# inverter capacity calculation

28 Posts
Nov 7, 2007 07:09 am
inverter capacity calculation

Hi All,  (I'm new here)

I was looking at the inverter calculation on page,
http://store.altenergystore.com/calculators/on_grid_calculator/

and I can't seem to figure how they get from a monthly usage of 100 kW, avg sun hours of 4.13 to Watts needed = 969

There is some kind of loss factor in there.

Any ideas about what the Watts formula contains?
Thanks

578 Posts
Nov 7, 2007 07:06 pm
Re: inverter capacity calculation

those numbers are about right.  i have been meaning to freshen up the calculators, but i can barely get through my vm and email.

anyway,

you dont have monthly usage of 100kW;that does not make sense.  It may be 100kWh, and I will make that assumption for the numbers below.

969w of solar x 4.13 sun hours average = 4000 watt-hours or 4kwh production daily

4kWh x 30 days in a month = 120 kwh per month

20% deration for module performance, inverter efficiency, voltage drop, etc.  it is an estimator, but that is where the numbers come from.  in this case that is just about right.

james - Alt-E staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854

28 Posts
Nov 8, 2007 10:06 am
Re: inverter capacity calculation

James,

Right, my numbers don't make sense and don't matter anyway, as its only the method used on the page (link) that I'm interested in clarifying.

As you have indicated, a 20% increase factor appears in the calculation, which I don't think was noted.  I was just making sure there wasn't another consideration that might have influenced that increase over the calculated value.

Thanks,

578 Posts
Nov 8, 2007 11:49 am
Re: inverter capacity calculation

cool, no problem.

generally it is because of the tendency for modules to perform closer to ptc ratings, in the real world, which are typically 80% of STC as well as the other factors i mentioned.

thanks for the question and input,

james Alt-E staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854

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