Sizing of a PV Plant

When sizing a PV plant, there are two approaches to obtaining the installed DC and AC capacity and AC capacity at the PoC:

For a given AC Capacity target at the PoC, determine the DC and AC needs to be installed OR

For a given installed DC capacity (limited by the available development area), determine the AC power available at the PoC.

When a project site has a limited area, we use the second approach by determining the maximum DC that could fit within the development boundary and then calculating the AC capacity available at the PoC. Note that this is based on assumed DC/AC ratios.

We consider mainly two DC/AC ratios for PV plants: the DC/AC ratio at the inverter and the DC/AC ratio at the PoC. Determining the optimum value at each point requires an optimisation study. Therefore, we assume a reasonable value based on experience at an early stage. However, these values should be refined later when the loss values are known with higher certainty. We typically expect a DC/AC ratio of around 1.2 at the PoC to account for all losses and reactive power requirements.

With this in mind, the image below shows an example of a plant with 6 MWp PV and 6 MVA inverter capacity installed (DC/AC ratio @ inverter = 1.0). Suppose we assume around 10% losses from the modules to the inverters. In that case, with a 5% loss from the inverters to the PoC and another 5% allowance for reactive power requirements, the expected AC capacity at the PoC is around 4.9 MWac. This can, of course, be further refined by running a yield simulation with the actual losses.

In other words, if we were to use approach no. 1 with a target of 5 MWac at the PoC, then our installed DC capacity would need to be:

5 MWac x (1 + 5% losses + 5% reactive power) x (1+10% losses) -> 5 x 1.1 x 1.1 = 6.05 MWp installed

Therefore, to obtain 5 MWac at the PoC, the project site needs sufficient area to fit 6.05 MWp.