# 9   Light

## Lighting home and work

The brightest domestic lightbulbs use 250 W, and bedside lamps use 40 W.
In an old-fashioned incandescent bulb, most of this power gets turned into
heat, rather than light. A fluorescent tube can produce an equal amount
of light using one quarter of the power of an incandescent bulb.

How much power does a moderately affluent person use for lighting?
My rough estimate, based on table 9.2, is that a typical two-person home
with a mix of low-energy and high-energy bulbs uses about 5.5 kWh per
day, or 2.7 kWh per day per person. I assume that each person also has
a workplace where they share similar illumination with their colleagues;
guessing that the workplace uses 1.3 kWh/d per person, we get a round
figure of 4 kWh/d per person.

## Street-lights and traffic lights

Do we need to include public lighting too, to get an accurate estimate, or
do home and work dominate the lighting budget? Street-lights in fact use
about 0.1 kWh per day per person, and traffic lights only 0.005 kWh/d per
person – both negligible, compared with our home and workplace lighting.
What about other forms of public lighting – illuminated signs and bollards,
for example? There are fewer of them than street-lights; and street-lights
already came in well under our radar, so we don’t need to modify our
overall estimate of 4 kWh/d per person.

## Lights on the traffic

In some countries, drivers must switch their lights on whenever their car
is moving. How does the extra power required by that policy compare
with the power already being used to trundle the car around? Let’s say
the car has four incandescent lights totalling 100 W. The electricity for
those bulbs is supplied by a 25%-efficient engine powering a 55%-efficient
generator, so the power required is 730 W. For comparison, a typical car
going at an average speed of 50 km/h and consuming one litre per 12 km

Device power Time per day Energy per day
per home
10 incandescent lights 1 kW 5 h 5 kWh
10 low-energy lights 0.1 kW 5 h 0.5 kWh
Figure 9.1. Lighting – 4 kWh per day per person.
Table 9.2. Electric consumption for domestic lighting. A plausible total is 5.5 kWh per home per day; and a similar figure at work; perhaps 4 kWh per day per person.