Nighttime losses: was Xantrex versus Sunny Boy [RE-wrenches]

Bill Brooks billb at endecon.com
Fri Aug 1 17:01:33 PDT 2003


Jeff,

It is best to take the load off the transformer at night, but that does add
cost (a big contactor) and some controls. Since there are many building
electrical configurations, Trace Tech decided not to go that direction. Kent
could probably give a much better history of that decision.

The question is does it cause load on the utility (not that we care). The
utility has literally hundreds of the thousands of transformers (PG&E alone)
running all the time. The real power load of transformers is fairly low even
though the volt-amps might be much higher (no load power factor can be 0.1
to 0.2) Power factor equals Watts/Volt-Amps. The Watts being consumed is the
heating in the windings. Amps are sloshing back and forth energizing the
transformer and inducing a voltage on the secondary side.

If the no load loss on a 30kVA isolation transformer is between 100 and 200
Watts of real power. Over 12 hours, that would consume 1.2 to 2.4 kWh of
energy (not insignificant). If the PV system produces 120 kWh on an average
day, that is a 1% to 2% loss (not 6%). If the system costs $250,000 and
loses and avoidable 1% of energy, the designer should allow an additional
cost of up to $2,500 (1% of system costs) for the addition of a contactor to
eliminate this loss.

I have measured 50 Watts of nightime power consumption on the Trace SW (with
batteries) and the Vanner RE4500 (without batteries). These are much larger
losses relative to the energy production.

All-in-all this is a very important issue and one that designers and
installers too often overlook.

Bill.


-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com]
Sent: Friday, August 01, 2003 4:14 PM
To: RE-wrenches at topica.com
Subject: RE: Xantrex versus Sunny Boy [RE-wrenches]



Bill,
Right you are, this was a basic V/A measurement at night and you're also
correct that most of the time this gets under the utility billing radar. I
now have a Davidge Controls kWh meter on it and I don't know if it will pick
it up, but I think so. Now, if I can just get the owner to LOOK at it for me
(I'm 3 hrs. away). Billed or not it is still a grid load is it not? Not
considering these losses is a NIMBY attitude. I think the best design takes
the maximum load off of the grid with the PV watts you have to work with.

We all await your test w/eagerness!

-jeff o

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