Electricity math question

[jimc91]

I am getting confused on this easy question. I`ve learned if before but have completely forgotten.

If I have a machine that is rated 4kWh (kilowatt-hour) and my machine is running for 20 minutes and the electricity price is 25c per kWh. How much electricity does the machine use?

So I think this machine needs 4kWh of energy per hour? Multiple that by 20minutes (or 0.33) and you get:
4000W x 0.33 = 1320W x 0.25c = 330c or 3.30 euro?

 

[yoscar04]

Your calculation is partially right but wrongly written. The units do not fit.
Try to separate your computation: 1. how many KW were spent and then 2. calculate the cost.

 

[tkhunny]

4000W x 0.33 = 1320W x 0.25c

This should NOT make sense to you. That "=" MUST tell the truth.
Use complete units. Let the units help you. They will save you.

What you have above is "W" = "W€". That cannot possibly be correct.

Write the WHOLE thing - every time.
4 kW/hr * 20 min * 25 c / (kW * hr)=
[4 * 20 min * 25 c/hr] (kW/kW) =
4 * 20 min * 25 c/hr =
4 / hr * 1/3 hr * 1/4 € /hr=
[4 * 1/3 * 1/4 €] (hr/hr) =
4 * 1/3 * 1/4 € =
[1/3 €] (4 * 1/4) =
1/3 €

You are off by a factor of 10, but his is mostly luck. Various errors managed to get this close.

Read and write very carefully:
1) "...this machine needs 4kWh of energy per hour" -- NO. It needs 4kW of energy per hour, not 4kWh per hour. Bad units.
2) "...electricity price is 25c per kWh"
----- 2a) Is this correct or is it 0.25c, as in your formula? I used 25c, not 0.25c.
----- 2b) Is the price really based on kWh or just on kW? I used kW?

 

[Dr.Peterson]

I am getting confused on this easy question. I`ve learned if before but have completely forgotten.

If I have a machine that is rated 4kWh (kilowatt-hour) and my machine is running for 20 minutes and the electricity price is 25c per kWh. How much electricity does the machine use?

So I think this machine needs 4kWh of energy per hour? Multiple that by 20minutes (or 0.33) and you get:
4000W x 0.33 = 1320W x 0.25c = 330c or 3.30 euro?

I don't think a machine would be rated at 4 kWh; that's a total amount of energy used, not a rate. I'd expect it to be just 4 kW. Check the problem.

But 4 kW would be the same as 4 kWh per hour, so you're right at that point. After that, you lost the correct units again. Multiplying that by 1/3 hour gives 4/3 kWh. Converting to watts doesn't help.

You also used decimals that you rounded too much, too soon.

 

[Deleted member 4993]

I am getting confused on this easy question. I`ve learned if before but have completely forgotten.

If I have a machine that is rated 4kWh (kilowatt-hour) and my machine is running for 20 minutes and the electricity price is 25c per kWh. How much electricity does the machine use?

So I think this machine needs 4kWh of energy per hour? Multiple that by 20minutes (or 0.33) and you get:
4000W x 0.33 = 1320W x 0.25c = 330c or 3.30 euro?

Is the question:

How much electricity (energy) does the machine use? .................................... or

How much money is spent for running the machine?