Re: (ET) Power Inverters

["Russ O'Rourke" <mailing_lists russorourke com>]

Bill,

11 Amps sounds a little high.  Is that the maximum start-up current or
truly the running power?

I've been working on a similar backup power scheme as well and from what
I can see overcoming the startup current surge requirements are what
make the job difficult.  Or at least difficult if you don't want to
spend a lot for something you may not get much use from.

As mentioned by others, 36VDC inverters are not very common.  In the end
I decided to go with a 12VDC inverter as it is the most common and most
likely available energy source in an emergency.

I tried an inexpensive 1800 watt inverter and discovered it wouldn't
start our furnace (oil fired, hot water baseboard) or freezer or
microwave.  It worked OK with incandescent light bulbs, however, all the
CF bulb equipped lights had trouble (flickering, mostly) and the
computer/TV UPS's never would come back online.

It was a disappointing experiment, but thought maybe my bad experience
was just a combination of poor quality inverter and/or sensitive loads.

Next, I tried a high quality 2800 watt sine wave UPS I had stumbled on
at a local surplus auction (its only problem was dead internal
batteries).  It was 36 VDC powered and even had an external battery
connection!  Much better - it powered up all the CF bulbs, TV and even
other UPS's just fine.  However, it had a "safety" feature that would
immediately shut the inverter off if presented with a large startup load
such as a microwave or the motors in the freezer compressor and furnace
oil burner.  Again, very disappointing.  

After many months of searching for a solution, I finally came across an
inexpensive 2400 watt, 12 VDC powered, industrial sine-wave inverter on
eBay.  The inverter was used, but not in bad shape - large and heavy
though!  Probably from all those huge caps and beefy transformer inside.
On this attempt, I was happy to finally have success!  The lights all
work without any flicker, the TV and computers are happy and most
importantly we can start and run the furnace, freezer and microwave
(start any two of the three simultaneously, at least).  An important
lesson learned here is that the rating of this inverter is an actual
continuous wattage.  It can go much higher (3-4 kW) for short periods of
time and as the manual says - will even attempt to power a dead short
load for several seconds before the controls will shut it down.

Along the way I've also been doing some power measurements to help
answer the question of how much power would we need during an outage in
the winter and what the levels look like depending on what is going on.
For us, it's a steady 400 to 600 watts with normal loads but it does
change dramatically when the furnace, freezer, fridge or microwave kicks
in, usually 1-2 kW extra load for anywhere from 5 to 10 minutes.  In any
given hour, this usually happens at least once.
  
As far as "real world" results go - on our EGT-120 we can get about 2 to
3 hours on each 12 volt "bank" in the 36 volt pack.  Did this test
recently in 20-30 F degree weather late afternoon/night time conditions,
so it's probably a fair test even if the sample size is small.  The
house load was at the same "normal" power levels as mentioned above:
furnace running as needed, lights, TV, freezer, fridge, microwave cooked
dinner.

The cable from cell to cell on the ET is too small for the currents
drawn at 12 VDC and it would probably improve the run time somewhat if
it was changed to something better.

Hope my ramblings prove useful...


-Russ