IQ8 PV Module Rating Larger Than Inverter Rating

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Why is my PV Module rating larger than my Inverter rating?

PV module and inverter selection are two of the most important decisions in PV system design. Ensuring that these

components will work together is important from a technical, reliability, and economic perspective. Goals and design

assumptions of dierent stakeholders can inuence the decision-making process. The following considerations may ease

the decision-making process:

• The DC: AC ratio is the relationship between PV module power rating and inverter power. Every PV system has a

DC:AC ratio regardless of architecture. Many inverters have DC:AC ratio limitations for reliability and warranty

purposes. Enphase Microinverters have no DC:AC ratio input limit aside from DC input voltage and current

compatibility.

• Higher DC:AC ratios always improve inverter utilization and the capacity factor. The measurement of inverter

utilization is capacity factor—the ratio between actual and maximum energy production. A signicant portion of

system cost is tied to the AC rating of the inverter (string or microinverter). Installing more DC on a given inverter

will increase the capacity factor and may drive down the overall dollar per watt system cost.

• DC losses in string inverter systems (including those with optimizers) are typically higher than in microinverter

systems. This means that string inverter system simulations may show lower clipping losses at a given DC:AC ratio.

However, these additional DC losses also impact the nominal DC:AC ratio and result in better nominal DC:AC

ratios for microinverters systems for a given pairing.

• Clipping losses in systems are typically very low compared to other sources of losses, such as orientation factors,

soiling, shading, and thermal losses. Additionally, clipping losses over time decrease as modules degradation takes

place, while other loss factors such as soiling and shading generally increase.

• Economic implications of various system performance metrics, including better inverter utilization and capacity

factor by designing with higher DC:AC ratios, are ultimately dependent on the economics of the local energy

market and system installation conguration. Economic simulation tools such as NREL System Advisor Model

(SAM)

allow stakeholders to make their own evaluations.

Background

Why is my PV module rating larger than my inverter rating? — This common question has a simple answer. In real world

conditions, PV module output rarely produces power at the rated output due to thermal losses. PV module power is a

product of DC current and DC voltage. In a PV module, the DC voltage is a function of PV module cell temperature. That

is, DC voltage goes down as cell temperature goes up. DC current is a function of the amount of available sunlight, called

irradiance, which depends on the position of the sun relative to the module orientation and to environmental conditions.

1) System Advisor Model. National Research Energy Laboratory.

Golden, CO. https://sam.nrel.gov/content/downloads