When determining ratios there is a need for a comparison value.
http://www.aaastudy.com/rat62bx2.htmAll we have to start with is the PV modules ratings at the STC or Standard Test Conditions which are only possible in a lab under strict guidelines, but this should do.
The other thing we need is ratings based on real world conditions averaged out over a specific period of time. Say, longest day to longest day. June 21 in my region. Then you formulate the ratio of panel efficiency as compared to STC ratings. Luckily someone has been recording solar radiation for nearly 30 years at various places of the North American continent.
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/Once there, chose: average, annual, flat plate tilted south at latitude then view map. Find your regions color, compared it to the legend. What this will give you is an approximation of the number of hours of equivalent full rated power of a PV module. Notice over the legend it shows kWh/m2/day. Thats an equivalent of 1000 watts per hour over a 1 meter square area per day. Obviously there are a lot of other factors involved here but this should give an approximation. Try other scenarios. Get to know whats going on with this. Try one similar to your PV array for this month and see how it compares to the number of hours of equivalent full rated power you are actually getting from your array.
Lets say for example you have a 1000 watt PV array facing south tilted at latitude plus 15 degrees in December and you live in central Nebraska. Go to the above site, click average, December, flat plate South tilted latitude� +15 degrees then view map. Look at the legend for central Nebraska and you see 3 to 4 kW hrs per day of potential energy from our Sun averaged out over the month. You have recorded the amount of actual power from your PV array and it equates to 2.5 kW hrs. per day averaged out for the month as well. Divided 2.5 by 3.5 = an efficiency of 71%, roughly speaking. That would make a ratio of what? 5:7 this tell us that you would need two more modules to reach 100% Or a 1:1 ratio. Is this right? Check my math please. I don't get the chance to play with ratios much.
