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There are lots of words out there about how to save energy. However, very few actually spell out how much energy and/or money you will save by making the changes. Some changes would require complex calculations and expertise to identify the savings, while some can be done by anyone. 1. Compact fluorescent lamps The old type of domestic light globe – tungsten incandescent (TI) – are notorious for their inefficiency, generating more heat than light. The new alternatives – like the compact fluorescent (CF) – are being advertised as saving money and energy. They cost a lot more to buy but use only 20% of the energy of the older type and last a lot longer. Some facts • a 11W CF provides the same amount of light as a 60W TI • a typical room in an Australian home would use two lamps of this size for adequate lighting. • • • • cost of electricity: 11.7 c/kWhr* cost of 60W TI: 80c* cost of 11W CF: $4.50* average lifetime of TI: 850 hours • average lifetime of CF: 7000 hours * as at April 2008 Questions


What would a CF cost to purchase and run for one full average lifetime? 2.

How many TIs would have to be bought to run for the same time as one CF? 3.



How much would they cost to buy? How much would they cost to run? What is the total cost of running the TIs? 6.

How long per day (on average) would lights be used for in the following room types? 7.

bedroom bathroom kitchen laundry lounge/living area For a house with 3 bedrooms and 2 bathrooms, how many hours would lights be on over a year?



How many kWhr would TIs consume in this house in a year? 12.



How much would this cost? 10.

Repeat Q8 & 9 for CFs. How many hours would a CF have to run to repay the higher purchase price? How many days in the (a) lounge (b) laundry would this be? Do you believe it is worthwhile to change over to CFs:

when the old TI gives up?

as soon as possible?

Energy Matters: Putting A $ Value On Energy Savings 2. Off-peak hot water systems Normal electricity supply is on demand: when you need it, it is there. This is because most applications require power on the spot. However, storage hot water systems (HWS) can use electricity at any time, since their insulation means that the energy used in heating the water is lost very slowly. Therefore, the off-peak system was devised so that the HWS only heats overnight when there is little demand for other uses. Energy suppliers encourage people to switch to off-peak for their hot water by charging a much lower rate. It costs nothing to change to off-peak. As long as the storage tank is large enough to cope with all demands during the day (how many teenage girls are in the household?), it can refill and reheat while everyone is asleep. Some facts • off-peak power costs 4.9c/kWhr compared to 11.7c for normal times • • a common HWS size is 240 L the standard temperature setting for domestic hot water is 65ºC • it takes 4186 J of energy to heat 1 L of water by 1 ºC • to convert from kJ to kWhr, multiply by 0.000278 Questions


Calculate how much energy (in kJ) would be required to heat an entire tank from a start temperature of 20ºC.

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Convert this to kWhr. Calculate the cost of fully heating the tank at the normal rate. 17.

Calculate the cost of fully heating the tank at the off-peak rate. 18.

Calculate the cost saving of using off peak hot water across a whole year. Assume the tank is 75% reheated each day. 19.

Do you think it is worth it? 20.

Does off-peak supply save energy? If not, why do energy suppliers push it?

3. Solar hot water systems There are various approaches to solar heating of domestic hot water, but the simplest and most common – passive – absorbs the heat. They replace the normal electrical or gas heating when the sun is shining, but do not replace conventional heating completely, eg when it is raining for a number of days or in winter when the hours of daylight are less. Energy Matters: Putting A $ Value On Energy Savings Some facts • • claims are made for energy savings of 60-80% the purchase and installation cost ranges from $2000-5000 • government rebates range from $500 1000 Let’s pick the midpoints of these ranges: • • • • 70% reduction in electricity $3500 cost $750 rebate (effectively a cost of $2750)


use the volume and usage values from the previous section

How many kWhr of supplied electricity are needed for the year? 22.

How much does this cost using off peak electricity? 23.

How much is saved per year (see Q18)?


How long does the unit take to pay back the initial cost?


If the unit lasts 15 years, how much money does it save?

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