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DC to AC Ratios: High, Low, or Just Right?
Posted 03/01/2022in Solar News
The DC to AC ratio is a ratio of the rated nameplate DC power of the modules versus the rated
nameplate AC power of the inverter. The DC value is based on the DC nameplate of the modules at a
site and on Standard Testing Conditions, (Solar Radiation: 1 kW/m2, Temperature: 25 C, No wind).
In an ideal world, the DC to AC ratio would be 1, however Standard Testing Conditions (STC) are
important when considering the PV module output because the modules will rarely operate at these
conditions, especially in the New England area. Operating conditions are often much warmer or offer
less solar radiation. This results in overall less power output from the solar modules than their
nameplate rating.
To accommodate this difference in PV module nameplate power rating and real power delivered,
solar designers undersize inverter AC power in comparison to the PV module DC power. This results
in a DC to AC ratio that is greater than 1. Without this immediate under-sizing of the AC side of the
system, the customer would pay for a larger inverter whose capacity would rarely be used to its
fullest extent. With a DC to AC ratio greater than 1, more of the inverter’s capacity is used more of
the time.
Clipping Losses
When the DC rating of a PV system is higher than the AC ratio a new form of loss is introduced to
the system. An inverter will only produce at the maximum power output of the inverter’s nameplate,
if more DC power is being produced than that nameplate the inverter will move below the maximum
power point (MPP) to reduce the power it pulls from the array to maintain the maximum nameplate
power output of the inverter. The remaining DC power will be lost, these losses are called clipping
losses. Clipping losses occur on the sunniest days of the year and typically during the summer
months. Clipping losses may start high for a system but will decrease over time as the PV modules
begin to degrade. There are some initial positive sides to a higher DC to AC ratio that results in
noticeable clipping losses. Excess energy will be produced in this scenario because of the higher DC
rating, any time the inverter is functioning it is producing more energy than it would have at a lower
DC to AC ratio scenario, which is illustrated in Figure 1. Design engineers must decide what is an
acceptable level of clipping versus the excess energy that is produced at all other times throughout
the year and determine what is the most beneficial scenario for the customer. It is up to the design
engineer to find the balance between clipping losses and excess energy generated outside of the
clipping window.
Figure 1: Clipping Losses and Gains, Bromberg Author, David. “Choosing the Right Size Solar
Inverter.” Aurora Solar, 14 Apr. 2021, https://www.aurorasolar.com/blog/choosing-the-right-sizeinverter-for-your-solar-design-a-primer-on-inverter-clipping/.
Recommended DC/AC ratio
In general, the industry standard for DC to AC ratios in New England is 1.2, however, this number
has continued to rise in recent years. The rise in the ratio has been documented by the United States
Energy Information Administration (EIA) and can be seen in Figure 2.
Higher DC to AC ratios are becoming more and more popular with time. This is primarily a result of
the financial benefits that arise with the higher DC to AC ratio. Cheaper PV modules make it
financially beneficial to pack in DC power and accept higher clipping losses. According to studies
performed by ABB, the best financial outcomes exist between a DC to AC ratio of 1.3 and 1.6,
however, to maximize system yield the DC to AC ratio should be closer to 1.2. It should be noted
that the maximum allowable DC to AC ratio for many common inverter manufacturers (i.e., SMA
America and SolarEdge Technologies) is 1.5, a DC to AC ratio any higher would void the inverter
warranty.
A high DC to AC ratio can result in an overall negative impact on the inverter and reduce its useful
life. This occurs because the inverter is operating at its nameplate rating for a longer period. This
results in an overall higher operating temperature for the inverter, particularly during the summer
months. The additional heat also results in the inverter being less efficient because of the higher
operating temperature. It is also noted that higher operating temperatures are linked to aging
components in inverters. See Why array oversizing makes financial sense By Kathie Zipp.
One positive use case for a high DC to AC ratio is DC coupled storage. This scenario allows for the
DC power that would normally be lost at the inverter to be stored in a battery system. Then the stored
energy can be used during a non-peak time when the system will have less power production from
the PV system.
High DC to AC ratios can also be beneficial for arrays that are north-facing or heavily shaded due to
site restrictions and space restraints. Conversely, if a customer has a non-export interconnection
service agreement with the utility then a low DC to AC ratio would be more beneficial to ensure that
only enough power is being produced to supply the building associated with the system and not
export to the grid.