Watts, kWh, Volts and Amps Explained Simply: A Beginner’s Guide to Home Electricity

An easy way to picture it: water in a hose

Many teachers explain electricity with a garden hose. It isn’t exact, but it helps:

• Voltage is like water pressure: the push available at the outlet.
• Amps are like the flow: how much water passes through the hose.
• Watts are what you get from both together: the force of the jet.
• kWh is the total water that has filled the bucket after a while.

The watt (W): what an appliance uses at any moment

The watt measures power: how much electricity an appliance needs while it’s running. It’s on the label. A watt is very little, which is why the kilowatt (kW), 1,000 watts, is used a lot.

To give you an idea, according to the table published by Argentina’s electricity regulator (ENRE): an LED bulb uses 9 W, a pedestal fan 90 W, a fridge-freezer 200 W and an electric kettle 2,000 W.

The kilowatt-hour (kWh): what you use and what you pay for

The kWh measures energy: how much electricity an appliance has used over time. It’s what your home’s meter measures and what your bill charges for.

The maths is power times time. The U.S. Energy Information Administration explains it with this example: a 40 W light bulb left on for 5 hours uses 40 × 5 = 200 Wh, which is 0.2 kWh. And a 1,000 W appliance running for one hour uses exactly 1 kWh.

Power isn’t usage: the most common mistake

An appliance with a lot of watts doesn’t necessarily use a lot: it depends on how long it’s on. A 2,000 W kettle used for 5 minutes uses about 0.17 kWh. A 90 W fan left on all night, 8 hours, uses 0.72 kWh: more than four times the kettle. That’s why, to know what an appliance costs you, you always multiply its watts by its hours of use. Our electricity usage calculator does it for you.

The volt (V): set by your country

Voltage is the push available at your outlets, and you don’t choose it: it depends on the country. Roughly:

• 110–127 V in the United States, Canada, Mexico and much of Central America.
• 220–240 V in the UK, Ireland, Australia, New Zealand and most of Europe.

Always check the label before plugging in an appliance bought in another country. If it says “100–240 V”, it works anywhere, like most phone and laptop chargers. If it only says “110–120 V”, plugging it into 230 V can burn it out.

The amp (A): the current flowing

The amp measures current: how much electricity flows through the wire. The three units are tied together by a very simple formula:

Watts = volts × amps

So if a label only lists amps, you can work out the watts. And that’s why the same appliance draws more amps in a 120 V country than in a 230 V one: a 2,000 W kettle draws about 8.7 A at 230 V (2,000 ÷ 230) and about 16.7 A at 120 V (2,000 ÷ 120).

Amps also tell you how much a circuit in your home can handle: the circuit breakers in your panel are marked with their limit in amps. A 15 A circuit at 120 V can handle at most 15 × 120 = 1,800 W at a time; plug in more and the breaker trips. Any change to your panel must be done by an electrician.

Batteries: Wh and Ah

A battery’s capacity is the energy it stores, measured in watt-hours (Wh) or kWh, just like usage. A 1,000 Wh portable power station stores 1 kWh.

Many batteries, especially 12 V ones, state their capacity in amp-hours (Ah). To convert to Wh, multiply by its volts:

Wh = Ah × volts. A 12 V, 100 Ah battery stores 12 × 100 = 1,200 Wh.

How long will it last? A cautious formula is: hours = battery Wh × 0.85 ÷ appliance watts. The 0.85 covers what’s lost converting the battery’s current into the kind your outlets use: according to the US NREL lab, an inverter is over 90% efficient near its maximum and less at low loads, so we use 85% to be on the safe side. A 1,000 Wh station running a 90 W TV lasts about 1,000 × 0.85 ÷ 90 = 9.4 hours.

Depth of discharge: don’t drain the battery completely

Depth of discharge is how much of the battery you use each time: using half of it is 50%; running it down to zero is 100%. The more you drain it each time, the fewer times you can recharge it before it wears out.

Australia’s national science agency (CSIRO) gives this example for lithium batteries: drained completely every time, they lasted 180 cycles before losing 20% of their capacity; drained only halfway, 1,300 cycles. That’s why, when you work out which battery you need, you plan to use only part of it.

What is a deep-cycle battery?

A cycle is draining a battery and charging it back up. A deep-cycle battery is built to do that many times, being drained quite a lot each time: it’s the kind used with inverters, solar panels or RVs. According to PVEducation, the solar energy course from Arizona State University and UNSW, a deep-cycle lead battery can be discharged more than 50%, even up to 80%, and it has thicker plates that last longer.

A car battery is the exact opposite: it’s built to deliver a lot of current for a few seconds to start the engine, then recharge right away. It handles shallow discharges well, but it wears out quickly if it’s drained often. PVEducation is clear about it: they shouldn’t be used in a solar system. The same goes for running an inverter during power cuts.

BTU and frigories: how much an air conditioner cools

Air conditioners aren’t sold by their watts but by how much they cool: in BTU or, in Argentina and Uruguay, in frigories (1 frigorie is about 3.97 BTU). That isn’t what they use. To get the watts, divide the BTU by the unit’s efficiency (its EER): a 12,000 BTU unit with an EER of 12 draws 12,000 ÷ 12 = 1,000 W.

Summary table