Is my calculation correct?

1. Oct 5, 2009

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

2. Oct 5, 2009

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.

3. Oct 5, 2009

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?

4. Oct 5, 2009

mikelepore

One watt is one joule per second.

5. Oct 5, 2009

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?

6. Oct 5, 2009

Staff: Mentor

A watt-hour is a watt for an hour, so 16.8 watts for an hour is 16.8 watt-hours.
No, 16.8 Watts x 2 hours = 33.6 Watt-hours. Make sure you follow through with your units!
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.

7. Oct 5, 2009

dvn

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

Do I divide 16.8 by 60 = 0.28 joules?

8. Oct 5, 2009

Staff: Mentor

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.

9. Oct 5, 2009

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?

10. Oct 5, 2009

Staff: Mentor

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!