# How can one calculate the DC capacity for a given solar plant and its corresponding AC capacity?

How can one determine the DC capacity for a given solar plant and its corresponding AC capacity?

In order to calculate the DC to AC ratio of a solar power plant, it is essential that one grasps what the term ‘DC/AC ratio’ stands for and how it works, and this is the comparative measure of direct current output from solar photovoltaic panels and the alternating current output from inverters.

## Understanding DC/AC Ratio

The DC/AC ratio is defined as:

DC/AC Ratio = PDC at STC /PAC

Where,

PDC at STC - Total DC power capability of all photovoltaic (PV) modules at STC (Standard Test Conditions)

PAC - Total AC output capacity of all inverters

Normally, capacitive support is greater than inductive because, inverters cannot convert all the produced DC power into AC power because of the efficiency losses and operational limitations.

A practice that has been widely used, for instance, is the creation of systems, the value of DC/ AC of which fluctuates within the range of 1. 2 to 1.5 depending on the site’s conditions, work capacity of the machinery, and achievable cost outcomes.

## Calculation Steps

### Determine the DC Capacity

Determine the overall capacity of the solar panels. This is normally in Megawatts MW and it is determined from the sum of the rated capacities all the modules under STC.

### Determine the AC Capacity

Determine the sum up capacities of the inverters in question, the value of which is measured in MW.

### Calculate the Ratio

Substitute the values obtained for the formula above to determine the DC/AC ratio.

Example

If you have a solar plant with:

DC Capacity: 100MW (from PV Panel)

AC Capacity: 80 MW the total generation of power by the inverters

The DC/AC ratio would be:

DC/AC Ratio = 100 MW/80 MW = 1. 25

This means that according to normal practice the design capacity of the solar plant has been set at 25% higher DC capacity than AC capacity to ensure that the production of energy is optimized while at the same time avoiding the overloading of the inverters.

## Efficiency Considerations

By considering the usable AC output, the efficiency of the inverter should also be add up to this and it varies from 90% to 98%.

The formula to convert DC watts to AC watts is:

AC Watts = (DC Watts x Inverter Efficiency)

Example

If your inverter has an efficiency of 95%:

AC Watts = 100 MW x 0.95 = 95 MW

This means that while the system has a DC capacity of 100 MW, the effective AC output available for use would be 95 MW after accounting for inverter efficiency losses.