cost per kWh [RE-wrenches]

[Bill Brooks]

Todd,

It would help more all things were equal (which they rarely are in the real
world). My two arrays are identical size and identical orientation. However,
one array (the one with the SunnyBoy) has a lot of morning shade.

When looking at the times when the arrays are both fully illuminated, the
SunnyBoy produces about 4.5% more energy than the Trace SW. Looking at the
operating points I found that they were both operating very close to maximum
power point for their respective arrays. For instance, today the SW was
running at 52.2 Volts (temp compensated from 53.6V) and the SB was running
at 261 Volts (exactly 5 times greater with exactly 5 times more modules in
series)--side thought--how is a max power tracker going to help me?

The difference in performance is measured by a Campbell Data Logger with two
separate Ohio Semitronics PC-5 watt transducers. Data is taken every second
and averaged over a 15 minute period.

The difference in performance is due to several factors. The SW has a C-40
charge controller in series with the array and the power loss through the
controller is 1%. Voltage drop is definitely higher with the SW given the
lower voltage--charts suggest 2% as the difference in voltage drop given the
wiring in the system. Finally, the conversion efficiency is higher in the SB
likely causing the remaining 1.5% difference in energy delivery. If the SB
is running at the published 94% under these conditions, the SW would be
operating at 92.5%. Not bad for an old dog SW with nearly 5 years on it.

Night-time losses for the SW are measured at a continuous 40-45 Watts. The
SB is one big donut (no losses) at night.

For what they are, I'm impressed with both machines. If backup is not valued
or desired by the customer, the SunnyBoy looks awefully good. If you want
backup power, the SW is not that big of a penalty in performance. It
certainly does not warrant a separate inverter for battery backup. Is the
the SW optimized? Only from the point that I have adjusted to the PWM angle
to get higher efficiency. Max power tracking would have marginal impact on
my array and nighttime shutdown would save about 0.5 kWh per day.

Qualifying statement: this is very new data and calibrations have not been
carefully checked recently. I do believe the numbers are very close to
reality. Another qualifyer is that my unit is an old ETL-listed workhorse. I
have not personally tested the GTI or the current UL-listed version to see
how their conversion efficiencies and night-time tare losses compare. My
results may not be indicative of how a new SW and SB would compare. I just
have a new SB. The new SW could be better or worse.

That's all for now. I'll keep you posted if I learn anything new. I just
love my Max Power Point Tracking temperature compensation probe on my SW
inverter. It is highly reliable.

Bill.


-----Original Message-----
From: Todd Cory, Mt. Shasta Energy Services
[mailto:toddcory at finestplanet.com]
Sent: Saturday, May 31, 2003 2:08 PM
To: RE-wrenches at topica.com
Subject: Re: cost per kWh [RE-wrenches]


Just curious Bill on the difference in delivered kWh was between your 2
systems.
This helps define (in real terms) what the penalty is in having battery
backup
in a grid tied system.

Todd

***snip***

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