• Graham Harris

What is a watt? Understanding the data from your solar PV panels.

Understanding the various units and quantities that are associated with your solar panel system is important to monitor its performance, and to understand its carbon credit potential.

However, as many of the terms used are similar sounding - and it may be some time since you studied high school physics! - it can be easy to mix these up. In this blog post, therefore, we're going to briefly explain what is what.

Or, perhaps we should say, we're going to talk about what's a watt! This is because power is measured in a unit called Watts (named after the Scottish engineer and inventor, James Watt, who was instrumental in developing the steam engine and ushering in the industrial revolution). Watts are shown by the abbreviation 'W'.

A watt, however, is a small measure of power. A single solar panel might be rated at somewhere between 200 and 400 W. Similarly, lightbulbs and small household appliances may also measure their electrical demand in watts.

When considering the output of a household's PV system, or the electrical power needs of a household, it is therefore more common to consider power in the units of kilowatts (kW). One kilowatt is one thousand watts.

For larger users or solar arrays, such as commercial, industrial and farming sites, power is further scaled up and measured in megawatts (MW), which are one thousand kilowatts (or one million watts).

Now, watts, kilowatts and megawatts are all units of power. They can tell us the maximum demand or production from an electrical system. But they don't measure that production over time. For that, we need units of energy. (Although power and energy are terms that are used interchangeably by many in everyday life, in an electrical context they have distinct meanings). Power is the instantaneous demand for electricity, whereas energy is an amount of electricity consumed or produced over a defined period of time (usually, hours).

Energy = Power x Time.

Hence, power may be measured in watts, kilowatts or megawatts, but energy is measured in watt-hours (Wh), kilowatt-hours (kWh) or megawatt-hours (MWh).

For example, if your house is consuming 6 kW of power constantly for an hour, it will have consumed 6 kWh of energy. And conversely, if you have a solar PV system that is producing 6 kW of power for an hour, it will have produced 6 kWh of energy - and if it does this for 1,000 hours in a year, it will have produced 6,000 kWh, or 6 MWh of clean energy.

To summarize, power is measured in these units:

  • 1 kilowatt (kW) = 1,000 watts (W)

  • 1 megawatt (MW) = 1,000 kilowatts (kW) = 1,000,000 watts (W)

And similarly, energy is measured like this:

  • 1 kilowatt-hour (kWh) = 1,000 watt-hours (Wh)

  • 1 megawatt-hour (MWh) = 1,000 kilowatt-hours (kWh) = 1,000,000 watt- hours (Wh)

When signing up for Solar Offset, we will ask for your system capacity in kW - in other words, the maximum power it is capable of producing.

A typical Alberta household solar PV system capacity is likely to be somewhere between 2 and 20 kW, depending on factors such as a) the size of that house b) whether or not it is fully electric and c) whether or not it is also charging electrical vehicles. Note that it won't be measured in hundreds of kW unless it is a large commercial, industrial or farm-based system.

If you're not sure what your system size is, you can usually find this on your installer agreement, installer invoice and/or the electrical drawing (single line diagram) of your PV system that is hanging at your fuse panel.

Of course, if you're having difficulties identifying this information, don't hesitate to contact us.

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