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I have read this page: How to convert amps to kilowatts

I often purchase electricity from a supplier to power computing equipment within a data centre. Some sell me 10 amps, some sell me 2.16 kWh.

When they say 10A, they essentially mean I can draw up to 10A of power (at a usual 240v, single phase AC), continuously, each month (as I am billed monthly). If I purchase 2.16kWh, that means constantly, for a whole month (again it comes from 240v single phase AC feed).

So even though that article linked above states you can not compare amps [*1] and kWh directly, as long as I am using them in the same frame of reference (i.e. the single phase, 240v), can I directly compare them when making my supplier price comparisons for those selling to me as 10A and those selling to me 2.16kWh (assuming they are both providing 240v single AC feeds [*2]) ?

[*1] I understand that amps are an instantaneous snap shot of current, so in this scenario I am really talking about continuous amp hours

[*2] All throughout I have assume a generic power factor of 0.9, just in case you wondered about my dodgy maths :)

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2 Answers 2

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"10 amps" and "2.16 kilowatt-hours" are not the same units and can't be directly converted. An amp is current, a kWh is energy.

However, "10 amps at 240 volts for 1 month" is energy. Google will happily convert the units, if you ask it "(10 amps * 240 volts * 1 month) in kwh": 1,753 kWh.

Coming from the other direction, since kWh is energy, it doesn't make sense to talk about using 2.16 kWh "constantly, for a whole month". 2.16 kWh represents the total energy usage, not a rate of power consumption. You could use 2.16 kWh up in a minute or in a month or a year.

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  • \$\begingroup\$ Yes thats what I am long-windedly saying :) I am correct in comparing my two values above if I give some scope to the amp's value? i.e. the voltage and duration? \$\endgroup\$
    – Baldrick
    Commented Sep 13, 2012 at 20:04
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    \$\begingroup\$ Yes exactly. Use the voltage to convert the amps into power (kW), and then use the duration to convert power into total energy (kWh). (Note that power factor complicates things just a tiny bit; it comes into play when converting current to power, as described in the first link you shared. For typical power factors it's not going to be a big difference if you're just trying to roughly compare two service plans) \$\endgroup\$
    – Jim Paris
    Commented Sep 13, 2012 at 20:09
  • \$\begingroup\$ Ah I see, and your addition was a great point about the reverse coming from kWh, thanks very much, very clear :) \$\endgroup\$
    – Baldrick
    Commented Sep 13, 2012 at 20:13
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    \$\begingroup\$ isn't the kWh going to be much more effective in the long term, since 10A is going to be the maximum instantaneous usage, while the kWh is going to be total energy usage over the month, and computer power usage can easily double (or halve) based on computer load. For instance, during slow times they will power down cores, and hard drives take more power to seek, etc. \$\endgroup\$
    – FryGuy
    Commented Sep 14, 2012 at 0:12
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Actually, it isn't quite that straightforward. The guy who's selling you kWh is charging you only for the real energy consumed, and he's assuming you have a decent power factor (greater than 0.9, as you say).

On the other hand, the guy who's selling you Amps is charging you for both your real power and your reactive power, regardless of the power factor.

So in effect, if your power factor really is 0.9, the kWh guy is giving you a bonus of 10% more Amps, or you can think of the Amps guy as penalizing you 10% of your kWh.

The prices compare directly only if the power factor is 1.00.

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    \$\begingroup\$ You may want to make that kWh/h or kW and the comparison is fair. (I know you know that but it may not come across clearly.) ie 230 VAC x 10A = 2.3 kW. 2.16/2.3 = 0.94 The 10A seller is selling VARs in the hope that your power factor is <= 0.94*, and the 2.16 kW man is selling kW (assuming he is buying Watts and not VARs :-) ). If they buy in VARs then the gamble goes the other way. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 14, 2012 at 1:36
  • \$\begingroup\$ @RussellMcMahon: I figured that is was clear from the original question, the other answer and all the comments that everyone understood how the units work out; I just wanted to make the point about what might actually be getting measured. Although I'm really betting that both suppliers are actually reading the same (power) meter, and it really is just a matter of terminology, in which case, the point is moot. \$\endgroup\$
    – Dave Tweed
    Commented Sep 14, 2012 at 1:45
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    \$\begingroup\$ Dave - We are looking in from opposite sides of the same glass :-). I decided that the original questioner would probably not have a good kW vs kWh understanding after reading the answers. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 14, 2012 at 1:54

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