kW, kWp and kVA: how to read the size of your commercial solar system

Ask three people how big a commercial solar system is and you may get three different answers. That's usually because each of them is quoting a different number. Knowing which is which makes any conversation about your system — including a buyback assessment — much simpler.

Panel capacity: kWp (DC)

The ‘peak’ in kilowatt-peak refers to the combined rated output of all the solar modules under standard test conditions. It is a direct-current (DC) figure, worked out by multiplying the number of panels by each panel's rated wattage. A system of 250 panels rated at 440 W each has a DC capacity of 110 kWp.

This is the figure we refer to when we talk about systems of around 100 kWp and above.

Inverter capacity: kW or kVA (AC)

Inverters convert the panels' DC output into alternating current (AC) for your site. Their rating is usually expressed in kW or kVA. It is common for the panel capacity to be somewhat larger than the inverter capacity — designers do this deliberately because panels rarely produce their full rated output.

So a system described as ‘100 kW’ might have 100 kW of inverters and more than 100 kWp of panels. The two figures are both correct; they measure different things.

Export limits

Your network connection agreement may limit how much power the site can send back to the grid. An export limit is a constraint on the connection, not the size of the system, and it doesn't reduce how much solar electricity you can use onsite.

Where to find each number

• The installation contract or final invoice usually lists the panel model and quantity, and the inverter model and quantity.
• Panel datasheets show the rated wattage per module (for example 400 W or 440 W).
• Inverter datasheets and the inverter nameplate show the AC rating.
• Your monitoring portal often shows the system size, though some portals display only the inverter rating.
• Network connection documents show any export limit.

Don't have the paperwork?

That's common, particularly where a business has moved into premises with an existing system. An approximate size is enough to start a conversation. When you request an assessment you can tell us you're not sure, and we can help establish the figures during the assessment.

This article is general information only and is not financial, legal or tax advice. Consider your circumstances and seek independent advice before making decisions.