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

[Allan Sindelar]

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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