STC, PTC, and Power Production

STC

STC (Standard Test Conditions) is the test condition used to give PV panels their power ratings. When determining the STC rating, the PV panel is hand selected (usually the best cells in the lot), flashed with a controlled light source at 1000 W/m2, with the cell and cell temperature at 25°C (77°F) and 0 m/s wind speed.

Basically, when the PV panel is flashed with the light source the power output is measured. So, a panel which has a rating of 165W on the rating label (typically shown as Maximum Power and represented as Pmax), will produce 165W of DC power when subjected to the STC test conditions. These conditions are, in fact, far from real-world.

PTC

PTC (PVUSA Test Conditions) are defined as 45°C (113°F) cell temperature, 1000 W/m2 solar irradiance, and 1 m/s wind speed. This test was developed in an attempt to simulate what happens in a real-world outdoor installation. Usually, the PTC rating for a PV panel is between 70% and 85% of the STC rating. The reason that the PV panels produce less power under these conditions has to do with the material properties of the panels themselves. Most PV panels become less efficient as their temperature increases, and so they produce less power to the inverter. This property is roughly linear in the temperature range under which PV panels are usually exposed. Each manufacturer assigns a value to this characteristic, and it is usually expressed as a percentage change of the total power per °C.

For example, if a panel has a temperature coefficient of power that is -0.50%/°C, the panel produces 0.5% less power for every 1°C increase in temperature. As you will notice, the difference between the STC temperature and the PTC temperature is 20°C. This equates to a 10% performance difference between the two conditions due to ambient temperature alone. The PTC rating (which is closer to most real-world conditions), will produce 10% less power than the power shown on the rating label of the PV panel.

Any material that is exposed to sunlight will heat up as it absorbs the infrared radiation that the Sun produces. Dark colored surfaces tend to become warmer than light colored surfaces. PV panel are typically fairly dark colored, and tend to warm significantly due to the Sun's insolation. A PV panel mounted in direct sun with no wind blowing can be as hot as 70°C or 80°C.

If we perform our calculations again, with the panel at 70°C, we arrive at a power output of 124W per panel, or 75% of the STC rating. This number must be multiplied by inverter efficiency to arrive at inverter output power.

Inverter Efficiency

Efficiency for any power producing device can be defined as "power-out divided by power-in" or PWRout/PWRin. The gasoline engine in your car is about 20% efficient. This means that 20% of the energy created by the burning gasoline is converted into wheel torque, with 80% given off as heat. This efficiency will vary, however. When you are sitting at a stop light and your car is idling, it is 0% efficient, because it is consuming fuel but is not doing any work (in terms of moving your car). Your Sunny Boy 2500U inverter is about 94% efficient when it is delivering 700W or more to the utility grid. Only about 6% of the electric power that you put into your Sunny Boy is converted into heat. At lower power levels, the Sunny Boy is less efficient, just like when your car is idling. This is true for all inverters. For a typical installation, the Sunny Boy will average about 90% efficiency across an entire day. This is much better than the efficiency of the engine in your car.

Going back to our previous example, if a single panel is only supplying 124W of DC power, the inverter will only be able to produce 94% of that or 117W of AC power. If we get an entire array of these panels, say 18 of them in 2 parallel strings or 9 panels each, then the panels will produce a total of 2232W DC power, and the inverter should produce about 2098W of AC power.

Now, this discussion assumes that there are no losses of power in any of the cabling, connectors, switches or other components throughout the system. In a real-world installation, there will be additional losses, and this will slightly reduce the output power as well.

So in conclusion, due to many factors, a PV system will never produce AC power equal to the combined STC ratings of the panels. A properly designed system will provide a predictable output power under a given set of conditions.