Why is electricity consumption billed in watts and not amperes?

Question

Basically the subject. Why would companies bill for wattage instead of amperes?

@SolarMike Are you sure? IIRC, here home lines are billed per kWh and larger companies are billed per kWh with restrictions on their power factor. Better power factor = better price, so indirectly billed on kVARh but not exactly. — Mast, Apr 06 '20 at 08:10
@Mast and the rates are variable below X is one rate, above another and then those rates can change 2 or 3 times per day - bills can be very complicated... — Solar Mike, Apr 06 '20 at 08:12
@SolarMike Power companies bill each other every 15 minutes. Power can be free on the middle of the day (sunny day, much solar power) and bloody expensive during/after dinner. But I think that's beyond the scope of the question. — Mast, Apr 06 '20 at 08:16
@Mast don't care what power companies do to each other, but having been involved in a wind turbine and paying / receiving payment for production then one finds out how much detail there is. — Solar Mike, Apr 06 '20 at 08:22
A lot of small power networks actually DO bill their customers per amperes. It happens in places where law and order are not strong and a sustainable measurement and billing infrastructure is an extra hassle. You just pay a fixed amount to be connected to a 6A or 16A or 25A breaker for a week and take care not to trip it by overload. — fraxinus, Apr 06 '20 at 10:58
Caravan sites and marinas are a common example of fixed-supply billing - in the UK stringent regulations and type-approval apply to meters used for energy resale, and it's a pain to have to read every meter every day/week, so a fixed charge is used. — Owain, Apr 06 '20 at 12:10
There's a distinction for paying for a hookup of a stated capacity, and paying for a metered commodity. With the case of an RV ('caravan' to you Brits), park, they are charging for the use of the hookup, but it's not metered. That's different than charging by the ampere-hour. — hacktastical, Apr 07 '20 at 01:05
You are aware that it is the same thing for customers with a constant voltage?? (If we ignore the hours in the kWh for a second...) — Peter - Reinstate Monica, Apr 07 '20 at 05:08
@Peter-ReinstateMonica The voltage is (slightly but measurably) higher if you're closer to the source, so there might be a difference in which house pays what depending on the type (Ah vs kWh) meter used. — Mast, Apr 07 '20 at 11:40
I rarely downvote, but -1 for a very poorly worded question that makes it sound like you don't know the difference between amps and watts, **and made zero effort to find out**. — Harper - Reinstate Monica, Apr 07 '20 at 11:46
Compare for cars: "why do you buy fuel by the gallon/litre rather than distance?" — Viktor Mellgren, Apr 07 '20 at 13:34
@SolarMike: It depends on where you are and what contract you got, like here in germany private persons are always billed in kWh and companies depending on the power delivery capabilites in kVAh, data center customers usually in kVAh too etc. etc. etc. — PlasmaHH, Apr 07 '20 at 14:38
@PlasmaHH exactly the point I was making, especially as the OP does not specify, home, business or industrial etc — Solar Mike, Apr 07 '20 at 14:40
@SolarMike ah, sounded like you wanted to say "no its not W its VA" ...in some countries even private persons are billed in VA and they don't even know its possible to bill based on watts... — PlasmaHH, Apr 07 '20 at 14:44
@ViktorMellgren If all cars had the same mpg (the way all households have the same voltage) the two would be equivalent. For example we frequently express weight in kg on Earth, or astronomical distance in years (with c as the underlying constant speed). — Peter - Reinstate Monica, Apr 08 '20 at 07:06
They don't. They bill in Watt.hours (or 1000's of Wh = kWh). They may also bill in V.A.h which are the same as Wh when power factor =1. As power factor drops 9current & voltage increasingly out of phase) the user gets increasingly-less Watts for the same VA. BUT the current causes losses in transmission lines and transformers. || Wh = energy. || W = power = rate of using energy. || A = current flow = proportional to W ONLY at fixed V and fixed power factor, which we have not got. Wh = energy. — Russell McMahon, Apr 09 '20 at 00:24

hacktastical · Answer 1 · 2020-04-06T08:00:48.840

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They don’t bill in watts (power) either. They bill in watt-hours, that is, energy consumed. (kilowatt-hours typically.)

