Solar systems are commonly described using a DC system size, but the equipment also has an AC output rating.
For example, a solar system might be described as:
10.0 kW DC / 8.0 kW AC
Those numbers do not mean that 2 kW is simply being “lost.” They describe two different parts of the solar system.
Understanding the difference makes it much easier to compare solar designs and understand inverter sizing.
DC stands for Direct Current.
Solar modules produce DC electricity when exposed to sunlight.
The DC rating of a solar array is generally calculated by adding together the nameplate wattage of all the solar modules.
For example:
20 modules × 450 watts = 9,000 watts DC
or:
9.0 kW DC
The module nameplate rating is measured under standardized laboratory test conditions. Actual module output changes throughout the day based on sunlight, module temperature, shading, orientation, and other conditions.
AC stands for Alternating Current.
Homes and the electric grid primarily use AC electricity.
An inverter converts the DC electricity produced by the solar modules into AC electricity that can be used by the home's electrical system or exported to the utility grid.
This DC-to-AC conversion is the fundamental purpose of a solar inverter.
With a string inverter, this conversion occurs at the inverter.
With a microinverter system, such as an Enphase system, each module has its own microinverter that converts that module's DC output to AC as it's leaving the solar panel.
It is very common for the total DC nameplate capacity of the solar modules to be greater than the maximum AC output capacity of the inverter system.
This relationship is called the DC-to-AC ratio, or sometimes the Inverter Loading Ratio (ILR). NREL (National Renewable Energy Laboratory) uses this ratio when modeling PV systems.
For example:
10 kW DC of solar modules ÷ 8 kW AC of inverter capacity = 1.25 DC-to-AC ratio
A ratio greater than 1.0 does not automatically mean the system is poorly designed.
In many systems, intentionally installing more DC module capacity than AC inverter capacity can be an efficient use of the inverter.
A module's nameplate wattage is measured under standardized test conditions.
Real-world conditions are constantly changing.
Actual module power can be affected by:
Because modules spend much of the year operating below their nameplate output, an inverter does not necessarily need an AC rating equal to the total DC nameplate rating of the array.
Adding more module capacity relative to inverter capacity can increase annual solar production.
During mornings, afternoons, winter conditions, hazy weather, or other periods of less-than-ideal sunlight, the larger DC array provides more available power to the inverter.
As a result, the inverter can operate at a higher output for more hours of the year.
NREL describes inverter sizing as a tradeoff between the additional cost and production of more DC module capacity and the increased clipping that can occur when inverter capacity is exceeded.
The goal is not necessarily to maximize instantaneous peak power.
The goal is generally to produce the desired amount of energy over time, measured in kilowatt-hours (kWh).
If the solar modules are capable of supplying more power than the inverter can convert at that moment, the inverter limits its output to its maximum AC capability.
This is called clipping.
For example, imagine a system with:
At a particular moment, the array might be capable of supplying the equivalent of 8.6 kW to the inverter.
If the inverter can output only 8.0 kW AC, its output will be limited near that level.
The top of the production curve may appear flattened during that period.
NREL defines power clipping as a reduction in inverter AC output caused when available input power exceeds the inverter's output capability.
Not necessarily.
A small amount of clipping can be and generally is a part of a well-designed system.
On the other hand, excessively high DC-to-AC ratios can result in unnecessary lost production during high-sun conditions.
The appropriate ratio depends on factors such as:
There is no universal DC-to-AC ratio that is correct for every solar installation.
Consider a solar array with:
20 × 450-watt modules
The array rating is:
9,000 watts DC, or 9.0 kW DC
Now suppose each module is paired with a microinverter capable of approximately 366 watts of maximum AC output.
Total theoretical AC capacity would be:
20 × 366 W = 7,320 W AC
or approximately:
7.32 kW AC
The DC-to-AC ratio would therefore be:
9.0 ÷ 7.32 ≈ 1.23
That does not mean that roughly 19% of the system's annual energy is being lost.
The modules rarely produce their full nameplate rating simultaneously for long periods, and the system must be modeled over an entire year to estimate actual clipping and annual energy production.
The inverter does have conversion losses.
Some energy is lost when converting DC electricity into AC electricity, primarily as heat.
Modern residential solar inverters and microinverters are typically highly efficient, but their exact efficiency depends on the specific model and operating conditions.
For example, current Enphase IQ8 and IQ8+ microinverters list a CEC weighted efficiency of approximately 97%.
Conversion efficiency should therefore be included when estimating system energy production, even though it's a small amount.
However, conversion losses are separate from the difference between the system's DC nameplate rating and AC inverter rating.
A properly designed system must stay within the inverter manufacturer's electrical limits.
For a string inverter, those limits can include:
For a microinverter, compatibility requirements apply to each module and microinverter pairing and are based on the voltage and current limitations of the microinverter being used.
For example, current Enphase IQ8 documentation specifies maximum module voltage and current limits and states that there is not an enforced fixed DC-to-AC ratio for certain IQ8 models as long as the module remains within the microinverter's compatibility requirements.
Always use the manufacturer's current compatibility and sizing requirements. Nearly all string inverters will list a maximum solar array wattage on the product specification sheet.
Yes, but this needs to be evaluated as part of the electrical design.
The solar system's continuous AC output current is important when determining:
A system with lower AC output may sometimes fit an existing electrical configuration that could not accept a larger AC system (inverter output).
However, choosing a smaller inverter does not automatically eliminate the need for a main-panel or service upgrade.
Whether an upgrade is required depends on the complete design, including:
This should be determined from the approved electrical design rather than from DC array wattage alone.
With an Enphase system, each module is paired with a microinverter.
The system therefore has two easy-to-understand ratings:
DC system size
Module wattage × number of modules
and:
AC system capacity
Continuous or maximum AC output of each microinverter × number of microinverters
The module and microinverter do not need to have identical wattage ratings.
Enphase specifically provides module-compatibility guidance for matching IQ microinverters with higher-power modules.
For example, a 440-watt or 450-watt module may be appropriately paired with a microinverter whose maximum AC output is lower than the module's DC nameplate wattage, provided the pairing meets Enphase's electrical compatibility requirements.
A useful way to remember the difference is:
DC rating tells you how much solar-module nameplate capacity is installed.
AC rating tells you how much AC power the inverter system can deliver at its rated limit.
Neither number, by itself, tells you how much electricity the system will generate in a year.
Annual production should be estimated in kilowatt-hours (kWh) using a project-specific solar-production model that considers location, orientation, shading, weather, equipment, and system losses.
It is normal for a solar system's DC rating to be higher than its AC rating.
This is intentional in many well-designed solar systems.
A higher DC-to-AC ratio can allow the inverter to operate at useful power levels for more hours of the year, while some output may occasionally be limited by inverter clipping during ideal solar conditions.
The best design balances these things:
A lower AC rating does not mean the difference between the DC and AC ratings is simply lost energy.
The correct way to evaluate the design is by looking at modeled annual energy production in kWh, not just comparing DC and AC nameplate ratings.