Buying BIG Uninterruptible Power Supplies

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Last week I wrote a guide on Buying and Selling Uninterruptible Power Supplies.  That guide, at least the "Buying" part, was oriented toward those who need an "ordinary" UPS for use with their computer or other home system.  I explained that there are some fine bargains available on eBay.  I also offered some guidelines for sellers to get better prices and communicate the information that the buyers need.  If you are selling a "BIG" UPS, please review the previous guide.  If you are interested in buying a "BIG" UPS, please read on.

Why You Need a Big UPS

We are used to thinking of a UPS as a device to power our computer during brief outages.  Suddenly losing power to your computer can cause a host of problems such as loss of your current work and hard drive corruption.  We are all aware of this and running without a UPS is known to be dangerous and just plain silly.  But electrical failures and outages can cause problems for other household devices, including entertainment products, devices with motors, and even simple light bulbs, which can be damaged by surges.  Have you considered getting a really big UPS for your entire house?  It may be simpler and cheaper than you think!

I started thinking about this when I was considering the purchase of a Prius hybrid car.  Reading the specifications, I realized that it had a high power, high voltage battery.  Equally important, this battery would be charged by the car's gasoline engine whenever its voltage got  low.  The Prius battery can supply many kilowatts of peak power, and the car viewed as a system can supply a continuous power of 3kW continuously as well as much higher peaks.  This is easily enough to power a small house, and enough for critical equipment (furnace, refrigerator, well, etc.) in a larger house.  If I could somehow use this power, I would never worry about my ice cream melting again.  I started looking at the larger UPS units available on eBay and was pleased to discover that as the power level increased, so did the battery voltage.  I decided that I would be able to find a UPS that could run from the Prius when the UPS batteries were getting low, and that such a UPS would be able to run the house.  Voila!  A car and an emergency generator in one purchase.

Decide on Your Project

In decreasing level of complexity:

  • You might decide on duplicating my project with the Prius!  Hybrid cars are getting very popular and you might have one or be thinking of getting one yourself. 
    You might decide to do a smaller version of my project - if you have a gas-electric hybrid, you can put a high-power UPS in your trunk and use its outlets for emergency power 
  • You might decide that you like the protection provided by a UPS and already have a small emergency generator that you can use to keep its battery charged.
  • You might simply want to have a UPS because your power company has a number of brief dropouts and it's inconvenient to reset your collection of '50s electric clocks (bought on eBay, of course) every few days.

Once you've made your decision, you can get started.  For the first three projects, you will have to provide for recharging the UPS batteries from an external source when the AC line is down.  This means your battery voltage must be matched to your ultimate power source, be it hybrid vehicle or emergency generator source.  For the last "project," since the UPS will be doing its own charging, any battery voltage will work.

Next, you need to decide how much power to provide, and at what voltage.  At least in the USA, and in non-industrial settings, you will want either 115V or 230V.  For higher power, 3kW or greater, and for permanent installations, 230V is the way to go.

This is the UPS I picked for my project.  As is typical with larger units, it comes in two cabinets, one for the UPS, one for the batteries.  The battery cabinet on the right in the photo  holds four trays, each with 10 12-volt batteries.  The 120V trays are connected in series/parallel to provide 240VDC to run the UPS if line power fails.

Here's the panel of another 12kW UPS, a Deltec, on eBay as this was written.  Typically these large UPS units have controls and status indications, including front panel voltage and battery configuration options.

Finding the Right UPS on eBay

The first step is to search "UPS" in the Computers and Networking category.  Order your search in terms of price, most expensive first.  This tends to put the larger units on page one.  IMPORTANT-Don't restrict your search to the first page returned!  Frequently very desirable units are offered at low prices.  Doing this just now, the first unit, offered at $90,000 was a 500kVA unit, which is probably just a bit too big for your basement.  But the second half of the page sports units from $500 down to $200, including many 2kVA and 3kVA units, and one 12kW unit that I would snap up right now if I didn't already have one!  Continuing to the lower prices, I found a 6000VA unit with a starting price of $9.99 and a 3kVA unit with a starting price of a penny.  Don't assume you will have to pay big bucks at the end of the auction.  I've seen monster units go for $10.

Once you've narrowed your search to a handful of candidate units, your research begins.  For whatever reason, possibly because they haven't read my first guide, sellers can't, don't, or won't list the critical specifications of battery voltage and battery condition.  Also it might not be obvious from the listing if the UPS will provide the input and output voltages you need.

  • Battery voltage is critical.  You must try to determine the battery pack voltage and current from the manufacturer's specs, which themselves can be confusing.  The manufacturers like to sell batteries in a package, and often won't tell you the basics such as voltage and Amp-hour rating.  Sometimes you have to figure it out from the manufacturer's web site using detective work, sometimes you actually have to <shudder> call them on the telephone.  For example, if you are going to replicate my Prius project (which I call, cleverly, the PriUPS), you will want a UPS that will work on a battery voltage from about 200V to 250V.  If you have a Hybrid Highlander, the voltage will be close to 300V. 
  • Batteries are expensive and heavy.  Unless the seller has reason to believe they're good or you're picking up the UPS from a local seller, you might want to assume the batteries are bad and ask if the seller will remove them to reduce shipping charges. 
  • Many larger UPS units offer a number of different input and output configurations.  For example, if you see a 208V rating, this is  3-phase power, which is probably unsuitable for your house.  However a given UPS may be configurable to the correct configuration (230V with a neutral, so you get two 115V separate outputs), so don't disregard units that may seem to be inappropriate.  Even if you buy a UPS for almost nothing, as is likely to happen, you must do your research to confirm it's usable since shipping will be expensive regardless of the price you pay for the unit itself.

eBay Battery Hint

A lot of eBay sellers specialize in batteries alone.  If you don't have a local seller of batteries, you can get good deals on them on eBay, but of course shipping can be expensive since they're made mostly of lead.


This may be the most important hint of all.  Although I didn't get my big 12kW UPS for "almost nothing" as I might have done, it was only a few hundred dollars, it had a good battery pack, and it was local so I could drive to the seller and pick it up.  (Which I mean figuratively; they're much too heavy to pick up - bring a hand truck at least!)  If you don't see exactly what you want on eBay, just keep looking and one will show up in a day, week or month, and probably next door. 

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