How do you figure out how many watts an appliance uses?

Question

I had to buy a backup power source for my Mom's life support equipment and it's small and has a 500 watts capacity. I saw on the chart at Home Depot that a 13-inch TV uses less than 100 watts. I don't think any of her equipment would use more watts than a small TV does.

There are four items that are essential so I need to add up their total in watts used and hope it's not more than 500.

But none of the devices lists watts. NONE.

Here's a typical list of details (this is for the little motor/air pump that's constantly on, keeping her air mattress inflated):

120V/60HZ, 1A

Is watts the voltage divided by the hertz or what?

Answer 1

Volts X Amps = Watts.

Answer 2

Watts = Volts x Amps

If you have an appliance running on 120 volts at 1Amp, it's using 120 watts.

If your 13" TV is rated at 3 amps, it would be using 3*120 or 360 watts.

Any backup power supply you buy should be overrated for the job you for it to do. You should nor run 500watts of appliances from a  500watt backup. You would be taxing it at it's limit. It is generally advised to use double what your requirement is. If the device you need to run uses 500 watts, use a 1000 watt supply.

Also take into consideration that if this is truly a backup that is used rarely, that it is close in ratings, that you can shut off anything else being powered from it. That is, turnoff the bed pump (maybe just intermittently) before running the unit at capacity and possible having it go out and then not being able to power a say, dialysis machine.

If the device you're powering has a motor, allow about 20% extra as large motors have a high inrush load when they're first turned on.

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Your 50mA device uses 50 milli amps, or .050 or 50/1000 Amps. At 120 volts it uses about 6 watts.

.

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Answer 3

OK.

Volts x Amp.s = Watts.

1 milliamp is 1/1000 of an Amp. so 50 mA. @120V. = 0.6W

That fan 1 A. @ 120V. = 120W.

All of the equipment will have a UL nameplate somewhere on it with load information.

The problem is that if you're buying a battery back-up power source, it will only last as long as the battery power.

You'll have to read the spec.s to find out how long the battery will last.

That will be in Watt/Hours.

The load You've given so far (120.6W) will drain a 500 W./Hr. battery

in about 4 hours.

A 1000W. Generator set is available from 'Heartland America' ,

a discount house for about $150. It might be a better solution than a battery unit.

Sorry to hear about your situation.

I had to take care of an ailing parent myself.

Hang in there.

Answer 4

Watts = voltage x amps.  So a 1 amp device @ 120 volts = 120 watts.

*mA means milliamp, or 1/1000th of an amp.

Answer 5

To get watts, multiply the amps times the volts.  

500 watts is 120x 4 (480) close enough.  The pump for the air mattress is not vital, move it. If it's a cpap, it's vital.  No lights, just life support in the backup PS.  You may need a second unit, if this is a battery type power supply with an inverter, it is rated for peak power at 500 watts. As the battery puts out power, it goes dead and the power drops as it dies.  To do a proper analysis of the project, you should call a qualified professional electrician. Since this is life support equipment, certain medical plans may cover the emergency power supplies.  If you use a generator ( Highly recommended) you need to notify the local power company and the hospital in case of long term power outage.  A small portable generator with a buss transfer switch dedicated to the life support use would be your best solution.  

Answer 6

Those answers all look good, but I have to kick in a bit of AC mathematics.  For plain DC, watts does indeed equal volts times amps.  For AC, you also have to multiply by something called the 'power factor', which typically ranges somewhere between 0.5 and 1.0.

Or, W = V x A x PF.

For a purely resistive load, the power factor would be 1.  For a highly inductive load, such as a lightly-loaded motor, it might be 0.6 or so.

All that being said though, many times a generator or backup power source is rated in volt-amps, because regardless of how many watts a device draws (which directly affects what you would be paying for the power, since your electric meter reads watt-hours), it needs the capacity to supply the max amps (or volt-amps).

I just wanted to straighten out the mathematics.  To get more generally to your questions, get yourself a neat gadget marketed as Kill-A-Watt, a plug-in device that will read out volts, amps, VA, watts, power factor, and calculate KW over time.  Pick one up of your very own at a place like newegg.com for some $20.  (Plus the darned shipping, but whattya gonna do?)