Let’s break this down a bit. Current alone doesn’t tell you power. You also need to know the voltage, as power is voltage * current. Then, you tally power over time to figure energy.

Now you could estimate power from current if you make an assumption about the voltage, and in the early days of electricity they did exactly that using ‘coulomb counters’, that is, they measured and tallied the current delivered over time and billed based on the tally.

This current-only method proved to be inaccurate because of line fluctuations, so for this and other reasons (notably, the adoption of AC power) they developed the motor-type meter, and later, the more familiar spinning-disc induction-type watthour meter. These meters also take voltage into account by design.

More about meter development here: https://www.smart-energy.com/features-analysis/the-history-of-the-electricity-meter/

And because it matters sometimes, a discussion of real vs apparent power and how utilities deal with it: https://www.electronicdesign.com/power-management/article/21806945/how-does-power-factor-correction-impact-your-utility-bill

edited Apr 06 '20 at 08:00

answered Apr 05 '20 at 17:57

hacktastical

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P = V x I if they are directly in phase. Where is cosine? – relayman357 Apr 05 '20 at 18:14
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More about apparent power. Utilities will add a power-factor penalty for users that have low power factors as their reactive loads still tax the grid despite not dissipating real power. This doesn’t apply to household users who don’t tend to have large reactive loads. – hacktastical Apr 05 '20 at 18:58
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That all said, if you marked me down because of this phase angle consideration, shame on you. It’s beyond the scope of the OPs question, and it doesn’t even matter as typical meters measure real power anyway. – hacktastical Apr 05 '20 at 19:07
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@relayman357 - if you integrate the instantaneous product of current and voltage the phase is automatically accounted for as well as any distortion in the signal. – Kevin White Apr 05 '20 at 19:44
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Hi Kevin, agree. Not just V x I as typical meter readings (which are typical RMS volts and amps). V times I at each point in time from waveform sample data results in power waveform. If phase angle between V and I is 90 (pure sinusoids) then P = 0. – relayman357 Apr 05 '20 at 19:50
In the UK the mains voltage seen by an appliance can be anywhere in the range 216.2 volts to 253.0 volts. In my house the voltage is 252V so I would be very happy if they just billed me by the Amp Hour - it would pay for all of the light bulbs that burn out after a couple of months use. The electricity supplier refused to reduce the voltage after they had logged it for a month as, while high, it was insde the legal limits. – uɐɪ Apr 06 '20 at 10:48
This also means that if the electricity company is unable to deliver the rated voltage, you pay less. – grahamj42 Apr 07 '20 at 12:23

Andy aka · Accepted Answer · 2020-04-07T18:16:09.367

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Why would companies bill for wattage instead of amperes?

Because amperes don't tell the full story about energy transfer from a source to a load. If you supplied a load that took 100 amperes at 1 volt, the power consumption (joules of energy per second) is 100 watts. If a different load took 100 amperes at 100 volts, the energy transfer per second is 10,000 watts.

If you only billed in amperes you bill both customers the same.

It's all about energy and power so, you calculate power delivered to the load and that accumulation of power with time (energy) is what you are billed on. To calculate power it is amperes x volts.

