Design efficiencies (was: cost per kWh) [RE-wrenches]

[Ezra Auerbach, DragonSun Consulting]

-----Original Message-----
From: Allan Sindelar [mailto:allan at positiveenergysolar.com] 
Sent: Monday, June 02, 2003 10:05 AM
To: RE-wrenches at topica.com
Subject: Design efficiencies (was: cost per kWh) [RE-wrenches]


Ray,
We have used that spreadsheet for years, because it expresses so much
information in a user-friendly form on a single page, and as an active
spreadsheet, values can be changed on the fly with immediate results.
You are the first Wrench I know who also uses it and has posted your
assumptions for various efficiencies.

We generally use:
95% for wiring, like you.
85-92% for inverter, like you.
92% for PV/battery mismatch with an MPPT controller (more conservative
than you), and 75% with a non-MPPT controller. 96% for battery
efficiency, however. This is based on something I learned years ago from
Ralph Heise of Bogart Eng., in the context of setting up battery
efficiency on the TriMet. He distinguishes between per-cycle efficiency
and system efficiency. Per cycle efficiency refers to the direct
relationship of how many Ahrs it takes to return a battery to the same
state of charge each cycle. I use 95-97% for this, and find that 97% in
my own home system (12 T105s in good shape) it is quite accurate (that
is, the accumulated error in the "% full" display will be very little
even if I haven't completely filled the batteries/reset the monitor for
a week or more). This is distinct from system battery efficiency, which
factors in the energy used to maintain float on a full battery and to
routinely equalize. I think that system efficiency, which is much lower,
is where we have gotten the 80-85% battery efficiency figure that most
of us use. But I figure that per-cycle efficiency is more accurate when
doing a load analysis.

Any other Wrench opinions on this?

Allan at Positive Energy

----- Original Message -----
From: "Ray Walters" <remotech at taosnm.com>
> Windy and I have been using a spreadsheet that you can punch in 
> efficiency numbers for  battery storage, inverter, wiring, and PV to 
> battery voltage mismatch. I have been using a battery efficiency of 
> 85% (consider day
loads
> are higher, night a little lower), inverter at 85 to 92%, wiring at 
> 95% (shouldn't be worse than 90%) and with MPPT, I set the mismatch 
> factor at 98%. This gives a combined factor of  64% to 73%.

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