What is Peak Power in Solar Panels?

What is Peak Power (kWp) in Solar Panels?

kWp is the rated peak output of a solar array under standard test conditions — used to compare system size, not energy produced.

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Peak Power in Solar Panels is defined by the metric KILOWATT PEAK: kWp. kWp represents the theoretical peak output of the system, used as a measure to compare one system against another. It is the headline metric used to indicate the size of a Solar Installation. For homeowners however, a focus on this particular metric can be misleading, since the stated peak output of a system will not be reached unless “ideal” conditions are achieved. When solar panels are manufactured, they undergo a set of measurements and tests to define, amongst other things, the power output of the panel. When researching Solar Panels, you’ll see that individual panels are given a power rating. For instance a panel might have a rating of 400 watts. This means that under Standard Test Conditions (STC) – an ambient temperature of 25°C, 1000 W/m2 irradiance and 1.5 AM spectral distribution – it achieves 400 watt output.

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So what are the Standard Test Conditions? The test conditions correspond to sunlight at an angle of 41.81° above the horizon hitting a panel tilted at 37° on a clear day. Why were these factors chosen? They represent solar noon in the spring and autumn equinoxes in the continental United States. Here in the UK your panels are unlikely to encounter these exact conditions. Kilowatts, Kilowhat hours and Kilowatt Peak kWh = kilowatt hours
A kWh is a unit of energy, representing the power output (kW) of a solar system over one hour of time.
In perfect test conditions described above, a 4kWp solar system would have an output of 4kW. After one hour, it would have generated 4kWh of solar energy.

Do solar panels ever reach their peak output?

In the real world, the output of each solar panel varies constantly. This is because solar production is affected by shading, cloud cover, temperature and a range of other factors. Furthermore, the exact geographical location, season, panel tilt and orientation all play their part.

So your Solar Array might have a theoretical 4kWp, but it is unlikely to reach that most of the time, if at all.

What this diagram shows is the output of same solar array in three different conditions – sunny (41.3 kWh), intermittent cloud (22.0 kWh) and overcast (4.7 kWh). In the UK, those cloudless sunny days are the exception, not the rule.

This is why pairing your solar array with a battery system makes so much sense. The battery ensures the power is available when you need it, not just when the sun is shining.

Does Peak Power in Solar Panels really matter?

In practical terms it doesn’t matter at all.

Other considerations

The inverter output is likely to be the limiting factor on peak output. You’ll often see that installers specify an inverter which is smaller than the theoretical kWp of the solar array itself. This is for exactly the reasons discussed above. It is a cost and efficiency saving to have a theoretical kWp which is bigger than the maximum output of the inverter.

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This might sound counter-intuitive at first. If you have a 6kWp system installed, but your maximum output from the inverter is 5 kW, it sounds like you are going to miss out on 1 kW of solar production. But the reality is that your solar panels will likely never actually produce 6kWp, because they are not in the design conditions of North America.

The most important thing when sizing a system is the expected annual kWh energy generation. After all, the total amount of energy produced is the reason for getting solar PV in the first place. Focusing on generation across the whole year is worthwhile, as output varies with daylight hours and temperatures throughout the changing seasons.

What about power output at breakfast time?

The other way to think about this is to not so much focus on peak outputs, rather focus on output at other times of day – the times when you are actually using electricity. If you have a 6 kWp south facing system installed, unless you are using all 6 kW at noon (or whenever the high point of production is), it is almost irrelevant what that peak production level is. Since 6 kW is actually a lot of electricity to use at one time. In a regular domestic setting, that is like having all your appliances on all at once – the kettle, the dishwasher, the washing machine and the oven.

It might be better to consider what the output might look like in the early morning, for example, when you are getting breakfast ready. At this time, your 6 kWp south facing system might be generating 1 kW, whereas your 4 kWp east facing system might also be generating 1 kW. Same output, smaller investment.

As with anything, when you lift the lid on it there are more factors to consider than simply focussing on one metric in isolation – especially if we are ultimately trying to maximise our return on investment.