edited Apr 07 '20 at 18:16

answered Apr 05 '20 at 18:00

Andy aka

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Do you know any power company that delivers power at 1 V? – The Photon Apr 05 '20 at 18:11
Power factor anyone? Where is cosine? – relayman357 Apr 05 '20 at 18:12
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USA 120V, most Europe and India 220V... so there are variations @ThePhoton – across Apr 05 '20 at 18:19
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France : before ~1974 it was 110V, and after 220V – andre314 Apr 05 '20 at 18:21
@beccaboo, and in the US (and probably elsewhere) an industrial customer might get 440 V, 10 kV, or whatever suits their needs. But nobody nowhere is delivering power at 1 V, as mentioned in the example. – The Photon Apr 05 '20 at 18:30
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Who asked the question has a very little amount of knowledge on electricity. So, if you guys tell him about power factor, higher charge for industrial load, etc then he might not understand anything at all. And 1 volt is just an example. Using 1 and 100 makes a statement easy to understand. – Sadat Rafi Apr 05 '20 at 18:49
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.... There's not a wattmeter in the world that would calculate power by taking the RMS of the voltage and multiplying it by the RMS of the current then calculating what PF is and doing a final multiplication @relayman357. To do so would be plain stupid and so much in error it would be laughed at. In my final line in my answer I said: *To calculate power it is amps x volts.* and that is the only way any wattmeter will calculate power: instantaneous voltage multiplied by instantaneous current. If I meant RMS I would have said Volts x Amps where the capitalization implies RMS (but I didn't). – Andy aka Apr 05 '20 at 19:40
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Hi Andy - actually they don’t do it like that (sample x sample). They calculate phasors and do the math and accumulate. For SEL 735 they use cosine filters to compute the fundamental frequency phasors. – relayman357 Apr 05 '20 at 20:04
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@relayman357 well I have designed a couple of utility power meters in the past and sample x sample was the method. Maybe you can link to one that calculates as you say - I'd be very interested to see how they cope with current harmonics accurately. Obviously, a technical link is important rather than some brochure. – Andy aka Apr 05 '20 at 20:25
SEL 735 document: *The SEL-735 accurately reports bi-directional energy even in the presence of harmonics and distorted waveforms. When tested with peaked waveform distortion, the SEL-735 reports with an error of just 0.006 percent.* - that appears to contradict what you say. If the voltage is distorted you have to do v x i. because calculating phasors and doing math for each set of phasors at each harmonic appears to be plain silly. – Andy aka Apr 05 '20 at 20:29
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Hi Andy - it doesn't calculate P at each harmonic, only the fundamental. It calculates the P in the presence of distortion. The basic 735 version can report harmonics up 15th (by phasor calculation of course), the Intermediate and Advanced versions up to 63rd. The relay has a front end anti-aliasing filter which is why it can accurately (your 0.006%) measure the fundamental frequency power in the presence of high frequency distortion. What utility power meter did you design? I'd like to read about it. – relayman357 Apr 05 '20 at 20:48
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Most people neither know about fluctuations. So, since each country/region has the same fixed voltage (in their eyes), why do they not bill amperage, when the voltage is the same for everyone (in said country/region)? That's not addressed by this answer. – Huisman Apr 05 '20 at 20:53
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Of course it is. – Andy aka Apr 05 '20 at 23:24
And so if they billed for amperes, some clever customers would surely game the system with the use of a cleverly arranged system of transformers. – vsz Apr 06 '20 at 11:16
So the historical reasons might have been that it costs in terms of infrastructure to move voltage up and down and the consumer must pay for it? – A. Munir Apr 07 '20 at 09:54
The historical reasons are more tied in to the personalities and politics of the main players back in the day. – Andy aka Apr 07 '20 at 09:57
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Actually companies does not charge by the watt (that is a power unit) but by the Wh (that is an energy). 1W = 1J/s, 1Wh = 3600J. – DDS Apr 07 '20 at 14:58
Yeah I think it's important to clarify what @DDS pointed out. Your last paragraph could be clearer. It's all about energy and power, and since they measure in kWh, they are actually charging for energy, not power. – JMac Apr 07 '20 at 17:49
I’ve made an amendment to make it clear that it is the accumulation of power with time that is the basis for billing, but, the crux of this question is not specifically about that and insisting that I spell out what you might be billed on is trying to make this answer suit a different unspecified question. – Andy aka Apr 07 '20 at 18:19
@Andyaka The crux of the question is based on a misconception (that you are billed by watts, and not watt-hours). I would say it is _very_ important to make the distinction clear; or else you've actually managed to mislead the OP more, by making them think that they were correct in saying we are billed by watts. – JMac Apr 07 '20 at 19:06
@andre314: it is 230V since 1986 in France. – WoJ Apr 08 '20 at 11:03
@ThePhoton I don't think the answer was suggesting power is supplied at 1V. It was an example to illustrate the point. – JBentley Apr 08 '20 at 22:21
Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/108839/discussion-on-answer-by-andy-aka-why-is-electricity-consumption-billed-in-watts). – Voltage Spike Jun 03 '20 at 15:03

