What does the 3.68kW limit per phase mean when installing solar panels?



The 3.68kW limit per phase (before permission is required) relates to the AC OUTPUT of the inverter not the CAPACITY of the solar panel system. 

The DNO (grid) has a limit on the amount of output you can connect to the grid without needing permission. Output and PV capacity are not the same or directly comparable. It’s important to understand the difference to ensure you get the most productive system possible and avoid missing out on £000’s in lost generation. We explain below. 

Ensure that your system designer or salesman understands the difference to avoid ending up with a significantly underperforming system.

First, let us explain what the capacity of a solar panel system means…

The capacity relates to how much energy the solar panels would produce under standard test conditions (Irradiance: 1000W/m2 at a temperature of 25 degrees celsius). Therefore a 4kW solar array operating within an environment of constant irradiance of 1000W/m2 @ 25 degrees would generate 4000Wh during those conditions (before any system losses).

In the real world, especially in the UK, irradiance fluctuates significantly. Here in Hove, we can expect irradiance to fluctuate from 0W/m2 (night) to 1200W/m2  (clear sky at midday around the height of summer). 

What does this mean in practice?

In the example shown (right),we have a south-facing, unshaded 16-panel, 6.2kW system with a good quality 3.68kW inverter.

According to the weather station at Birmingham airport, the maximum irradiance the array will receive is 1006W/m2. Irradiance fluctuates significantly throughout the day/year.

Therefore, during the summer we can assume, on a clear day from 12-1 pm the array will produce 6.23kWh. (for illustration purposes, excludes system losses).

The inverter can deliver 3.68kWh to the AC side. 

2.55kWh is ‘AC clipped’ - lost as heat via the inverter and/or module.

The simulation software calculates the yearly losses to ‘AC clipping’ for us: 

190 kWh is lost per year to AC clipping - worth £6.65 - £32.30 depending on whether or not you can use the energy.

Total output of 6.2kW array (after AC clipping) = 5908 kWh

What if we installed a 4kW solar system instead “because 4 is closer to 3.68”

The same model (right) but with a 4kW system installed with the same inverter.

AC clipping: 0 kWh

Total output: 4095 kWh

Resulting in 1813 kWh less production per year.

Or enough energy to power an average family home for 11.9 YEARS over the lifetime of the system.