Jump to content

Solar panel output


Recommended Posts

On our new van there is solar panel fitted and looking at the control panel it is feeding juice into the batteries.(only a tiny amount as van not in direct sun) There aren't any instruction manuals so just wondering if there is any way of telling how many watts it is rated at. Is there anyway of telling? On a sunny day what amperage would you expect to see and how do I know if that is feeding much juice into the batteries? Does the formula Watts= Volts x Amps apply? So if I multiplied the number of amps registering on the control panel by 12 would that give me the wattage?
Link to comment
Share on other sites

If you cannot read the label (and the supplier doesn't know), then you can calculate a very approximate power rating from the dimensions of the panel.

As a rough guide, measure the length and width (in cm), subtract 5cm from each (to account for the frame and space between cells) and then multiply the two numbers (length - 5cm) x (width - 5cm). This gives the approximate area of the cells. For a low/moderate quality monocrystaline panel, divide the above result by a "fudge factor" of 56 to get the approx. wattage. For higher quality panels that generally have a higher efficiency, this fudge factor is more like 48. These are just approximate numbers but should give you a result that is close to 10% of the actual rating.


Just as an example, this (low cost) panel [url=] https://www.photonicuniverse.com/en/catalog/full/476-120W-monocrystalline-solar-panel-with-5m-cable.html [/url] measures 118cm x 67cm so (118-5)x(67-5) / 56 =125W.


It is not unusual for manufacturers to underrate their panels when new as they know they degrade over time. After say 5 years, they can still guarantee 80% (or what ever) of their specified rating.


"Watts = Volts x Amps" is only useful if you have a MPPT controller (not a PWM controller). Even then, the MPPT controller will have some inefficiency and the power you get out will be around 95% of the solar power input (panel Volts x Amps).


For a PWM controller, the power output is more like 14V x the solar panel current. The 14V is just the battery voltage when on charge so is just a nominal figure. The excess panel voltage gets wasted with a PWM controller



Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...