Some help with electrical math please- UPS

Joconfire

Hi guys, I have a 3W candelabra CFL lamp. I was wondering what size UPS I would need to run this small lamp for 12 hours? How would I do that math to find out what size UPS I need? Thanks!

Missouri Bound

Got an UPS in mind? Look at the specs and it will tell you how long it will power any given wattage. Most UPS are much shorter than 12 hours. They are designed to allow time to shut down equipment before they run out of "juice"

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Joconfire

They don't list run times for such a low wattage. For example, how long would a 300W UPS run a 3W load?

I'm sure there is someone here who knows how to calculate this? I also assume due to inefficiencies that I may need a bigger UPS than advertised, no?

Missouri Bound

Look here. There is a chart which shows the run time with no load to full load. You aren't going to find any which will last 12 hours at a reasonable cost. A flashlight with a backup is a better investment.APC Back-UPS 350, 120V, without auto-shutdown software

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Joconfire

So you are saying that they have some type of timer that won't allow them to continue to run even if there is power left?

I would think that a UPS that could run a 500 watt load for 1 hour would be able to run a 3W load for at least 12 hours.

mpoulton

What are you actually trying to accomplish? Running a 3W fluorescent lamp on a UPS is probably not the right solution. As you guessed, UPS's are not designed for very small loads. It will be very inefficient and will waste most of the energy in the batteries.

Joconfire

Surely there is a way to approximate and account for any inefficiency?

joed

It all depends on the size of the battery connected. 3w @120 volts = .025amps. Since most batteries are 12 volts that's about .25 amps at the battery.
.25 amps times 12 hours = 3 amps hours. So any UPS with 6 or 7 amphour battery should run them. 7AH is a very common battery size used in UPS units and emergency lighting.

If it is critical that last this long I would suggest doing test by unplugging the UPS and seeing how long they actually last. As the battery ages the time will become less.

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Joconfire

Exactly what I was looking for, thank you sir

ddawg16

A 3w bulb is going to use a lot less engergy than the inverter circuit in the UPS.

If your looking for battery backed light, i would suggest an LED light running off a battery. You could use an AC powered relay that would enable a simple charger for the battery...when you loose AC, the relay de-energizes and turns on the LED. You will get a lot more run time that way.

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Missouri Bound

Well that is good logic and acceptable electrical math. However UPS's have internal circuitry, and very often fans which will draw more than your load, in this instance. The circuitry itself is the deciding factor in a non-load situation.

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curiousB

There is. Power up the UPS for a couple days with no load. Let it fully charge battery. Then unplug it and monitor when it goes dead due to batteries being drained. That will be how much power the supply consumes with no load. As stated by others just running the UPS DC/AC converter will consume power, I suspect significantly higher than a 3W LED bulb.

If you are looking for a 3W LED light to run off of a battery during a power outage there are better ways to handle this. Charge a sealed Lead acid battery with a charger intended for that type of battery (3-step triple charger bulk/absorption/float). Then power a 12VDC LED lamps which are becoming very common for LV yard lighting (Malibu) and LED bulb replacements for "puck" style halogen lamps. It won't be both 120 and 12 but it will run a heck of a lot longer off the battery without all the losses of a DC/AC conversion and then an AC/DC conversion inside the bulb.

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