What's the difference between kW, kWh and kWp?

If you're looking into solar panels you're probably seeing kW, kWh, and kWp. Unless you've got a background in electronics it can feel like acronym-overload, but understanding the differences is key to grasping how solar power works and what to expect from your system. Let's break them down from a non-technical perspective.

1. kW (Kilowatt): How Much Power Can It Produce Right Now?

Think of kilowatts as the instantaneous power a solar panel or an entire solar array can produce at a given moment. It's a measure of power, not energy over time.

2. kWh (Kilowatt-hour): How Much Energy Have You Produced (or Used) Over Time?

Kilowatt-hours (kWh) are the units we use to measure energy. This is probably the most familiar term, as it's what your electricity bill is based on. In a solar panel context, if you're generating 4kW at that moment in time, you'll generate 4kWh over an hour. 

But while your solar panels might have a peak power of 4kW/kWp, but they won't always be producing at that level. Over the course of a day, they will generate a certain number of kWh. This is the actual electricity you can use or export to the grid. How much energy does a solar panel array produce in a day? I explain using my real-world data.

3. kWp (Kilowatt-peak): What's the Maximum Your System Can Potentially Produce?

Kilowatt-peak (kWp) is used in the solar industry to denote the maximum theoretical power output of a solar photovoltaic (PV) system under ideal, standardised test conditions (STC).

Why is it "peak"? Because in the real world, these ideal conditions are rarely met continuously. Factors like shading, cloud cover, panel temperature (panels become less efficient when very hot), and the angle of the sun all reduce the actual power output. How much do you really produce at different times of the day and year? I explain using my real-world data.

Common source of confusion:

One regular source of confusion is that systems are sometimes rated by any of kW, kWh and kWp. This can depend on standard practice in different countries, but also because in many cases people just don't think it matters. In all cases, bigger means more generation capacity. 

In all cases, the number might not be helpful for understanding how it actually compares to your daily energy use. Comparing daily energy use to solar panel system ratings is comparing apples to oranges! A 7kWp system might produce 15kWh on a cloudy day, or 48kWh on a bright day. Unfortunately, the name (and in my case, the solar panel sales teams) often couldn't help translate ratings into daily production. But you can look at my data!