Energy Saving Electronics

Saving energy for a better tomorrow

Electricity Basics

What you need to know

To understand energy efficiency and energy saving, a small amount of knowledge about electricity is needed.

Electrical power is measured in watts.  Watts is the product of the voltage applied to a piece of equipment multiplied by the current consumed by that equipment.  Voltage is measured in volts, current is measured in amps, volts x amps = watts.

In a household situation the voltage is fixed at circa 115 volts in North America and circa 230 volts in Europe.  It is the current that varies.

Actually, the situation is more complex than this because of something known as phase angle. Strictly, volts x amps = VA and watts is calculated using the current phase angle. However, because your electricity meter automatically compensates for phase angle, we shall ignore it within this explanation.

The electricity that you use and are charged for is measured in killowatt-hours, which is the product of watts x time (one killowatt = 1000 watts).  A two killowatt heater switched on for three hours will have used six killowatt-hours of electricity.

Your Electricity Bill

Typically an electricity bill is made up of some level of fixed charge plus a charge per 'unit' of electricity.  A unit is one killowatt-hour as described above.

Electricity costs have risen sharply recently, but the cost varies widely from country to country.  In the USA a typical figure might be $0.1 per unit while in the UK it is around twice that.

Watts and Power Consumption

Many pieces of electrical equipment tell you their power consumption in watts.  Example are light bulbs, heaters, and vacuum cleaners.

Because the electricity you use is the product of power (in watts) multiplied by time, the watts tell you only part of the story. 

Take for example an iron rated at 750 watts.  This is a high level of watts, but the actual energy consumption will be lower than might be expected.  This is because the 750 watts only applies while the iron is heating up; once it is hot the power falls to near zero until the iron cools below a particular temperature.

During one hour of ironing, the iron may be taking power for only 20% of the time, so that instead of consuming  0.75 units of electricity, it actually consumes around 0.15 units.

A light bulb, however, uses power all of the time that it is switched on.  A 100 watt conventional light bulb switched on for ten hours will use 1 unit.

Power = Heat

A convenient fact which is helpful in guessing power consumption, is that all electrical power is converted into heat.  Generally speaking, if a piece of equipment runs cool, it will not be using much electrical power.

To apply this rule, size must be taken into account.  The temperature rise for an object is roughly proportional to the power consumed divided by the surface area.  Thus a large object that is slightly warm may be using more power than a small object that is running hot.

Cooling fans further confuse the issue as they remove the heat from the object by creating a hot air flow.

As a rule of thumb, if the equipment is cool to the touch and is not generating warm air, it is not consuming much power.

Measuring Power Consumption

There are many devices now available for measuring the power consumed by by individual appliances.

Go the the Power Measurement page for details. 

volts x amps = watts

Basics of Electricity - Sunelco


Percentage energy useage

Percentage use of electricity by type of electronic device (US Households) -
Total = 82 Billion kWh