring it back to life and its instantaneous power will rise immediately.

If, at any particular moment, everything in an office building is switched on, that office building might be using 42 kW of power. That's 42 kW of instantaneous power. If, at any particular moment, everything in the office building is switched off, that building should be using 0 kW of power. That's 0 kW of instantaneous power.

The same applies to your home. The instantaneous power of most properties varies all the time. People are constantly switching things on and off, and many items of equipment within the building have instantaneous power consumption that is constantly changing too.

Average Power

The best way to compare the power use is to find the average power used and compare them on that basis.

Average power enables you think of complicated things like buildings as if they were simple things, like light bulbs...

Average figures smooth out the constant fluctuations of instantaneous power, and make it possible to compare the efficiency of different periods, like for like. For example:

  • A house used 4.1 kW (on average) across the whole of last week.
  • The house used 1.9 kW (on average) across all the Saturdays and Sundays since March 2012. (We don't need to care how many Saturdays and Sundays there were since March 2012.)
  • The house used 3.8 kW (on average) between 07:00 and 08:00 on 7th May 2012. That's double the average kW of a typical weekend.

You can easily use these average-kW figures to compare the energy consumption of different periods and even different buildings (we use the term "energy consumption" loosely because really we're talking about average power, not energy). It's a bit like comparing the fuel consumption of cars:

  • On long journeys my car does an average of 7 litres per 100kms. My brother's car does an average of 9 litres per 100kms.
  • Around town my car does an average of 10 litres per 100kms. My brother's car does an average of 13.

Average power (load or demand), typically measured in kW, is a great way to look at the energy usage of a property. In many ways average-kW figures are easier to work with than kWh figures.

The beauty of average-kW figures is that you can compare them fairly in an instant. The length of the time period doesn't really matter. So you can look at the average-kW figures from 15-minute interval dataand compare them directly with the average-kW figures from 60-minute. Or you can instantly compare the average kW from last month with the average kW from yesterday and the average kW from the whole of last year. 

A highly qualified customer of The Lines Company, David Reid, prepared an interesting commentary about the use of kW load for us which can be viewed by clicking here.

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Useful Tip

Tips for holiday home owners with demand meters

Holiday home owners can experience considerable increases in their line charges because of their distinctive kW load pattern. While you are staying at the residence it is advisable to maintain a base warmth in the home during the day to avoid the load required to heat the home from a much lower temperature in the evening. Have hot water and non-essential appliances on ripple control.