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Question

An energy efficient air conditioner draws 7 amps in a stadard 120 volt circuilt. it costs $40 more than a standard air conditioner that draws 12 amps. if electricity costs 8 cents per kilowatt hour, how long would you have to run the efficient air conditioner to recoup the difference in price?

Answer 1

The less-efficient a/c uses 600 W more power.
That is 0.6 kW, so every hour the less-efficient one uses 0.6 kWH * $0.08/kWH = $.048 more than the high-efficiency.

Therefore the break even is at $40 / ($.048/hr) = 833 1/3 hours.

At $.08 per kWH I'd like to buy electricity where you live! It is nearly double that where I am.

Maybe it is easier for you to think of it this way: $40 buys 500 kWH of electricity. Since the less-efficient one uses 600 WH = 0.6 kWH more in every hour of operation, it can run for 500 / 0.6 = 833 1/3 hours additional for the $40 price difference.

Answer 2

difference in price = 40$

the amount of power that efficient air conditioner uses :
120*70*60= 50400 = 50.4 kw/hour

(assuming 8 cents per kw/h )

50.4 * 8 =4.032 $

so , just in almost 10 hours the difference will br recouped.
but please pay attention that this efficient air conditioner wastes near 4$ in an hour , but that non-efficent air conditioner wastes 6.9 $ in an hour(120*12*60* 8cents = 691 cents=6.9$) .
so it is reasonable to buy that efficient air conditioner !!
please note the following calculation :

6.9 - 4 = 2.9 (saving money in an hour)
40$ / 2.9 =13.79

it means in 14 hours of working , the non-efficient air conditioner wastes 40 $ more than the efficient one .
after that it consumes 2.9 $ each hour more than the efficient conditioner.