What Inverter Size Do You Need for Your Flat Roof Solar System?

When designing a photovoltaic (PV) system for flat roofs, choosing the right solar inverter size can significantly impact both your system’s efficiency and overall cost. This blog post explores key factors to consider when selecting an inverter for flat rooftop solar systems. It also highlights the differences in inverter sizing between conventional tilted PV setups and vertical photovoltaic (VPV) installations.

Inverter connection at a rooftop solar site - after selecting the right size inverter

Should I Oversize the Inverter?

Many solar installers and planners believe that the safest approach to inverter sizing is to select an inverter that has a power capacity slightly above the total power capacity of the solar array. It seems like a safe choice, right? However, a professional solar planner will know how to “undersize” the inverter and find the optimal compromise.

Why Undersize the Inverter?

1. Price and size correlation

The price of an inverter generally correlates directly with its size. Larger inverters capable of higher power output come at a higher price. Thus, strategically minimizing inverter size - without significantly reducing yield -can lead to substantial cost savings.

2. Temperature effects

If you have a solar array rated for 10 kWp, will it likely produce 10 kW of power any time in it’s lifetime? Probably not. Even for optimally oriented solar panels when the sun rays hit directly and orthogonally on the panel surface, the temperature effect will reduce the power. The solar panel operating temperature will likely be much higher than the 25 °C that is used in rating the panels, so the power output will be reduced to about 80-90% when the sun hits directly on the panel surface. So the 10 kWp array will probably not deliver more than 8 or 9 kWp if the sun shines directly (perpendicular) onto the panels.

3. Peak power and panel orientation

For conventional flat roof installations (often at 10° tilt), and VPV systems, panels are in practice never directly perpendicular to the sun during peak sunlight hours. This means maximum theoretical power output is never reached.

4. Inverter efficiency curves

A regular solar inverter is most efficient when operating around 40-60% of the rated power, and the inverter efficiency drops off very quickly when the power from the solar panels is less than 10% of the rated AC power. So, to maximize the output of the solar installation, you want to avoid the inverter operating at the lowest power. The larger inverter you chose, the more it will operate at the low power range, so this is a fourth reason for undersizing the inverter.

A typical efficiency curve peaks at around 40% of the rated power (Figure adapted from major inverter manufacturer by Over Easy Solar)

What Happens When Solar Panel Output Exceeds Inverter Capacity?

Peak production occurs only very few hours of the year. And since we now know that inverter efficiency is low for low power ratios, we can conclude that it is optimal to select an inverter that is smaller than the theoretical maximum panel output. It pays off to undersize the inverter so peak production on the best days of the year are cut off, because the overall inverter efficiency would be better.

So, what happens if your inverter is smaller than your solar array’s peak potential? Don’t worry—it’s not going to explode. It just means the inverter will "clip" the peaks of solar production. In other words, any excess energy beyond its capacity will be trimmed off. Check out the example below.

With an ideally sized inverter, you “clip off the tops” on the best days of the year. The lost energy for this 5 kWp simulated system is compensated by higher inverter efficiency when the radiation is low during winter and cloudy days.

If you've used tools like PVSyst or PV*Sol, you’ve probably seen how you can experiment with different inverter sizes to see how they affect system performance. Choosing an inverter that clips just 0.5–1.5% of your system’s total annual energy is often a smart trade-off—it keeps costs down without significantly impacting overall performance.

A typical measure used for inverter sizing is the “DC-to-AC ratio” which is the label power of the system divided by the size of the inverter. A DC-to-AC ratio of 1.5 means the PV array size (kWp) is 50% larger than the inverter size in kW. Showing the DC-to-AC ratio versus the energy loss by selecting a smaller inverter in an Inverter clipping diagram helps in showing the effect of inverter sizing for a given system. Some solar installers may be more used to talking about the AC-to-DC ratio in %; a DC-to-AC ratio of 1.5 equals an AC-to-DC ratio of 67%.

Inverter clipping diagrams comparing two different orientations of VPV systems with a conventional system for two different locations.

Optimal Inverter Sizing for Conventional and VPV Systems on Flat Roofs

We hope this article has helped you understand how inverters should always be undersized for flat roof solar installations, whether they are conventional 10 degree east west systems, 15 degrees south or vertical solar installations.

The table below shows some recommendations from our PV engineers on inverter sizing for various systems and locations, based on our actual measurements and simulations in PV*Sol and PVSyst. As you can see, the ranges are quite wide, but within this range, the actual output and specific yield of the solar installation will vary with less than 1%. It may pay off for the customer to go for an inverter as small as possible, to save on inverter costs and get a better total financial payback for the system.

Location	System type	DC-to-AC ratio	AC-to-DC ratio
Oslo, Norway	10° tilt EW	1.2-1.6	62-83%
Oslo, Norway	VPV	1.2-1.6	62-83%
Berlin, Germany	10° tilt EW	1.1-1.5	67-91%
Berlin, Germany	VPV	1.2-1.6	62-83%
Madrid, Spain	10° tilt EW	0.9-1.3	77-111%
Madrid, Spain	VPV	1.0-1.4	72-100%

Partners of Over Easy Solar will always get a recommendation for minimum inverter sizing directly from our software VPV Planner.

Learn more about our partner program.