If you have concerns about the amount of power your solar system is producing, don't worry, you're not alone. There are many contributing factors that govern and limit how much the panels on your van will net you and there is even more conflicting information that can be found online and most of it is misleading and unclear.
The two most common mistakes made when trying to work out what your panels can produce are:
1. The calculations are done using STC ratings and not considering NOC ratings and losses. Standard Test Conditions are lab controlled environments and not typically replicated in Normal Operating Conditions (Irradiance of 1000w/m² @25°c w/ Air mass of 1·5 and angled at 90° from panel). Source
2. The calculations are performed with incorrect information or assumptions concerning temperatures and system losses/efficiencies, often ignoring that they exist at all.
It’s important to note that a standard P=V*I calculation won’t work for solar as it doesn’t consider panel efficiency, weather conditions, system losses, solar regulator DC-DC conversion or modulation, temperatures of panels, batteries, charging algorithms, etc.
There is a simplified calculation that can be used with a reasonable amount of accuracy.
This is Watts x 0·7-0·8 (losses) / VMP = Input to regulator.
In a case with 3 x 170w panels, this would be:
170(w) * 3(units) * 0·8 (losses) / 18·8(vmp) = 21·7(amps)
Of course, it’s a ballpark figure and not to be taken as Gospel, but it does give you a more realistic figure of what your system will produce under Normal Operating Conditions. Things such as cloud cover, panel temperature, latitude, dust, leaves, shadows (A/C, Antenna, Trees) will all reduce the amount produced. Other things like the State of Charge and battery temperature will affect what can be put to the battery directly and must be factored in as well.
Further information can be found in the following articles: