![]() ![]() Finally, residential arrays can go up to 380W with clipping losses under 1%. ![]() In commercial arrays (with lower temperature losses), the module power can go up to 365W while still keeping clipping losses under 2%. Since an ILR of 1.25 in Atlanta represents just 0.1% to 0.6% clipping losses, this raises a question: What would the losses be if we increased DC/AC ratios more dramatically? See below:Ĭlipping losses are zero for DC/AC ratios of 1.15 in both system types. As a result, the Atlanta system with a 1.25 DC/AC ratio has total clipping losses of just 0.6% for commercial arrays, and 0.1% for residential arrays. Because residential systems are flush-mounted, they run hotter, and therefore the temperature losses are even larger. This effect is even more extreme with residential arrays. Looking to the Atlanta example again: For the 212 hours when the modules are producing greater than 80% of rated power (the cutoff point for a 1.25 DC/AC ratio), the average power is just 6.8% over the limit. So even when the actual DC power is 10% over the max AC power, the losses are just 10% for that time. ![]() Generally, when an inverter is in over-power mode, it simply means that it will sacrifice the excess power. Production does not go to zero when the DC power is greater than max AC power. This time, there are just 212 hours (4.5% of the operating hours) when the modules are producing over 80% of their rated max power.ģ. Note that the top end of the distribution is even thinner because this data also includes the temperature losses, which will be greatest at the times of highest sunlight. Therefore, the array produces less than the rated power, and it doesn’t reach over-power conditions at the inverter.īack to the Atlanta example: Let’s look at how often the modules are producing close to their rated power. In particular, temperature losses make a huge difference here: modules are typically hotter than 25✬, particularly when the array is receiving maximum sunlight. In addition to the fact that the array rarely gets full sun, there are other system losses between the module surface and the inverter. Temperature losses reduce the high-power times even further. In fact, just 422 hours (9% of the operating hours) see more than 800W/m2 (equivalent to clipping at a 1.25 DC/AC ratio).Ģ. Note that the array rarely sees full sunlight. In practice, systems rarely receive these idealized conditions.įor example, the chart below shows a frequency chart for a solar array in Atlanta facing south at a 15° tilt. Full “standard” conditions (1,000W/m 2) are rare.Ī module is rated at “Standard Test Conditions” (STC), which is sunlight of 1,000W/m 2 (basically noon on a summer day). Why is this? Three factors help explain these low losses:ġ. This actually keeps losses extremely low, usually under 0.25%. Many designs start with an assumption of a maximum 1.2 DC-to-AC ratio (in other words, 20% large module power rating versus the inverter max power rating). ![]() Why a 20% DC/AC ratio results in minimal clipping losses During times when the DC input power is too high, the inverter will raise the operating voltage of the modules to pull the array off of its max power point and reduce the DC power. Inverters will generally never output more than their max-rated AC power. Second, at the system level, the home’s AC panel (and/or the grid connection point) are designed with a specific max power in mind. First, the component ratings of the power electronics in the inverter are often designed with a specific power and voltage range in mind. Instead, design values of 1.2 often result in minimal losses, while a 1.25 or 1.3 value can improve project economics, especially when a project size is constrained by the AC capacity.Įvery inverter has a maximum rated power. Many people think DC/AC ratios of 1.1 are ideal, with 1.2 as slightly aggressive. We at Folsom Labs have found that many designers are overly conservative when thinking about DC/AC ratios. The key driver here is the “clipping loss”: when the DC power feeding an inverter is more than the inverter can handle, the resulting power is “clipped” and lost. For example, a 6-kW DC array combined with a 5-kW AC rated inverter would have a DC/AC ratio of 1.2 (6 kW / 5 kW = 1.2). The DC to AC ratio (also known as the Inverter Load Ratio, or “ILR”) is an important parameter when designing a solar project. But far fewer designers and engineers understand what are the practical limits. We all know that the module rated power can be larger than the inverter rated power (within reason-inverters do have a max input current). Pop quiz: What happens if you pair 6 kW of modules with a 5-kW inverter? How much energy will be lost due to “clipping?” Read an updated story on solar inverter clipping from 2019 here. ![]()
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