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I checked the net and found that one horse power is equal to 735.5 watt. I thought there should be a time factor in this calculation.

I have a 0.5 HP motor. I would like to know if the motor runs continuously for 30 days, how much power would it consume. I was thinking HP * time = watt but several web sites all say 1 HP is equal to 735.5 watt. Then, where does the time factor come in to play? Surely if a motor runs for two hours, it'd consume twice as much power as if it runs for one hour. So, how do I factor in the time to get the watts the motor consumes?

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    look at your electric bill .... what are you paying for? – jsotola Dec 26 '19 at 1:37
  • Converting To what? Watt? – andrewmh20 Dec 26 '19 at 5:29
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    I was thinking HP * time = watt that's your trouble right there. That is false. – Harper - Reinstate Monica Dec 26 '19 at 6:10
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    0.5 HP motor for 30 days == 372.85 Watts for 30 days == ( 372.85 * 24 * 30 ) Wh == 268,452 Wh == | 268 kWh. Assuming the motor is 85% efficient then that's a consumption of about 316 kWh from your electricity supply, and assuming you're paying $0.12/kWh for electricity then running the motor for 30 days non-stop will cost you about $32.21 USD. – Dai Dec 26 '19 at 13:03
  • @dai your calculation was the most accurate of any of the answers. You should have received more up votes. – Ed Beal Feb 1 at 17:53
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I thought there should be a time factor in this calculation

Both watt and horsepower measure power. They are the same thing, just with different units, hence the direct conversion.

You seem to be confusing power with energy. Power is an instantaneous value, while energy (e.g. joules or watt-hours) is what you use if you consume power for some time.

Note that the HP rating of a motor doesn't take into account efficiency, and so tells you only how much power the motor will produce, not how much it will consume.

how do I factor in the time to get the watts the motor consumes?

You don't. Time has nothing to do with how many watts the motor consumes. Watts is a measure of how much power the motor is consuming (or producing) at any given instant in time.

If you want to know how much energy the motor consumes, first you have to figure out how much power the motor is consuming, rather than how much it is producing (and a 0.5 HP motor doesn't produce 0.5 HP at all times...that's just its nominal maximum output), and then multiply that by the number of hours it consumes that much.

Having done that, you get a figure with the units as watt-hours, which is the amount of energy you've consumed. This will translate to e.g. a cost on your electric bill, or the capacity of a battery, etc.

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  • your bill is based on kWh or thousands of watt hours part of your answer says time is not part of the calculation and part of the answer includes it it is quite simple for calculation as all other answers show 736 * .5 = watts then * hours used would be the way to calculate the cost of running a motor. You don’t bring up up power factor of efficiency at all . – Ed Beal Feb 1 at 17:34
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As another answer already explains, horsepower and watts are two units measuring the power being used (demand) at any point in time. Watt-hours - or more typically kilowatt-hours or kWh is the common measure of the usage by electric utilities.

Watts = HP * 735.5 (Actually, the formulas I have found use a slightly different number - 745.7 - but close enough.)

kWh = (watts/1000) * number-of-hours

So 0.5 HP for a month is approximately:

((0.5 * 735.5)/1000) * 24 * 30 = 265 kWh

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    What you forgot was power factor and efficiency since the size was given in HP your answer will be off but it will be in the ballpark, on my state test we were always instructed to use 736w. – Ed Beal Feb 1 at 17:50
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HP is the measure of output power. The efficiency of motors is terrible, often near 50%. For instance this 1 hp motor has an input of 6.1 amps at 230 volts, which is about 1400 watts. (Just a quick note, that's about the same amount of power as a portable heater on full heat mode.)

You are billed by kilowatt-hours, or hours of using power at a volume of 1000 watts. Break it down a little further, watts are volts x amps, so kwh is (volts x amps x hours)/1000. So for the motor above for 1 day you get 230v x 6.1A x 24hrs = 33,672 watt-hours, divide by 1000 to get kwh, 33.672 kilo-watts hours. Then look at your utility bill, in the US you will probably be paying somewhere between .08 and .18 per kwh, with average around .13, so a little more than $4 a day.

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  • This answer brings up a very important point. Make sure the motor isn't overheating. There will be significant heat coming off a motor that's consuming over 300 watts – xyious Dec 26 '19 at 16:31
  • Most motors are in the 80+ range , I support close to 20 motors over 150hp and I can put my hand on every one of them. If these guys were only 50% they would have melted down. – Ed Beal Feb 1 at 17:40
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An educated guess would be as follows. A 1 H.P. motor would produce about 746 watts of work or energy, but it would consume about 1000 watts of electricity. This is an arbitrary amount but close enough for your answer. The 1000 watts is the nominal power used when you add for the motors running efficiency. So, a 1 horse power motor running non-stop for 24 hours would consume 24,000 watts (1000 X 24hours). Since you are billed by the kilowatts of electricity used 24,000 watts divided by 1000 = 24 KW/day of electricity used. Now go to your electric bill and see what you pay per kilowatt Hour. A 1/2 HP motor would consume 1/2 the power of a 1 HP motor & etc. Some motors are more efficient than others but if you are just looking for an approximate number this will do.

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