Size a grid tie system based on monthly utility bill seems flawed.

[mauricerichter]

Any online size calculator I've found so far wants to know monthly kwh on electric bill. The contractor I called asked for and I gave same info (I'll try to keep it simple with monthly average of 929kwh). Grid tie, no battery, he says 9 kwh. (24 panels x 380w).

I think the flaw is:
1. Some of that POCO usage is at night when sun don't shine.
2. I've seen online that central Indiana averages 4-1/2 hours of sunlight - hey, its online so its gotta be true, right?
3. My REMC (POCO) allows me to graph my hourly usage. I printed the whole year and checked. My one hour peak of all all year is 7.5kwh. I have only had 78 hours out of a whole year that was 6kwh or higher (0.89%). And only 11.27% of hours are above 3kwh.
4. My REMC (POCO) charges me $0.1058 per kwh during "daytime" and would pay me $0.04389 per kwh for excess solar power.

I can't see that those 4 pennies are ever going to payback a 9kwh system. Where am I wrong?
Thanks!

[SunEagle]

You may want to look into a website called PVWatts.nrel.gov. You can perform a lot of calculations that helps determine how many panel watts will generate kWh for your home.

[mauricerichter]

SunEagle, Thank you, yes, intersting site. However. bottom line of my question is if 1% of the hours I go at or over 6kwh, then why do I need a 9kwh system?

[SunEagle]

SunEagle, Thank you, yes, intersting site. However. bottom line of my question is if 1% of the hours I go at or over 6kwh, then why do I need a 9kwh system?

Well a 9kW system seems pretty big for someone that uses less then 12000kWh a year so I think your questions are valid.

When you run the cals try using a smaller kW system and see how it does to cover at least 80% of your consumption. IMO unless you have a great NetMetering contract or plan on expanding your usage most systems that are in excess of 80% end up not paying for themselves.

[J.P.M.]

Where am I wrong?
Thanks!

Numbers 1 to 4 above would be my short answer.

They are the wrong questions because you have a basic non understanding of how grid tied PV works. But that's a relatively easy fix.

Get a free download on line PDF download of "Solar Power Your Home for Dummies". Come back here for answers to questions your self education creates.

Residential PV may or may not be for you but you'll need the basics to understand why. The book is a good primer even if a bit dated.

After the read, and as SunEagle suggests get familiar with PVWatts by reading all the help screens a couple of times before you attempt any runs.

[Mike 134]

As stated above much depends on your Power Company net metering rates. I'm northern Illinois with ComEd and it's one for one so If I can generate 47 KWH during the 8 hours of summer sun my entire day's usage is covered and then some unless I turn on the A/C. Any excess can be used later in the year, comes April 1 any leftover credits are zeroed out and the process begins again.

[peakbagger]

In order to have a discussion folks need to speak the same "language" and you it would help to get a bit of education. Either search the internet and find an older version of Solar Power your Home for Dummies or spend the big bucks $15 for a used hardcopy.

It does sound like your net metering options are poor so my guess is once you understand the basics you can see if smaller system or a hybrid system with a battery would make sense.