A 100-kW inverter may seem the obvious choice for a 100-kW solar photovoltaic array, but this is a common misconception. If you check the specifications of highly engineered projects, you will notice that the DC output of solar arrays is specified higher than the AC output of inverters, and there are many technical reasons for this.
The nameplate wattage of a solar panel is determined under carefully controlled laboratory settings, which are very different from the real operating conditions of commercial solar installations:
The conditions above are called Standard Test Conditions (STC), and they reflect an idealised scenario that is rarely achieved with commercial solar power systems. For example, a solar array rated at 100 kW DC under STC will operate below 100 kW in real-life applications.
Since photovoltaic systems operate below their nameplate DC output, an inverter of matching AC capacity will end up oversized. Considering that inverter price is determined in great part by nameplate capacity, this represents a waste of capital. Also note that other AC components like protection devices and transformers are based on the inverter rating, which leads to even more oversized equipment.
There is a very simple solution for this issue: using an inverter with rated AC output lower than the nameplate capacity of the DC array. The DC/AC ratio or inverter load ratio is calculated by dividing the array capacity (kW DC) over the inverter capacity (kW AC).
The advantage of a high DC/AC ratio is that you can boost energy output without increasing costs for the AC portion of the solar installation.
When you use a high DC/AC ratio, the output of the photovoltaic array may exceed inverter capacity on the sunniest days. However, inverters are smart devices that can “clip” excessive power by adjusting the operating parameters of the solar array. In simple terms, the inverter is smart enough to avoid drawing power above its rated capacity.
Although the calculation procedure for the inverter load ratio is complex and requires simulation software, the logic is very simple: If the extra energy output from a higher DC/AC ratio has more value than the extra power loss, it makes sense to use a higher ratio. To illustrate this, assume you have a solar power system operating under the following conditions:
Of course, there is an upper limit for the inverter load ratio. A larger array means more current, and also a higher short circuit current if there is a fault. You can increase the DC/AC ratio as long as you are not exceeding any of the parameters established by the inverter manufacturer.
The cost of solar panels has been decreasing rapidly in recent years, and now it is possible to install a utility-scale solar farm for less than $1,000/kW. However, solar panels used to be much more expensive, and the first photovoltaic arrays were designed to avoid power clipping. In other words, the target was to maximise the kWh obtained from every kW of solar panels. The traditional design approach leads to an inverter load ratio ranging from 1.1 to 1.2 in most cases.
Now that solar panels have a much lower cost, the design goal changes to maximising financial return, even if some generation capacity is clipped occasionally. This can often be achieved by increasing the DC output of the photovoltaic array while specifying the same inverter capacity. Inverter load ratios of up to 1.5 are common with this design approach.
Solar farms that sell their output through the power network can increase profits with a higher DC load ratio. Consider that solar panels are more productive during daytime, when power demand is high and the spot price of kilowatt-hours is increased. With a higher load ratio, electricity sales can be boosted considerably for a relatively low cost increase in project cost.
Large-scale producers of solar power get additional benefits from a high DC/AC ratio. The capacity of transformers, substations, interconnection equipment and other expensive project elements is defined by the AC output, and their cost is not affected when using an array with a higher nameplate capacity.
Like with most engineering decisions, it is impossible to recommend an ideal DC/AC ratio for all commercial solar power systems. A general rule of thumb is a 1.2 Load Ratio or 80% inverter (AC) to 100% solar panels (DC). To optimise the capacities of your solar array and inverter, there is no substitute for a professional assessment of your site.