score 11 · Answer 3 · answered Apr 05 '20 at 18:00

I think because they care about energy consumption, energy is what costs money and resources to generate, also you can have different voltages when using 2 and 3 phases which would make the charge measurement by itself useless

-- edit, and yes, they measure in watt*hours for obvious reasons

Solomon Slow · Answer 4 · 2020-04-07T01:32:25.313

One reason why they bill Watt-hours is because that's what is directly measured by the classic, electro-mechanical service meters that were used all over the world for more than a century. Many of those meters still are in service today.

Modern electronic meters measure voltage and current separately. They could be programmed to total up the Amp-hours, but it makes sense for them to calculate Watt-hours, for compatibility with the older meters.

skvery · Answer 5 · 2020-04-06T23:25:30.313

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Most utilities both bill energy [kWh] and demand [kVA] as well as a fixed monthly charge for large customers. There are various other costs that can be billed.

The demand component is a way to bill the current, \$ [\textrm{A}] = \frac{[\textrm{kVA}]}{[\textrm{V}]}\$ because the voltage is mostly constant.

The current is integrated over the integration period to get an average.

edited Apr 06 '20 at 23:25

answered Apr 06 '20 at 23:19

skvery

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score 2 · Answer 6 · answered Apr 08 '20 at 08:12

Companies bill for the Watt.hours (Wh) product because that's the thing that makes up the irreducible cost of supplying the energy. If you use more Wh, then the supply company has to burn more fuel. There's a direct proportional relationship between Wh supplied and kg of fuel burned.

Within that Wh product, or Volt.Amps.hours product, it's possible to make any of those terms big or small without impacting how much fuel is used. You can trade off Volts against Amps with a transformer. You can trade off power against time with storage.

score 1 · Answer 7 · answered Apr 08 '20 at 10:22

kWh provides part of the basis for a method of billing which reasonably reflects the actual cost to many utilities and is reasonably fair to the customer in a majority of situations. There are, however, situations in some parts of the world where the current capacity of the system is limited and the grid connection limited or non-existent and there is only limited capacity to modulate the delivery of power -- e.g. hydro-electric schemes in remote areas with limited or non-existent grid connection. In that sort of situation even domestic premises are billed according to maximum current draw and there's a large ammeter on the kitchen wall with a red line indicating the maximum permitted. Go over that and the financial penalties are severe.

score 0 · Answer 8 · answered Apr 07 '20 at 15:03

Actually companies charge by the Wh (Energy), not by the W ([instantaneous] Power).

1W measures the "rate" at wich you use power. 1kW heater uses 1000J/s (1000J every second)

1Wh measures the energy. 1kW electric heater running for one hour uses 1kWh = 3,6MJ Energy.

A 100W light bulb would use the same kWh [3,6MJ] in 10 hours.

score 0 · Answer 9 · answered Apr 07 '20 at 16:11

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Many devices spend part of each cycle taking more energy from the utility than they're going to use, but then give the unused portion of the energy back during another part of the same cycle. If a meter were to merely measure RMS current rather than power, it would not only fail to credit devices for the energy that's given back to the grid, but it would actually bill users for that energy as though they were consuming it.

