Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

I'm a little confused about the whole kilowatt hour thing. I get its 1000 watts in 1 hour but I don't get how you figure out how much voltage and current is running through the house. I did a little research an found that houses are wired for 200 amps. This solves how much current can go through your house but not how much really is. Plus I don't think anyone will ever get close to 200 amps. Could anyone tell me how to figure out how much current and voltage is going through a house every second (or hour if it's easier). If it helps my house has an average of 600 kwh going through it. Thanks :)

share|improve this question
What are you doing with 600kWh per month? Heating house? Air conditioning? –  Kamil Aug 22 '14 at 22:16
Be careful with your terminology! You can't have 1000 watts in 1 hour, since watts are a rate of energy use. It sounds like you mean 1000 watts for one hour, ie. using 1 kW constantly for an hour. –  Kynit Aug 22 '14 at 23:10
Voltage doesn't go through your house, current does. Voltage is just what pushes it through. The voltage never changes (within a few percent, anyway). –  gbarry Aug 22 '14 at 23:34
kW = power as in horsepower = INSTANTANEOUS measure. Energy = power x time so kW.hour = power x time = energy. | water analogy: Voltage = pressure = pumping head. | Current = current = flow rate. | Power = volts x amps = pumping head x flow rate | Energy = Power x time = flow rate x pumping head x time. | 600 kWh = energy per month | 200 A = MAX allowed flow rate. Actual flow rate is measured by meter . | IF you read those through carefully enough to understand them you will have answers to the questions you asked. –  user10162 Aug 23 '14 at 1:39
.... Sadly, experience shows that most people given this information will fail to take the effort to allow them to gain understanding. What you do is up to you. –  user10162 Aug 23 '14 at 1:41

8 Answers 8

Energy (measured in kWh) is simply the sum of power over time (measured in Watts, or Volts * Amps).

So, if you use 600 kWh in one month (I'm assuming that's what you meant), you can simply divide energy by time to get average power. Google is actually great for these type of conversions (you get 821 Watts): https://www.google.com/search?q=600%20kwh%20%2F%201%20month

Now, since Watts are Amps * Volts, you can just divide your average power by your voltage (120 Volts if you're in the US) to get your average current. Going to google again, it looks like 6.8 Amps: https://www.google.com/search?q=600%20kwh%20%2F%201%20month+%2F+120+volts

Of course, this ignores the fact that some of your power is probably used at 220 volts, such as washing machines and some furnaces and water heaters, but hopefully it is helpful anyway.

share|improve this answer
Okay so the bill says this month I used 600 kwh. Does this mean I used an average of 600 kilowatts per hour or does it mean I used 600 kilowatts last month. –  Trevor Aug 22 '14 at 20:32
It means that you used an average of 821 watts for the whole month. I think you are thinking about kwh/watts backwards. Watts are the speed at which you use watt-hours (which are what they bill you for). kWh is kilowatts times hours, meaning you could have used either 600 kilowatts for one hour, 300 kilowatts for 2 hours, or 821 watts spread out over the whole month. –  Chriszuma Aug 22 '14 at 20:35
Ok so you said there is 120 or 220 volts going through my house? So does that mean that every outlet is wired in parallel? –  Trevor Aug 22 '14 at 20:44
Simplistically, yes, all the outlets are wired in parallel. More completely, there are actually two different 120-V incoming wires with different phases, and some outlets are on one phase and others are on the other phase. But all the outlets on one phase are in parallel. –  The Photon Aug 22 '14 at 20:46
@TDHofstetter No, 821 watts is correct. 600 kW * h / (30.42 days * 24 hours/day) = 821 watts. –  alex.forencich Aug 22 '14 at 22:50

I get its 1000 watts in 1 hour

It's also 1 watt for 1000 hours. The point is that the kWh is a measure of energy used but, by itself, it does not tell you the rate of energy usage or power. A small amount of power over a long time is as much energy as a large amount of power over a short time.

Could anyone tell me how to figure out how much current and voltage is going through a house every second

If your energy usage over some number of hours \$N\$ is \$600 \mathrm{kWh}\$, then your average power is simply

$$p_{avg} = \frac{600}{N} \mathrm{kW}$$

For example, there are 720 hours in 30 days so, if your energy usage for a 30 day period were 600kWh, the average power delivered to your home in that period is

$$p_{avg} = \frac{600 \mathrm{kWh}}{720 \mathrm{h}} = 833 \mathrm{W}$$

Since most homes in the USA have a 240V service, this means the average AC current 'drawn' by your home is

$$I_{avg} = \frac{833}{240} \approx 3.5\mathrm A$$

Of course, the power and current actually fluctuate greatly around this average. For example, the power and current will increase dramatically when, e.g., an electric water heater or an electric stove is on. At night, with the lights off, the power and current can be much less than the average.

share|improve this answer

You are sure confused about a few things, but at least you did not take the maximum rating of 200 A for the actual current. BTW assuming you are on a 120 V net, 200 A means 24 kW. That's not your average house, are you sure about that figure?

