http://www.ck12.org Chapter 2. Numbers, Not Adjectives
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 100W. The electricity for those bulbs is supplied by a 25%-efficient engine
powering a 55%-efficient generator, so the power required is 730W. For comparison, a typical car going at an
average speed of 50 km/h and consuming one litre per 12 km has an average power consumption of 42000W. So
having the lights on while driving requires 2% extra power.
TABLE2.6:
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 kWhElectric 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.
What about the future’s electric cars? The power consumption of a typical electric car is about 5000W. So popping
on an extra 100W would increase its consumption by 2%. Power consumption would be smaller if we switched all
car lights to light-emitting diodes, but if we pay any more attention to this topic, we will be coming down with a
severe case of every-little-helps-ism.
The economics of low-energy bulbs
Generally I avoid discussing economics, but I’d like to make an exception for lightbulbs. Osram’s 20W low-energy
bulb claims the same light output as a 100W incandescent bulb. Moreover, its lifetime is said to be 15000 hours
(or “12 years,” at 3 hours per day). In contrast a typical incandescent bulb might last 1000 hours. So during a
12-year period, you have this choice (figure 9.3): buy 15 incandescent bulbs and 1500 kWh of electricity (which
costs roughly £150); or buy one low-energy bulb and 300 kWh of electricity (which costs roughly £30).