Note that utilities do often bill big customers for current as well as for power, because the utility will lose a fraction of the power that flows through its wires. A utility might have to generate 1,010 joules in order for 1,000 to reach the customer. The fact that there are 10 joules the customer isn't directly paying for wouldn't be a problem if the customer would pay for 1,000, but if the customer gives back 990 joules, and 980 reach the utility, the utility would end up with 30 less joules of energy than it started with, while only getting paid for 10. For small residential customers, this isn't a big issue, but for large industrial customers it can be huge.

answered Apr 07 '20 at 16:11

supercat

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There is no difference in measuring negative current or negative power. Both would be possible, and since mains is pretty stable it is just a linear factor difference. At least as long as rms current and apparent power. – eckes Apr 07 '20 at 18:44
@eckes: Whether power is positive or negative depends upon whether current and voltage are positive and negative simultaneously, or whether voltage is positive when current is negative and vice versa. Sampling current alone would not convey that information. – supercat Apr 07 '20 at 18:58
The same mechanism used to detect if power is consumed or provided can be used to determine if it's a consumer or provider current. – eckes Apr 07 '20 at 19:09
@eckes: Suppose a device connected to a 60Hz mains causes 1 amp to flow from the hot wire to the neutral for one millisecond every 16.6667 milliseconds (1/60 of a second). If the hot wire happens to be at +170V at that time, the device will take 0.17 joules from the utility. If it happens to be at -170V, the device will push 0.17 joules back to the utility. If it's at e.g. +30V, the device will take 0.03 joules from the utility. If it's at essentially 0V, the device won't take nor deliver any meaningful amount of energy. The amount of energy transferred by moving... – supercat Apr 08 '20 at 16:18
...a certain amount of current depends upon precisely when within the AC cycle the transfer takes place. One could perhaps build a meter that measured current and the phase of the line voltage without measuring the magnitude of line voltage, but it's simpler to just measure power. – supercat Apr 08 '20 at 16:19

score 0 · Answer 10 · answered Apr 08 '20 at 16:25

Consider two customers who are using electric heat and who both need precisely the same amount of energy to heat their homes. Imagine this is the United States and the official voltage is 120V.

Say one customer is a bit further from the transformer though and they are getting 117 volts while the other customer is a bit closer and is getting 121 volts. The customer with the lower voltage will average a higher current level for the same amount of power. Is it reasonable to charge that customer more money for the same amount of energy when the difference is due to factors entirely under the control of the power company and over which the customer has no control?

score 0 · Answer 11 · answered Apr 09 '20 at 00:29

They don't.
They bill in Watt.hours (or 1000's of Wh = kWh).

They may also bill in V.A.h which are the same as Wh when power factor =1.
As power factor drops (current & voltage increasingly are out of phase) the user gets increasingly-less Watts for the same VA.
BUT the current causes losses in transmission lines and transformers.
At a power factor of zero the customer would consume NO watt.hours but the supplier would still have to send current to them and incur real energy losses in lines and transformers.

Wh = energy.
W = power = rate of using energy.
A = current flow = proportional to W ONLY at fixed V and fixed power factor,
which never occur in real world situatins. So Wh are the real measure of the generation effort and A.h a measure of the line losses.

Wh = energy.

score -1 · Answer 12 · answered Apr 07 '20 at 10:10

As for the "less-techy" comparison as stipulated in one of earlier replies, the comparison of electrical flow to water flow can help, handwaving away the finer nuances.

The current (Amps) is the amount of water that would flow, the Voltage is from how high it falls or flows, the Wattage (per hour) is the energy consumed by the user is how fast your useful water-wheel is turning to grind the grain or power some other machine. (And the resistance, Ohms, might be the loss of original flow to obstacles until it gets to your wheel - freefall in vacuum vs. scraping the riverbed, or hitting a dam altogether).

You can have a bucket of water (X Amps) lying still (0 Volts) and so turning nothing (0 Watts => 0 kWh). Or you can pour it from different heights (1 or 100 Volts as in an earlier example) and give more or less spin to the water-wheel over the same amount of time, allowing you to grind more or less grain into flour. As an end user, you are not billed for the bucket of water per se, but for how much grain it helps you to grind in the overall contraption.

In real life, same amount of water falling from different height converts a different amount of its potential energy to kinetical (in water-wheels of the old, or hydro-dam turbines of the modern age - hence the tall dams making large reservoires).

Why is electricity consumption billed in watts and not amperes?

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