I'm avoiding the Harbor Freight (Horror Fright) units until I hear
that they aren't fire hazards.
That's what I do, using rug-safe jumpstarters and computer UPSs I can
leave at the points of use to avoid extension cords across the unlit
floor. They are also easier and safer than a 60 Lb truck battery to
carry out back to recharge, and are more useful the rest of the time.
This laptop draws 20-35W and my 22" HDTV+monitor takes <30W with the
backlight off. The bigger faster desktop draws around 180W so it stays
off. I take many other measures to keep useage down to 4-5 KWH / day,
those are the easy ones.
The Tripplite UPS recharges in ~2 hours but the others can take all
These show you the demand of appliances and have a memory that saves
the accumulated KWH when a battery inverter drops out, a good measure
of its remaining life:
(Amazon.com product link shortened)
Unfortunately, (and contrary to what you may expect) UPS boxes normally don't
make good sources of emergency power. They are made for bridging small power
interruptions, not running equipment for hours off-grid. You will find that
they have small batteries. If you try to connect a larger outboard battery it
may work, or you may discover other problems such as insufficient heat sink or
low efficiency. Also, they have small chargers so take a long time to recharge.
On Wed, 29 Dec 2010 10:42:21 -0500, "vaughn"
While what you say is undoubtedly true for a UPS power strip that you
might pick up at Wally world, it is certainly generally not true for
UPSs large enough to use for standby power. That is, 1kW and above.
And even though I say that I've been quite surprised at some of the
cheap UPSs once I got inside 'em. Perfectly adequate thermal and
electrical design for continuous duty.
Instead of such a broad generalization, perhaps what we ought to do is
list some features that a good UPS should have.
*) Adequate size. Blivet packing just doesn't work with power
electronics. Has to be room for air flow and heat sinks. My 2 kW
unit is about the size of 2 microwave ovens sitting side by side.
*) Fan cooled.
*) Moderate battery voltage. 24 to 48 volts DC. the trend for short
term (10 minutes or so) large UPSs is 96 to 144 volts' worth of small
AGM batteries. Not very practical unless you have an unlimited budget
*) weight. 30-50 lbs per kW. Iron, copper and heat sinks are heavy
so a lightweight UPS isn't going to contain very much of any of those
*) Connector for external batteries. I would not exclude a unit that
met all my other requirements but didn't have an external battery
connection but if that connector is there then you know for sure that
it can handle continuous duty.
*) ability to disable the internal charger. This is usually a labeled
jumper on the main circuit board. the reason is that built in
chargers are typically simple one stage float chargers that do not
baby the batteries. It's better to disable the internal charger and
use a smart external charger. I have an 80 amp 24 volt multi-stage
smart charger from Iota Engineering that treats my batteries with kid
*) while not absolutely essential, it is important in my book to have
an adjustable DC undervoltage trip. Just about every UPS that I've
ever worked on take the batteries down to 10.0 volts per 12 volt
battery or even lower before the unit trips. This is done for obvious
reasons - to get the most time out of a given battery pack size. It
also kills the batteries.
I have my undervotage trip set to 11.75 volts per unit. Allowing for
a little voltage drop through the connecting cables, that trips when
the batteries are just under 12 volts per unit. I have a 600 amp-hour
24 volt battery pack. Just thinking about having to replace those
give me heart flutters. Compared to killing that string, I could care
less whether I get that last 30 minutes out of the string. If the
unit trips on UV, all I do is start my generator.
Note: my trip voltage is specific to my setup and was selected after
consultation with a battery factory engineer. Whatever type of
batteries you get, I highly recommend having that chat if their
technical literature doesn't specify a minimum discharge voltage.
The only thing that I do NOT like about a UPS for standby power is
that most (mine included) require AC power to be present to
initialize. That is, you can't be in a blackout, turn the unit on and
expect it to come up. If the unit is plugged in and turned on all the
time, as it will be if your vital loads are powered through it, then
this isn't a huge issue.
In making my "broad generalization", I was thinking mostly of consumer grade
boxes, which don't generally comply to John's list above. To John's advice I
would add; avoid ferro-resonant UPSs, which tend to have fairly high core losses
and can be intolerant of the frequency excursions of all but the best
generators. Over the years, I have junked several of those beasts, even though
I could easily have dragged them home for free..
Good to have Neon John showing up here again.