Question

Someone decides to replace six 60 watt filament bulbs with six 15 watt low energy bulbs . If energy costs 10 cu per unit and the bulbs are used , on average, for 4 hours per day what will the annual saving be?

Answer 1

Gives

Bulb =6

Initial pawer

$P=\frac{W}{t}$

$E=P\times t$

$E=60\times4$ watt $\times hour$

Number of bulbs=6

$$E=6\times60\times4=1440watthour$$

Monthly $E=30\times1440 =43200watthour$

Annual

$$E=43200\times12 =518400watthour=518.4kwh$$

1 unit =10rupees

Total cost

$Ru=518.4\times10=5184rupees$

Bulb replacement 15watt

$E=15\times4 watthour$

$E=6\times15\times4=360 watt hour$

Month

$E=360\times30 =10800watthour$

Annual

$$E=10800\times12=12960 0watthour=12.96kwh$$

Total cost

$$Ru=129.6\times10=1296ruppes$$

Saving(ru)=

$$5184-1296=3888ru$$