Understanding Power and Energy Calculations for Electrical Output

[dvn]

Hello,

I have been researching on electrical output and it power and working out how much power I use at home and have been doing a project to make power.

I wanted to make sure if this calculation is correct...

I use a 12V DC motor with an output of 70mA which creates 0.84 Watt according to PIE method. 12 X 0.07 = 0.84W

So I use 20 x dc motors and combined of 20 motors will make 16.8 Watt?
= 20 X 0.84 = 16.8W

If I get the motors running for an hour then it will create 1.008 kWh??
= 16.8 x 60 = 1008W

Please correct me if I am wrong.

Thank you

 

[MATLABdude]

Everything's correct, except for the last line. You'd use up 1008 J, but only 16.8 Watt*hour. Why? You're multiplying 16.8 watts by, well, 1 hour!

It won't actually produce 16.8 Watt*hour of energy, since a portion of the input power goes to heat.

 

[dvn]

I multipled 60 because of 60 minutes in an hour. So you are saying that the energy 16.8W is applied as an hour?

 

[mikelepore]

One watt is one joule per second.

 

[dvn]

So if I get the dc motors running for 2 hours then it will produce 33.6W?

= 16.8 x 2 = 33.6W

So how do I work out the watt for minutes and seconds?

 

[russ_watters]

I multipled 60 because of 60 minutes in an hour. So you are saying that the energy 16.8W is applied as an hour?

A watt-hour is a watt for an hour, so 16.8 watts for an hour is 16.8 watt-hours.

So if I get the dc motors running for 2 hours then it will produce 33.6W?

= 16.8 x 2 = 33.6W

No, 16.8 Watts x 2 hours = 33.6 Watt-hours. Make sure you follow through with your units!

So how do I work out the watt for minutes and seconds?

Watts are a unit of power - they are already in terms of seconds (as already stated: joules per second). Energy is power times time. Watt-hours.

 

[dvn]

If I want to measure the power output for 1 minute.

Do I divide 16.8 by 60 = 0.28 joules?

 

[russ_watters]

No, watts is power: 16.8 watts. It doesn't matter if you measure it for a second, an hour, or a day, 16.8 watts of power is 16.8 watts of power. And a watt is a joule per second, so you can express it as 16.8 joules per second.

If you take a book that weighs 1 N and pick it up off the floor and put it on a 1m table, that's 1 N-m -- 1 Joule of energy. If you do it in 1 second, that's still 1 joule of energy, but expended at a rate of 1 joule per second or 1 watt.

Take your car: if your car is moving at 60 miles per hour, does measuring it for 2 hours mean you were driving at 120 miles per hour? No, it is still 60 miles per hour.

 

[dvn]

Now I understand what you mean. Sorry I was thinking about recharging theory.

Yes you are right about the constant energy output that the dc motor make. What I have forgotten to tell is that if I recharge the battery or secondary battery or so, so if I use 16.8W and recharge the battery in 2 hours then it will recharge as 33.6w or something?

 

[russ_watters]

No, still 16.8W. Watts is power. Power is energy per unit time. If you want to know how much energy it takes, that's watt-hours.

This could all be cleared-up if you write down your math and include the units when doing the math. You'll see that multiplying a watt by an hour can't yield a watt, it must yield a watt-hour!