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Question about solar panel wattage

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Member level 3
May 16, 2006
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solar panel wattage

Hi All, I have a quick question about solar panels. When the specs say that the panel will generate 1000w at 12v, does that mean that it can supply 83 amps (1000watts/12volts)?

Also, how can i then workout how many watts i will have after I will have after the inverter to change it to 240VAC?

Thanks, Mark

solar panel wattage computation

The watt rating is how much power (rated amps times rated voltage) the panel will produce in full sunlight ..
That is the "best case" scenario and in real life on average you can expect about 10-30% less for most panels ..

Next thing is the efficiency of an inverter ..
This usually ranges from 70% to 95%, say, 80% average ..

1000W (advertised rating) * 80% (real panel) * 80% (inverter) ≈ 640W .. or something like that ..

Thanks IanP

So, the system output will be 640W. If i then use a inverter to boost the power up to 240VAC, will i have (amp=watts\volts = 640\240) 2.666 amps?? Is this right, or am I on the wrong track?

Thanks, Mark

That's true, but remember that this estimation is valid for resistive loads (power factor = 1) ..


A solar panel is considered to be nearly an ideal current source and as such it will put out a constant current over a wide range of load voltages from perhaps 10 to 17 volts for a 12 volts system. This constant current is called the rated current under load il. The wattage from a solar panel will depend on the load voltage i.e. in most cases the battery charging voltage. If the load voltage is at the Rated Voltage under Load (also given in the spec and typically around 17 volts for a 12 volt panel) the panel will put out its maximum power which in your case is 1000 watts. The rated current under load in this case is approximately 1000watts/17 volts = 59 amps (and not the 83 amps you expected). If you are operating with a nearly completely discharged battery at 10 volts the panels will put out only 10Volts x il = 590 watts in full sunlight - far below the 1000 watt rating. Note that all of the above assumes full sun light.
If you are using an MPPT (Maximum Power Point Tracker) regulator the rules change and the MPPT will ensure (for a lot more money) that the panels put out their maximum power under a wide range of load voltages(again under full sunlight) , which in your case is 1000 watts. In this case the amps will increase above il which is why the MPPT is also called a current boost regulator.

Once you understand this then you must figure out what the insolation of the sun incident upon your panels is at you latitude and cloud cover. During the summer time in June at noon in Annapolis you can expect to receive approximately the 1000 watts per meter square that is the standard for all solar panel ratings and you should get close to il amps out of your panel as described above. I have included an attachment that shows how the amps varies through out the day for a 1000 watt system in June in Annapolis. Note that with cable losses etc you can expect a max current of about 49 amps.

If you go to my web site at **broken link removed** you can download for free the documentation for my Solar Panel Power Estimator that provides the power out of a user defined rig at any time of the day at any latitude in the world and for any cloud cover. This documentation will explain the mechanics a little further on what to expect from a typical rig. If you download the excel spreadsheet you will be able to see how the power varies with latitude and cloud cover for a fixed system.

Hope this helps

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