I paid for a 10.8kWp Solar PV system, but only got a 7.6kW inverter. Am I being cheated?!

 

When comparing Solar PV system offers, most people only look at the number of PV Panels, brand, price and the savings. But some people are very detailed, they noticed the inverter power is smaller than the PV Panels power. For example, a 10.8kWp Solar PV system. Shouldn’t a 10kW or bigger size inverter be used? They think their Solar PV system isn’t fully utilized when the Inverter is not one-to-one in terms of power!! Did you notice that? Do you insist on using the Inverter one-to-one power with PV Panels power? Actually the truth is the opposite! This is why!

We all know that the sun works like the following diagram (for Malaysia). It rises around 7am, the sun's power gets stronger till the afternoon (around 12pm). 

And then the sun’s power reduces until sunset at around 7pm.

The area under the curve will be the Solar energy you harvest with your PV Panels.

As you can see from the diagram, although you installed said 10.8kWp Solar PV system. It is not always generating power at this 10.8kWp. 

Most of the time your Solar PV system generates at lower kWp, and only reaches the maximum during the afternoon (for a short period of time).

The inverter efficiency will become low when operating at lower power than the design power..

 

Meaning that a 10kW Inverter will harvest less Solar energy during the time where your Solar PV system is generating at a capacity lower than 10.8kWp compared to a 8kW inverter.

Therefore, Solar contractors will always size the Inverter at a power lower than the PV Panels Power. In this case the DC:AC ratio is 1.35 (10.8kWp : 8kW).

Yes, sizing the Inverter to 8kW for this case will limit your PV system to generate at Maximum capacity (10.8kWp) during the noon time.

Thus you lose some energy or we call it clipping loss. 

However, this clipping loss caused by the 8kW Inverter in fact is much lower compared to the efficiency loss caused by the 10kW Inverter.

Therefore, in Solar PV System design, the sizing of Inverter is always lower than the PV Panels power. Usually the DC:AC is between 1.2 to 1.35.

Sizing the Inverter at DC:AC ratio more than the above range will also cause energy loss in your PV system.

The following are energy yield analysis from PVsyst for 3 scenarios for your reference.

You can see that using a 10kW inverter for a 10.8kWp system will generate 13.668MWh per year. Which is less than if you use a 8kW Inverter - generate 14.867MWh per year.

Also pushing the DC:AC too high, say 1.42 (10.8kWp : 7.6kW) for the same 10.8kWp system will also generate less energy (14.751MWh per year) compared to the 8kW inverter.

So, please don’t argue with your Contractor if you saw the proposed Inverter power is lower than the PV Panels Power.

Forcing them to give you a higher power inverter only costs them a little bit more. But the long term loss is borne by you….

10kW Inverter for 10.8kWp PV system

8kW Inverter for 10.8kWp PV system

7.6kW Inverter for 10.8kWp PV system