Originally Posted by alano
Your customer is leaving energy on the table! 400 W / 13.6 V = 29.4 A and 400 W / 12 V = 33.3 A. Even if you account for their typical converter inefficiency of 4%, that should still generate 27.6 A at 13.6 V and 31.3 A at 12 V in optimal conditions. The manual says this converter will current limit the output to 25 A so you're leaving on the table between 10 and 25% of the peak energy. I wouldn't specify this configuration with 400 W of solar input.
You are looking at this from a theoretical engineering view and not a real world one. There is a significant difference between STC ratings (Standard Operating Conditions) and NOCT (Normal Operating Cell Temperatures)
I have included an explanation from AM Solar's web site so others can see the difference. Blue Sky also calls for derating their open circuit current and short circuit current by 1.25% to account for the real world
factors of solar panel output. From their manual:
Voltage, current and power produced by PV modules fluctuate widely with operating conditions. As a result a set of test conditions referred to as Standard Test Conditions (STC) are used to rate modules in a meaningful manner and accurately predict real world performance. STC ratings are not maximum or optimal ratings. Conditions can be present where VOC and ISC approach 1.25 times STC ratings which is why National Electrical Code and our recommendations call for 1.25 derating of both VOC and ISC. Yet in real world conditions IMP actually seen is commonly only about 75 – 80% of IMP at STC.
- Solar Panel manufacturers use what are called STC (Standard Test Conditions) when they evaluate solar panels in a solar simulator called a flash tester. During flash testing, solar panels are exposed to artificial sunlight with an intensity of 1000 watts per square meter. (Bear in mind, 1000 watts per square meter of sunlight would only be reached around solar noon, with the panel squarely facing the sun, just after a rain shower has washed all the dust out of the air.) The temperature is 25°C (77 °F) and the atmospheric density is 1.5. The output of the panel under these Standard Test Conditions is what gives the panel it’s rating. All solar panels are rated using this same method, meaning that a 100 watt solar panel from one manufacturer will produce that same 100 watts under STC as a 100 watt solar panel from another manufacturer. Think of the STC rating of a solar panel as similar to an EPA mileage rating for a vehicle. How often does your mileage exactly match the stated EPA rating?
NOTE: Atmospheric density is the optical path length through the Earth's atmosphere for light from a celestial source. As light passes through the atmosphere, it is attenuated by scatter and absorption; the more atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies at the horizon appear less bright than at their zenith. An atmospheric density of 1 is what you would find when looking straight up from sea level at the sun when it is directly overhead (which never occurs in North America).
If you are anywhere north of the Tropics (23.5 degrees of latitude), the sun is never going to be directly overhead and you will never see an atmospheric density of 1. Since solar panel manufacturers operate at varying degrees of latitude and elevation, some sort of standard had to be developed so all solar panels could be tested and rated under identical conditions and customers could have a reliable basis for comparison. A number had to be agreed upon, as a result, 1.5 was adopted for use in the STC. This is roughly equivalent to 500’ above sea level where the majority of the Earth’s population lives.
– Because Standard Test Conditions hardly resemble "Real World" conditions, utilities and municipalities have adopted what they call NOCT (Normal Operating Cell Temperature) ratings in order to more accurately issue rebates and tax credits. NOCT incorporates more reality into their conditions by assuming the following: 800 watts per square meter of sunlight irradiance, an average of 20°C (68°F) air temperature and an average wind velocity of 1 meter per second (2.24 miles per hour) with the back side of the solar panel open to that breeze (as opposed to being on a roof where heat builds up under the panels).
When you consider that solar cells are dark blue to almost black, they soak up sunshine and get quite hot. Under these conditions, average cell temperature (not air temperature) was found to be about 48°C (118.4°F). Some panels operate a little warmer and some are a little cooler. What you need to know is that ALL solar panels experience some degree of voltage drop when exposed to heat. Since Volts x Amps = Watts, the power output of the solar panel is reduced when the cells get hotter.
Since most solar panels in use on RVs are laying flat on the roof, they are operating at even higher cell temperatures than what would be expected from NOCT (as described above, with the back of the solar panel being open to airflow). This is why you want to keep the panels raised up off of the roof by a few inches. Even then, cell temperatures as high as 70°C (158°F) have been measured on a day when the air temperature was only 80°F.
The point of this is not to imply that panel manufacturers are purposely trying to deceive you. It is because "real world" operating conditions are so variable that they had to come up with some standard test conditions so that all panel ratings are derived after being subjected to the same conditions as every other panel. On average, you will be getting about 75% to 80% of the power you pay for. Know that the 100 watt solar panel you are about to buy will only give you about 75 to 80 watts of power during peak sun hours on an average day. There will be days when you get the full rating of the panels but those days will be few and far between. Likewise, there will be days when it is dark and cloudy and you will get only about 10 to 20 watts out of that 100 watt panel.
Considering that one rarely sees full output from ANY solar module, there is very little, if anything, that is left on the table from using the 2512iX-HV and a 400 watt array. If you look at the readings that the iPN-PRO give you for the individual solar charge controllers, you will generally see real-world input voltages approaching 20VDC from the array. Using your math, 400 watts (ideal) ÷ 20VDC = 20 amps input to the controller. The input will actually be less than that most of the time as you rarely see the full 400 watt output from the array.