# WATT PROBLEM: HELP!!!

#### francesco.ciani88

##### New User
Hi all,
I am dealing with photovoltaic system, but I am not expert with that. I have the solar exposure in W/m^2 in a specific hour of the day and I want to translate in Volt and Ampere, but I am not sure about how I can do it. Watching the DATA-SHEET of my panel. I found the attached curves, but now I don't Know which curves I should use (1000W/m^2 , 800W/m^2 , 600W/m^2 , 400W/m^2 , 200W/m^2).
Focusing on my case, if I know that at 10:00 I have just 5 W/m^2, how can I find whic are Voltage and ampere?

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#### Besoeker

##### Senior Member
Hi all,
I am dealing with photovoltaic system, but I am not expert with that. I have the solar exposure in W/m^2 in a specific hour of the day and I want to translate in Volt and Ampere, but I am not sure about how I can do it. Watching the DATA-SHEET of my panel. I found the attached curves, but now I don't Know which curves I should use (1000W/m^2 , 800W/m^2 , 600W/m^2 , 400W/m^2 , 200W/m^2).
Focusing on my case, if I know that at 10:00 I have just 5 W/m^2, how can I find whic are Voltage and ampere?
You can't from the data you provided.

#### GoldDigger

##### Moderator
Staff member
Given a volt versus amp curve for a particular insulation value (w/m^^2) you can determine Voc, Isc, Vmp and Imp.
But you will not know the combination of V and A for a particular load unless you know the corresponding V vs A curve for the load. (Hint: for a resistor it is a straight line.)
In general, for a crystaline silicon panel, the Voc value will be almost independent of insolation while the Isc will be directly proportional to insolation.
When the insolation value you want to work with is outside of the range of values given you can extrapolate carefully, but that will break down for extreme values.
You could also try to derive the shunt and series resistance parameters for the panel and apply that info to the insolation you need.
5w/cm^^2 will not give any useful power in any case.

#### ggunn

##### PE (Electrical), NABCEP certified
Hi all,
I am dealing with photovoltaic system, but I am not expert with that. I have the solar exposure in W/m^2 in a specific hour of the day and I want to translate in Volt and Ampere, but I am not sure about how I can do it. Watching the DATA-SHEET of my panel. I found the attached curves, but now I don't Know which curves I should use (1000W/m^2 , 800W/m^2 , 600W/m^2 , 400W/m^2 , 200W/m^2).
Focusing on my case, if I know that at 10:00 I have just 5 W/m^2, how can I find whic are Voltage and ampere?
What is it that you are trying to do? If you are trying to predict the output of a particular system use PVWatts (http://pvwatts.nrel.gov/pvwatts.php) and put in your system info.

The curves you show are only part of the story. The 1000W/m^2 curve is the solar power hitting the earth for "full sun"; the others are for... less sun. 1000W/m^2 is the accepted standard for noon on the summer solstice (best case), but your modules have to be pointed due south and tilted at whatever degrees latitude where you are located to get that, and it's a theoretical number, not real. The actual strength of sunlight may be less or (less commonly) more than that, and it's changing as the sun (apparently, of course) moves through the sky. And then there's temperature. Those curves are generated at STC (standard test conditions), one of which is 25 degrees C. 1000W/m^2 sun strength when the module temperature is 25 degrees C almost never happens in the real world.

PVWatts, on the other hand, uses real climactic condition data collected at scads of NREL weather stations all over the country over a 20 year span to predict what a system will produce over a year's time at your specific location. It takes into account cloud conditions, temperature, sun angle (which obviously is changing continuously) as well as your system configuration variables, and if you are precise with the information you put into it, it will do a pretty good job of telling what your system will produce.

The curves you show are worthless for predicting system performance.

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#### BillK-AZ

##### Senior Member
The curves you have are for a fixed cell temperature, not often applicable as the sunshine warms up the PV module and reduces the voltage for a specific irradiance level. If these PV modules are in series and connected to an inverter (not charging a battery), the inverter will adjust the current in order to operate the PV modules at the peak power point (top of the power curves) for the specific irradiance and cell temperature.

The "just 5 W/m^2" you mention is a very low light level, 1/2% of full sun. I do not know how you arrived at such a reading. Most testing is done with irradiance over 800W/m².

If your PV system has more that two modules in series, it is difficult to safely measure by opening the circuit to insert an ammeter to measure current. Most PV inverters use 8 to 14 series modules and can produce serious arcs if the circuit is opened. Be careful. If your system uses micro-inverters, you can do this safely.

Computer programs such as PVWatts calculate hourly values of power by using Solar Insolation Models to get specific hourly irradiance on an array for a location/date/tilt/azmiuth, then use the resulting data and the PV module parameters ( Voc, Isc, Vmp and Imp) along with temperature coefficients to get a specific power for that hour. Then the hours/days/months are summed up in reports. Most programs do not directly report the hourly values that you seem to seek to compare to your readings.

#### Besoeker

##### Senior Member
The "just 5 W/m^2" you mention is a very low light level, 1/2% of full sun. I do not know how you arrived at such a reading.
I wondered about that too. To the extent that I thought it might have been a typo.

#### ggunn

##### PE (Electrical), NABCEP certified
Hi all,
I am dealing with photovoltaic system, but I am not expert with that. I have the solar exposure in W/m^2 in a specific hour of the day and I want to translate in Volt and Ampere, but I am not sure about how I can do it. Watching the DATA-SHEET of my panel. I found the attached curves, but now I don't Know which curves I should use (1000W/m^2 , 800W/m^2 , 600W/m^2 , 400W/m^2 , 200W/m^2).
Focusing on my case, if I know that at 10:00 I have just 5 W/m^2, how can I find whic are Voltage and ampere?