Power Usage Calculations

[zander1976]

Hello,

I feel a little stupid for asking this but I am very new to renewable power. I am trying to read the data sheets from a solar company:

The sheet says:
Nominal Max Power at STC: 235W
Optimum Operating Voltage: 29.8V
Optimum Operating Current: 7.90A
Open Circuit Voltage: 36.9V
Short Circuit Current: 8.48A

Is that per minute, day, year? How much power is that per day assuming i have 5 hours of sun?
Optimum volts and currents I assume is when the sun is perfectly on the cell at the right temperature right?
What is Open voltage and short current?

I want to start out by feeding power into the grid. Assuming conditions were perfect how do I figure out how much A/C power I would feed to my power company?
Do I have to invert the power before feeding it to the power company ( I assume this to be true )?

I know my current power usage. Its about 700Kw per month. That number is going to go up since I want to switch to electric heat and on demand electric hot water. Space isn't really an issue for solar and wind power generation so I want to go all electric if possible and not use any natural gas.

This is a lot of questions I know. I have searched for many of them but I can't seem to find the right calculations I am looking for.
Thanks a lot for all your help.
Ben

[zander1976]

Also what is the most efficient voltage to convert DC power to AC power?

[zander1976]

Series = More voltage = Connecting Positive to negative in series
Parallel = More amps = Connecting Positive to positive in series.

Why is that the case? I would think that if you connect positive to negative then all the electrons would make it to the positive post and have completed there journey.

[dnix71]

STC means laboratory test conditions that are repeatable and verifyable. PTC would be "real world" conditions. California used to publish a comparison of the two, but the link there from this page is dead. http://www.gosolarcalifornia.ca.gov/equipment/pvmodule.php

STC is 70F I think. Most panels output goes up if it's colder and falls when it's warmer. If you use thin film in very cold conditions you better watch the voltage carefully esp. if you use a MPPT. Don't connect more panels than 75% of the MPPT max voltage rating in a cold climate, 2/3's if you live where it might hit -40, esp if there is snow. With reflected light and cold from snow you might just push your panel's output so high you fry the controller.

In your case, with an MPPT that's rated 150v max, you couldn't string more than 3 panels anyway.. If the panels are thin film they fade worse in the heat and may not be intended for series connection. Poly and Mono panels can be stacked to give higher voltages. Over 60 volts and the NEC rules apply for wiring, 60+ is considered 'high' voltage.

Higher voltages mean lower impedance losses for a given wire size. Copper costs money, so the trend is as high a voltage as possible in commercial use. BUT, dc has a nasty arc that a/c doesn't. Breaker and disconnects for dc are different. Enphase makes a dc to ac micro converter that can be paralleled as a workaround if high voltage dc is undesirable, but the Enphase inverter costs as much as some panels, so it's a lot of extra money upfront.

If you got full rated power for 5 hours you would get 5 x 235 = 1175 watt-hours [ 1.175kwh of electricity] per day per panel. That's worth about 6.7 cents worth of grid power. You aren't going to get rich selling it back that way without the govt giving you the panel through tax cuts or forcing the utility to pay you more that they would charge you for the same electricity.

48 volt inverters do a pretty good job making 120vac if that's what you want to do with the power. But the bigger inverters have larger idle power draws so you might consider 2 inverters, one for the largest load you have and a small one for other uses. I have 2 sine inverters, a 300 watt and a 1000 watt.