Practical Problem 3

Electricity and Magnetism Level 2

Problem statement:

You decided to GO-Green in your house and have 3000 credits to spend on an adequate renewable system. Monthly (31 days) you pay 1275 credits on household power consumption and you would love to reduce this by 30%. By looking at the following options and your current credits, choose the most appropriate products and conclude your answer by the actual percentage of monthly saving

Assume the following

One light uses 0.1 kWh
One geyser (100 liters) uses 2.0 kWh
One geyser (250 liters) uses 3.5 kWh
One pump uses 1.0 kWh
Price per kWh is 0.2016 credits

In your house you have

10 Lights
1 geyser (100 liters)
1 geyser (250 liters)
2 pumps
Total power consumption of 8.5 kWh

The Options you have are below

Replace all lights for 650 credits, system uses 0.4 kWh
Replace a 100 liter geyser for 1150 credits, uses 0.8 kWh per geyser
Replace a 250 liter geyser for 2350 credits, uses 1.3 kWh per geyser
Variable speed drive for a single pump at 800 credits, uses 0.4 kWh per pump

*Choose the best options to fit your budget of 3000 credits and calculate your actual percentage savings. *

76,4% savings 30% savings 15% savings 31.76% savings 17,8% savings

1 solution

Brandon Louw
Aug 4, 2016

By just observing and making a small comparison between every item on the given list, you can see the following.

10 standard lights results in 1 kWh of power consumption and replacing them will cost you 650 credits and save you 0.6 kWh
Replacing a 100 liter geyser will cost you 1150 credits and save you 1.2 kWh of power
Replacing a 250 liter geyser will cost you 2350 credits and save you 2.2 kWh of power -Replacing all pumps will cost you 1600 credits and save you a mere 1.2 kWh of power

By comparing the saving of energy amount, you can see that the 100 liter geyser will be the best investment, but will leave you with a total of 1850 credits, thus only allowing you to either reach a maximum energy savings of 0.6 + 1.2 or 1.2 + 1.2. This gives an option of a maximum savings of only 2.4 kWh.

The 250 liter geyser might seem the most, but is essentially more cost effective than replacing the globes or the pump drives compared to the power consumed per credit. By using all your credits, you can replace the 250 liter geyser and all the globes, reaching an energy saving of 2.8 kWh.

This results in a total consumption of 5.8 kWh, with a total of 5.8 * 31 * 24 = 4240,8 kW/month and resulting to a 4240,8 * 0,2016 = 869,9 credit per month expense.

However the maximum total savings in percentage would then lead to

Saving(%) = $\frac{854}{1275}$ Saving(%) = 1- 0,682 = 31,67%

Practical Problem 3

1 solution

0 pending reports