# Measuring power, current and voltage

I guess the way that the power consumption in our homes is calculated, is by calculating the amount of power that each appliance in the home is using. Since power is current times voltage, does it mean that the current that is drawn by each appliance is multipled by the voltage and this is how the power consumption is calculated? is this how we get billed for the electricity each month?

"is this how we get billed for the electricity each month?"

No. A meter keeps track of the actual energy (kiloWatthours) that all of your electric devices have used. Typically, for a home, they calculate the kWh used * $/kWh to arrive at your energy cost. Then they calculate taxes + surcharges + other fees & add that to your bill. They (the electric company) don't know what devices you have connected to your home wiring, so they don't have any means of computing the usage based on the electric facts that are provided on device labels. Label data is consumer data, so you can determine how efficient one device is compared to another device, & so you can estimate how much power & cost the device may consume as you intend to use the device. • How would they know the time that is the appliances are on...? For example there is a difference between my bills if I use the 100 watt lamp for an hour or two hours – Jack Commented Feb 6, 2016 at 22:20 • The moment your light switches on, the meter is measuring the energy that is consumed. The reason there is a difference when you use your light for twice as much time is because your device is consuming energy for twice as long when it is on for twice as much time. Commented Feb 6, 2016 at 22:50 • Consider & compare: 100W * 1h * .125$/kW = .0125 $. 100W * 2h * .125$/kW = .025 $. That's 1.25 cents compared to 2.5 cents of energy cost. Keeping the light lit for twice as long doubles your cost. Ideally, you would replace that 100W bulb with a low wattage CFL or LED light. You can buy such lights with comparable light output (i.e. ~1200-1400 lumens) that use ~15W (which would cost you about ~1/6th as much$ (for the energy) for the same amount of light used). Commented Feb 6, 2016 at 23:01
• But how does the meter measure the energy that is consumed?
– Jack
Commented Feb 7, 2016 at 2:00
• It uses a Wattmeter--which is a completely different question & subject that relates to instrumentation design & operation. You can read about the various methods of how electric power is measured here: en.wikipedia.org/wiki/Wattmeter Commented Feb 7, 2016 at 8:39

Multiplying current by voltage gives you the apparent power S which is the same as active power P if the load is resistive. If it is inductive(motor in a washing machine), S is different (bigger than) P. But you're not billed for reactive because it's not that big of a deal for homes. Industries however are billed for reactive power.

• how would the power meter would know the power is reactive or real power?
– Jack
Commented Feb 7, 2016 at 21:14

Some of the appliances in a home aren't purely resistive because they contain circuitry which is reactive.

When the voltage across one of those reactive loads is measured and multiplied by the current measured through the load, the product isn't watts, it's volt-amperes and, because of the reactive current shuttling back and forth across the load, volt-amperes will be greater than if the load was purely resistive.

For residential service, the energy company charges for watt-hours, not volt-ampere-hours, and in order to do that the meter cleverly gets rid of the reactive term, measures the actual power being used by the load, and reports that as billable.

• Reactive power can be a large issue to the utility too though, right? Because they would have to adjust their generators to be leading or lagging to make sure the power factor is as close to 1 as possible. Around from where I am, there is actually a lot of scepticism over whether the utility charges you for reactive loads or not. Especially with the rise of computers. Commented Feb 7, 2016 at 3:14
• I'm sure it's a huge issue. If it weren't, then there'd be little need for power factor controlled hardware and the zillions of dollars needed to implement it [almost] universally, these days. Commented Feb 7, 2016 at 7:30
• @ EM how would they get rid off the reactive term..?.why don't they just charge us for it. According to my text book watts and var are the same units just different names so we would know one is natural and other one is reactive
– Jack
Commented Feb 7, 2016 at 20:06
• @MaryE: Part 1. There are basically two ways of getting rid of the reactive term(s), and Wikipedia does a far better job of describing them than I could in a comment. They don't charge us for it because what they're selling us is real power (watts), not apparent power (volt-amperes). If your textbook says that watts and VAR are the same thing, then your textbook is wrong. Commented Feb 7, 2016 at 22:10
• @MaryE: Part 2. Watts is power dissipated by a purely resistive load, where the voltage across, and current through the load are in phase, while VAR is the power dissipated by the resistive part of the load plus what appears to be power dissipated by the out-of-phase or reactive part of the load impedance. Commented Feb 7, 2016 at 22:15

It just measures the total power coming in -- it doesn't measure each individual appliance. It's basically voltage * current, but since it's AC, it is slightly more complex to calculate than that.

• yeah, you could say that it treats the whole house as a single appliance. Commented Feb 6, 2016 at 21:48

Speaking for my area: Each building has power lines running to the building, before the lines enter the building there is a meter placed in series with the line. The meter would have some kind of current/voltage sensing means.

So when you turn on your toaster in the morning, the toaster will require current, and the overall current draw through the meter will increase. The meter at my place will show how much power I consume, and a worker from the utility will come by and record how much power I consume to charge me accordingly.

Additional information: the utility has to face large challenges called "peak power". Essentially this occurs when everyone turns everything on in the morning when they wake up, so the power grid needs to supply a lot of power in a short amount of time. But for most of the day the power grid wouldn't need to supply as much power when people are at work or away from home.

This can be fixed by shifting loads with the means of power storage such as heaters that store heat in bricks or a battery bank.

Also lights are typically a very small percentage of your load. Keeping the lights off is a good idea, but tends not to make much of a difference compared to things like washer/dryer, heaters, fridge, etc.