The difference between the wattage of the panel and the wattage of the inverter is called the DCratio. Here's how to calculate it:
With Enphase: Divide the wattage of the panel by the wattage of the inverter (one inverter per panel).
Other Inverter Types: Take the total DC system size and divide it by the total AC system size.
Current Panel and Inverter Offerings (2024)
Peimar 400W with IQ8+ (295W) gives a DC ratio of 1.35
Bauer 450W with IQ8A (366W) gives a DC ratio of 1.23
Bauer 500W with IQ8H (384W) gives a DC ratio of 1.30
Why is the Panel Wattage Higher than the Inverter?
Using the 500W Bauer with the Enphase IQ8H (384W) as an example:
Real-world Performance: Panels only produce their wattage rating under laboratory conditions. A 500W panel may produce 480-490W in real life except under ideal conditions (very cold and sunny).
Energy Yield: Higher panel wattage increases energy yield on cloudy days or during morning/evening sun. The more "potential" energy you capture, the better the performance on bad weather days.
Degradation Over Time: Panels degrade over time, typically by 15% over 25 years. A 500W panel will produce a maximum of 425W after 25 years, which is still sufficient for the 384W inverter.
However, if the panel were 400W, it would produce a maximum of 340W after 25 years, less than the IQ8H inverter can handle. This would result in progressively lower production year after year, affecting ROI calculations.
Sizing Panels and Inverters
Oversizing Panels: Sizing a panel too high for the inverter can lead to diminishing returns. For instance, a 500W panel with an Enphase IQ8+ inverter (295W peak) gives a DC ratio of 1.69, which is not recommended.
Provincial Regulations: In Ontario, systems larger than 10 kW AC are not allowed, leading to DC ratios of 1.4-1.5.
Clipping
"Clipping" occurs when a panel produces more power than the inverter can capture. Below is a chart from an Enphase technical brief:
A DC:AC ratio of 1.31 has an annual clipping loss of 0.3% in year one.
A DC:AC ratio of 1.68 has an annual clipping loss of 5.1%.
Consequences of Oversizing
SolarEdge, a string inverter manufacturer, warns against excessive DC:AC ratios:
Excessive oversizing can negatively affect the inverter’s power production. Inverters are designed to generate AC output power up to a defined maximum, which cannot be exceeded. The inverter limits or clips the power output when the actual produced DC power is higher than the inverter’s allowed maximum output, resulting in energy loss. Oversizing the inverter can cause it to operate at high power for longer periods, affecting its lifetime. Operating at high power increases inverter internal heating and might heat its surroundings. Inverters reduce their peak power generation when overheating.
SolarEdge Inverters: Maximum DC ratio of 1.50 (may void warranty if exceeded).
Fronius Inverters: Maximum DC ratio of 1.55 (will void warranty if exceeded).
SMA Inverters: Maximum DC ratio of 1.50 (may void warranty if exceeded).
Summary
The ideal DC:AC ratio is between 1.2 and 1.4. Systems below 1.2 do not account sufficiently for panel degradation, and systems above 1.4 may experience diminishing returns due to clipping.
