Voltage and Efficiency

The efficiency of an incandescent lamp is measured in terms of the amount of light produced per watt of power consumed. As the temperature of the lamp decreases, the light output per watt decreases. Thus, at a lower voltage (brownout), the efficiency of the lamp is very low.

• saying that the efficiency is "very low" is purely subjective - "very low" compared to what?

The tungsten filament normal operating temperature is selected to minimize the net cost of running lighting fixtures, balancing efficiency and lifetime. Hotter filament temperatures cost more because they wear out the filament faster and require more frequent replacements. Colder filament temperatures cost more because they require more electrical power for a given amount of visible light.

The luminous efficiency of any black-body radiator increases with temperature up to 6300 °C (6600 K or 11,500 °F). Tungsten melts at 3695 K (6192°F), where it, like any black-body radiator, would theoretically have a luminous efficiency of 52 lumens per watt.

A 50-hour-life projection bulb is designed to operate at 50 °C (90 °F) below that melting point, where it may achieve up to 22 lumens/watt.

A 1000 hour lifespan general service bulb typically operates at 2000 K to 3300 K (about 3100-5400°F), achieving 10 to 17 lumens/watt.

Fluorescent Lamps

The tungsten lamp has been replaced in most applications by fluorescent lamps. Fluorescent lamps have a power factor close to 0.25. Fluorescent lamps typically rate about 40 W, and they provide much more (about 5 times) light compared to an incandescent lamp of the same wattage. they also give out less heat.

Passive Control

Early fluorescent lamps used a ballast (also called a choke coil), which was essentially an inductor to control the current in the lamp. Also, the lamp was started by using a starter, which is essentially a neon thermistor which heats up and closes a circuit. With the choke coil in series with it, the lamp has a relatively small voltage drop across it so that the starter doesn't close again. As the starter is in parallel with the lamp, the same starter can be used to start several lamps. One particularly annoying aspect of the electromagnetic ballast is the 60 Hz flicker produced. While it does not bother most people, some find it extremely irritating. Also, the electromagnetic ballast increases power consumption by about 25% when on utility power.

Active (Electronic) Control

Modern lamps use electronic circuits to control the current, so that both the starter and the choke coil are redundant, and they behave much better on both inverter based and utility power. Many electronic ballasts will boost the frequency to something in the range of 20 kHz, so that there is no flicker problem.

CRT Based Appliances

The other major source of power consumption are CRTs (Cathode Ray Tubes) like computer monitors and televisions.

Computer Towers

The towers of a modern computer draw their power from a SMPS, which has been detailed below. The most popular computers today (running P4s and 3D cards) consume several hundred watts of power.

Other Electronic Loads

Other electronic items in a household draw their power from the mains using a wall wart. The steady state power consumed by each component is pretty low, and in many cases (likeprinters, scanners etc.), they don't work continuously.

Control Elements

Control elements are the switches, dimmers, and regulators which are connected to the circuit. They are, by their very nature non linear elements and their behavior is quite complicated, and not quite well represented by their simple schematic symbols.

Light Dimmers

Light dimmers work by cutting off parts of the input sine wave. While this works for resistive loads, even here it has side effects.

Energy Meters

Most households are on the grid, i.e. their electricity comes from a utility, which installs an energy meter on the premises. The meter is then read either manually or by phone line connection to the utility offices.

The utility wants your power factor to be as close to 1 as possible, and businesses are penalized if they cannot achieve a target set by the utility, as the transmission losses are nearly the same for both active and reactive power consumed. For home users no such rule exists, and it is interesting to see the changes in the power consumption patterns now that most of the home electricity use is not lighting, and even the lighting is by fluorescent lamps which are not resistive in nature. The utility only charges the home uses for active power, so that a low power factor is not an issue from an economic perspective, and transmission losses within the household are negligible.