Solar Load Ratio is an important aspect of designing a PV system. Often overlooked and somewhat misunderstood – solar load ratio can make a mediocre solar project turn in to a star through skillful design.
The basic of solar load ratio involve the DC/AC power conversion that takes place in all solar PV systems. All solar PV modules produce DC power and inverters convert that into AC power, which we use in our homes and buildings. A 4.8kW (DC) array combined with 4kW of (AC) inverters has a solar load ratio of 1.2 (4.8kW / 4kW = 1.2). The difference between 4.8kW/ 4kW is called the “clipping loss” as the inverters clip the power output from the modules. Many designers use load ratios of 1.0 to 1.2, but we design our projects between 1.25 to 1.4.
How do inverters clip power?
Every inverter has a maximum power load that it receives from PV modules. This is important for two reasons. 1) output ratings of inverters have specific power and voltage ranges and 2) building AC panels also have maximum power ratings. Inverters do not output more than their maximum rated AC power. When DC input is higher than AC capacity, the inverter raises operating voltage of the modules to pull the array off of its max power point – reducing DC power output.
Why do 1.25 and above DC/AC ratios produce better performing projects in Alaska?
Two reasons. 1) Solar PV module outputs are rated at standard test conditions (STC). These conditions are rarely met in the field, but adjusting higher load ratios keeps modules performing at closer to STC conditions. 2) solar PV modules output at higher voltages in colder temperatures (and conversely lower in hot temperatures – lowering production). When modules are producing strong power outputs on a cooler day in Alaska this can produce close to STC conditions for short periods of time. Those times offer peak performance and clip power, but generally occur 10% or less of the time.
During the rest of the time having a higher load ratio along with our cooler air temperatures keeps modules perform at higher output levels for longer periods of time.
In an analogy, this allows a solar designer to keep the fuel tank (inverter) filled longer by the engine (PV module) to deliver better performance over time (kWh).
Let’s take a look at a 1.2 vs 1.4 load ratio. A 4kW AC system with a load ratio of 1.2 produces 4,238kWh a year. That system with a load ratio of 1.4 would produces 5,309kWh. That additional output ends up to be a lot over time. The more kWh you produce – the quicker your return.
1.2 Solar PV Load Ratio
1.4 Solar PV Load Ratio
This highlights how and why you can build solar in Alaska and have it perform and return just as well as anywhere in the United States. Producing power at rates at least half that of utility rates, returns of 8-10 years, and providing 50%-100% of annual power with solar.
We make solar specifically for Alaska. That is the Arctic Solar Ventures difference.