kWp, kW, and kWh

When you start to talk to sales people you will start to hear some terms that you might not be familiar with. Anybody with a basic understanding of electricity will probably have a rough idea of the relationship between Volts, Amps (Amperes), and Watts. However the kWp will be new to many.

So here is a quick primer (apologies to any physicists out there for the simplifications):

  • Voltage, measured in Volts is the "electromotive FORCE" that pushes electricity around a circuit. It is analagous to the "head" of water behind a dam in a river.
  • Current, measured in Amperes (Amps), is the FLOW of electricity around a circuit. It is analagous to the flow of water over a dam or down a river.
  • Energy, measured in Joules or Watt Seconds, is a measure of work (of sorts, but don't worry about that - you can think of it as a way of totting up power).
  • Power, measured in Watts, is a measure of the rate of transmission of energy from one place to another or from one form to another (electric power converted to heat, for example).
  • micro-, milli-, kilo-, and mega- are prefixes which can be applied to any of the units mentioned above and they are, respctively one millionth, one thousanth, one thousand, and one million.

The joule (Watt-Second) a rather small unit of energy. It represents a power of one Watt transferred (or consumed) for one second. The size is not a barrier for physicists - they just apply prefixes such as giga- or nano- and "hey presto" a bigger or smaller unit. For domestic power consumption or generation and everyday practical electrical work the term kilowatt hour is preferred. It is a power of one kilowatt transferred (or consumed) for one hour. A much easier quantity to get your head around when dealing with an electric heater running for a couple of hours. It is equivalent to 3,600,000 Watt seconds (Joules). This is because there are 1000 Watts in a kilowatt and 3600 seconds in an hour.

There are some important related, but similar, terms which it is worth getting your head around

  • The kilowatt hour, kWh - a unit of energy
  • The kilowatt, kW - a unit of power (rate of transfer of energy)
  • The kilowatt peak, kWp - not, as the name suggests, a unit of peak power but rather a unit of power produced under certain specific laboratory conditions (which happen to coincide very roughly with the output of a solar panel on a very bright day)

It is worth trying to understand the subtle difference between these last three. They are easy to get mixed up. The easiest way to distinguish them is how they are used.

  • kilowatt hours are what tick up on your electricity meter and what you pay for each month. A 1kW electric kettle running for 6 minutes (1/10 hr) and a 100W light bulb burning for one hour both consume 1/10 of a kWh
  • kilowatt, kW - a unit of power (rate of transfer of energy). a 1kW electric heater consumes ten times as much electrical energy as a 100W light bulb if left on for the same time. The heater consumes the SAME amount of electrical energy as the light bulb if left on for one tenth as long as the bulb.
  • The kilowatt peak, kWp - not, as the name suggests, a unit of peak power but rather a unit of power produced under certain very specific test conditions. A 4kWp solar array will generate twice as much as a 2kWp array under the same conditions*

This is only STRICTLY true under the very bright, standard, conditions. There are other "standard" conditions which help to compare panels under low-light conditions. If you want to understand this properly, Google STC and NOCT. Comparing the rated power of panels undet STC and NOCT conditions allows a more nuanced understanding of their relative merit. Having said all that, my south-facing panels (at 30 degrees to the horizontal) do peak a bit above the STC value - and that is on a sunny day in mid-March.

Some people struggle with the relationship between kilowatts and kilowatt-hours. I think that it can help a little if you think as the kWh in the old terms of a "unit" and the kW as "units per hour" or "units/hour". So units/h is kilowatt hours per hour. The hours cancel and you get kilowatts, Maybe that just makes it more complicated?

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