An optimal DC to AC ratio for solar inverters

The DC to AC ratio of a solar power system is the ratio between the SC output power of the solar array and the ac power of the inverters. The role of the DC to AC ratio is to find an optimal balance of cost vs. performance. The declining prices of solar panels make it beneficial to design solar power systems with higher DC to AC ratio. While the rule of thumb is a ratio of 1.2, we’ve seen recommendations as high as 1.5 or higher.

Calculating the DC to AC ratio of a solar power system
For a solar array in which:
STC Wattage of the solar panels: 350W
Number of modules in the array: 15
The DC power of the solar array is 15*350W = 5250W
Using an inverter with a maximum AC power of 4200W, the DC to AC ratio of this system is 5250W / 4200W = 1.25

Pros and Cons for a large DC to AC ratio
The benefits of a large DC to AC ratio are economical, as the solar array production is typically below the inverter’s rating most of the day, even when the DC rating is larger than the inverter AC power rating. Given the solar market trend of the reduced cost of the solar panels, designing a system with a high DC to AC ratio will result in more bottom line generated solar energy for a lower cost. However, there is a limit to how high we can go with this ratio. High DC to AC ratio results in inverter clipping. This happens at the times of peak productions, in which the solar panels produce more energy than the inverter’s maximum AC power. During these times the extra solar energy is “clipped” by the inverter and is lost.

The Bottom Line
Cheaper solar panels make it cost-effective to add more PV modules to the array, despite clipping losses. Taking into account the new time-of-use rates, a further increase to the DC to AC ratio is recommended, to generate more solar energy at times in which its value is higher. While the traditional design has been of a DC to AC ratio of 1.2, the financial trends in the solar industry are increasing the recommended ratio to be as high as 1.6 in certain cases.