From September 1, 2021, A is the highest energy efficiency class, while G is the lowest.

A class bulb has the longest operating time and the lowest power consumption.

The power consumption of the bulb in kilowatt hours (kWh) for 1,000 hours is indicated on the bottom of the label.

The luminosity of a light bulb is characterised by the luminous flux, with the lumen (lm) as the unit of measurement. The higher the lumen, the brighter the bulb shines.

For example, a 60 W incandescent bulb can be replaced with a 702 lm halogen bulb, a 741 lm compact fluorescent bulb or a 806 lm LED bulb, but the light that is perceived by the human eye will be the same.

The unit of power of the bulb is watts (W).

To calculate the daily, monthly or annual electricity consumption of a bulb in kilowatt-hours (kWh), the power (W) must be divided by 1,000 and multiplied by the bulb’s service life in hours (h).

The colour temperature of light bulbs is measured in degrees Kelvin and designated by K. The lower the degrees Kelvin, the more yellow the light, whereas the higher the degrees Kelvin, the more blueish the light.

The light emitted by bulbs tends to be different: yellow, white or bluish. The warm white light is in the range from 2,700 to 3,200 K, white light – from 3,200 to 4,000 K, bluish white light is more than 4,000 K and bluish light is over 6 000 K.

More yellow light is advisable for living space, while white light is suitable for workrooms and corridors.

The colour rendering index is designated by Ra and it represents the colour rendering of the object in the light of the bulb compared to the same colours in the daylight.

The closer Ra is to 100, the more precise the object’s colour in the light of the bulb.

In places where lighting has to be switched on or off frequently, bulbs with more switching cycles should be used.

6,000 switching cycles are most commonly indicated of the packaging of compact fluorescent bulbs, while LED bulbs have from 20,000 to 200,000 cycles.

The bulb’s service life forecast by the manufacturer is given in hours (h); however, at least 50% of bulbs also operate after exceeding the specified time.

This indicates the time in which the bulb will reach a 100% light output – whether it will happen gradually or with a delay of 1 second, or in any other time unit.

When purchasing a new bulb, pay attention to the bulb socket type, bulb size and light emitting angle. Therefore, it is recommended to take a burnt out bulb with you to the shop.