Solar PV System Design: Why Would you Oversize?

3-minute read

Put simply, oversizing means adding more solar panels to your solar PV system than you think you would need. Solar panels capture DC power, and the inverter converts the DC into usable AC power. In an oversized system, you add a greater capacity on the DC side (the panels) than the inverter rating. It sounds counterintuitive, but when you consider that for most of the time, solar panels are working below their maximum level, it begins to make sense.

Take a look at the graph below. It represents the amount of power that a typical solar PV system will yield during a day.

The only time the inverter is producing at its rated value is at the top of the curve, so for most of the day you’re not delivering the full capacity to the inverter and you’re missing out. By oversizing the system, it will generate more electricity during the morning and evening, when it’s cloudy and at points during the year when the sun is lower in the sky.

What happens when the sun is at its maximum?

During peak sunlight hours, the panels of an oversized system will generate more than the inverter can handle. That extra DC is ‘clipped’ (dissipated as heat). Effectively you lose it, but when you consider the amount of time that the system is working below its maximum, for those of us living in Northern Europe, the gains outweigh the losses.

Installing Smaller can be Better

A smaller rated inverter should be more cost-effective than its larger counterparts, and the installer can potentially cut down on paperwork. In the UK, if you have any inverter that’s rated at more than 3.6kW, your installer will need to submit a G99 application. However, if you oversize your system to say 5kW, with oversizing (and the appropriate inverter), you can stick with a 3.6kW inverter without needing the G99 documentation.

North - South & East - West

If you have the right roof and the right type of solar inverter, it makes sense to have more than one set of solar panels. As the position of the sun moves throughout the day, each orientation will reach its peak at different points, so with an oversized system, you have the opportunity to produce more from your PV overall, for more hours of the day.

What’s the manufacturers’ take on oversizing?
Many manufacturers design inverters so that they can be oversized and, as long as you remain within the limits, there’s no impact on the operation of the product or its lifetime, and the inverters remain within warranty. If you look at an inverter specification, you’ll often see that oversizing is one of the parameters listed.

It’s important to note that you should get a solar PV system designed by a reputable industry professional. To oversize your system, your installer should select an inverter with generous oversizing capacity, which not all inverters offer. The newest SolarEdge residential inverters allow for 200% oversizing. Click here to learn more.

As we move into winter, and the sun sits lower in the sky, an oversized PV system will make the most out of the winter sun – but there are a lots of other factors that will impact the amount of electricity that a PV system will generate throughout the year including time of day, clouds, soiling – such as bird droppings - even high temperatures can reduce a solar panel’s output, so if you have the space available, find out if oversizing would help you harvest even more clean energy from your rooftop.