Why A 100W Solar Panel May Only Give You 50W
There is much to do about solar panels on RV’s. They are affordable, virtually maintenance free and easy to install. But do they deliver, what they promise?
Actually, most of these panels produce the indicated watts, with a big caveat: in summer under ideal weather circumstances. That’s fine for the average holiday traveler, yet poses obstacles for the more hardened RVer, who needs the power especially during the colder months of the year.
What does that mean? While some places will have extended periods of this perfectly blue sky without clouds all year long, more often we all have to deal with cloudy days or rain that doesn’t seem to stop. That means that the expected output from the solar panel suddenly has disappeared. What makes it worse, is that winters and often spring and fall as well, expose the panels to less sunshine and consequently less daily output.
Then there is the game of averages. There is usually a minimum of 2% voltage drop in the wiring and sometimes much more. A similar loss can be attributed to the controller, fuses, circuit breakers, etc. Inverters are even worse offenders.
But Hurray! Finally that clean power you produced, gets stored in your batteries and is available for your use. But how much do you put into your batteries and what comes out? More losses that seem to disappear in thin air.
And there are more weaknesses in the system. You may still use a PWM solar controller, that squanders the higher voltage of your solar panels. And don’t forget that lead-acid batteries need to be constantly topped off, effectively changing the 100W output to a trickle.
So what’s actually left, when the juice gets into the battery? Getting to the answer, requires many assumptions. Your calculation of how many panels are needed, should be seasonaly adjusted and based on the 5 to 6 solar hours available on a winter day. That’s about 25% less than many of us expect on a long summer day. Another assumption we all have to make is that clouds or rainy weather will reduce the effectiveness of the panels with another 25%. If 25% of the energy is used by 120V appliances, your inverter will create an additional 10% loss. Assuming the use of an efficient MPPT controller, voltage drop in the wiring, solar controller and other devices will add another few watts to the list and I haven’t even considered other issues of living in a van, such as being parked in the shade.
+ 100W 100W solar panel
– 25W Winter adjustment
– 19W Cloudy days
– 2W Inverter losses
– 1W Wiring voltage drop
– 1W Controller voltage drop
There are many assumptions and misconceptions, but we all tend to look for the best and most ideal circumstances, yet when you derive your solar power requirements, you have to base it on the least favorable circumstances and the shortest days of the year. That means that you might need twice as many panels, than you opportunistically thought, you needed.