PV System Worksheet -- offering to the Wrenches [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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>Windy & Wrenches (& windy Wrenches...and sunny boys, too, I guess),
>I should have been more clear. I have unprotected the sheet and gone into
>the formula bars to tweak here and there. Sometimes I succeeded at getting
>the info I wanted, sometimes not. What I suggest in a companion sheet is
>descriptions of the logic of the equations from which the results are
>derived (huh?). For example, "watt-hours per day equals quantity times watts
>each times hours per day times days per week divided by 7". That's a very
>simple example. Some of the formulae are quite complex and yet insufficient
>to cover all situations, such as calculating generator run time. The
>inverter-generators have a very different efficiency than the contractor
>specials.
>This is a request, Windy, not a criticism.
>Allan


Dearest Allan,

I wrote the guts of this 6 years ago. You think I remember the logic? 
As for the generator run time, that's the only formula that's 
severely twisted. It came to me in a rare flash of brilliance, to wit:

=IF((G27-(G38*G32*(K20+(K20*B32))))/(J17*B30)<0,0,7*(G27-(G38*G32*(K20+(K20*B32))))/(J17*B30))

What's the logic? I couldn't wrap my head around it 6 SECONDS after I 
wrote it! I just tested it up and down with examples to see that it 
worked, sighed relief, and nobody's reported any problems.

I CAN tell you that it's simplified. It assumes the battery is 
accepting the full "Battery Charge Amps" for the entire charge cycle. 
(I will add notation of this to the instruction sheet.)

There are way too many factors to handle here, like how full is the 
battery when the generator starts up, and how far does the current 
taper down before the generator is stopped? And, of course, the 
C-rate (ratio of battery AH to charge rate). This would take another 
whole sheet, and too many assumptions in my opinion.

However --

I can easily add a "Charge Acceptance Factor", with a default of say 
75%, to show the run time /.75. This would reflect a realistic 
average for a period of bulk charge at full current, then some 
tapering. Sure it's a guestimate, but better than assuming 100%.

Oh, I guess we could enter a C-rate and it would adjust the factor 
further automatically?

Tell me what you think.

Thanks!

Windy

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