How much current goes through all the appliances in your house depends on those appliances. I can't predict it, you'll have to measure it.

Voltage does not go through something, it is 'over' something.

kWh is a measure of energy, not of power. To calculate back from 600 kWh and my assumed 120V to an (average) current, you must know over how much time that 600 kWh was consumed, to calculate the (average) power consumption. Think of energy (in kWh) as liters of gasoline, and of power (in Watt) as liters burned in your engine per hour.

share|improve this answer
In the USA, most (all?) houses built in the last few decades have 200A mains service. Then, the branch circuits are generally 15A (#14AWG wire) or 20A (#12AWG) each. The power is provided by two opposing legs of 120VAC, which can be used to create 240VAC for large appliances or garage loads. I doubt anybody ever actually pulls the full 200A, though :) –  bitsmack Aug 22 '14 at 21:32

At the point the electric cables enter the building, there is an electricity meter. It measures energy passing through it by integrating voltage and current over time.

Historically, this was done in electromechanical meter that spins an aluminium disc (by eddy current induction) placed between two coils: one for current and one for voltage. A mechanical counter then counts revolutions and therefore electric power consumption.

Nowadays it is done electronically with ADC and MCU, the current being sensed either by hall sensor or by current shunt resistor.

You can even use the optical interface of your meter to read out its stored parameters, which also include instantaneous power and total consumed energy. There is nothing illegal about it.

share|improve this answer

Your REAL power limit at house are fuses. See your fuses ratings.

Wires are oversized for safe handling failure conditions, like short circuits etc.

Thicker wires mean less resistance.

Less resistance helps fuses by causing higher short-circuit currents. Fuses blow off faster and there is smaller risk of fire if you have short circuit.

Thicker wires also means less voltage drop and less power loss on wire.

At 110V, one phase 200A would be 22000W (22kW). What could use that power? 10-20x clothes irons, 220 100W lightbulbs, ~500 notebooks or 30HP electric motor (really big, 130kg/270lbs), but that big motors use 3-phase systems and current splits over 3 wires.

share|improve this answer

Could anyone tell me how to figure out how much current and voltage is going through a house every second (or hour if it's easier).

Your house has 120v in every wire, all the time. If you measure between the black and red phases instead of black (or red) to white you will get double that value.

The current is highly variable on a minute-by-minute basis, there is no point at all trying to handle it on a short-term basis. Your computer will range from 10w on standby to 300w when busy, the refrigerator will consume nearly nothing or several amps depending on wether it's in a cooling cycle or not. Most heating / cooling devices do NOT throttle their consumption, they are either on at full power or off - the duty cycle determines the temperature. Heaters will often cycle so fast you don't notice.

So if you are trying to get a handle on what is drawing all that power, I suggest you go and get a clamp-on ammeter. But you cannot just look at the instant reading - put it on a wire* and sit there for 10 minutes to see what happens.

  • has to be a single wire, they don't work on normal power cords.
share|improve this answer

If you carefully follow the approximate equivalences here it will help demystify the subject by expressing unfamiliar concepts in common terms.

KiloWatt = kW = power as in horsepower = INSTANTANEOUS measure.

Energy = power x time
so kW.hour = power x time = energy.

All the rivers run - water analogy:

Voltage = pressure = pumping head. (Volts -> metres or feet or psi or N/m^2, ... )

Current = current = flow rate. (Amps-> gallons per second, litres per hour, ...)
[Litres and gallons ~= charge but don't go there for now]

Power = volts x amps = pumping head x flow rate
= psi x gallons per second ...

Energy = Power x time = flow rate x pumping head x time.
= gallons per hour x psi x hours = gallons x psi (note rate vanished)

So your 600 kWh = energy per month your 200 A = MAX allowed flow rate.
Actual flow rate is measured by meter.

share|improve this answer

Also, you can convert KWH to Joules. 1 Watt = 1 Joule per Second. So 1 KW = 1,0000 Joules per Second. 3,600 Second in an hour so.... 3,600,000 Joules in a KWH or 3.6M Joules.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.