Equation that calculate output power

Re: Equation that calculate output power

Using data from this source:

Nominal Operating Cell Temperature: 45ºC±2ºC
Temperature Coefficient of Pmax: -0.34 %/ºC
Temperature Coefficient of Voc: -0.48 %/ºC
Temperature Coefficient of Isc: 0.017 %/ºC

Assume Ratings:

Standard Air temperature: 25ºC
Standard Solar Irradiation: 1,000 W/sq.meter
Panel Rating: 200 Watts

Ambient Air Temperature: Tamb
Solar Irradiation: Si

The equation below should work as an approximation. You can substitute your measured values (actual panel rating, cell temp measured vs ambient, etc.). Note the signs are important (-*-=+).

Power = 200 Watts * (1 + (25ºC-Tamb)*(-0.34 %/ºC)) * (Si/1,000 W/sq.m.)

-Bill

Re: Equation that calculate output power

There are several ways of getting an answer...

One is there are several moderately priced weather stations (LaCrosse and others) out there that have optional solar irradiance meters. Many come with a USB to PC interface for data downloads

Or, just get a small solar panel that is the same type as your "main panels" (i.e., silicon mono or poly crystalline) and measure the output current with a DVM (short the output with your amp meter). The output current is pretty porportional to the irradiance (over time and temperature).

In this thread "Jeff Fendison" offered on his website information on how to build your own irradiance meter (basically short the output of a small solar cell with a current sense resistor and a panel DVM)--and he also sells kits that will log the data over months just using a 9 volt battery and a usb dump to a PC.

http://www.microcircuitlabs.com/SIM.htm

I have not used any of his equipment or had any discussions with Jeff, but his kit/turnkey logger looks very reasonable.

In the end--you (TnAndy) have a Grid Tied Power system--and as long as everything is working OK--your system pretty much is logging the solar irradiance + temperature coefficient to a reasonably accurate number.

If you have logging software for your system--can it log Solar Array Current? That will be a much more accurate representation of Solar Irradiance vs the Array Power reading:

Temperature Coefficient of Pmax: -0.34 %/ºC
Temperature Coefficient of Isc: 0.017 %/ºC

A 10C (18F temperature offset) will affect Pmax by -3.4%. The same offset will only affect Isc (~Imp) by +0.17%

Both numbers are probably more accurate than you can expect from a "garden variety" GT Inverter's data panel readings (many systems seem to be off in the +5-+10% range--read more power than is really generated--The Xantrex, from what others have said, is probably closer to the +/- 1-2% accuracy range--"Nobody is unhappy when their system produces more power than nameplate" :roll: ).

Don't get too mired in the data and assume that you have better than ~10% accuracy unless you have done some calibration/conformations.

-Bill

Re: Equation that calculate output power

Can you give us more information on what you need?

The formula I provided is just a first approximation written in linear form based on how temperature of a solar cell affects its output power by using a temperature conversion factor from the panel manufacturer based on standard temperature and solar radiation levels.

It will break down at lower illumination levels--there is a parasitic resistor that becomes significant at lower power levels.

-Bill