NZ Level 6 (NZC) Level 1 (NCEA) Energy of Consumption
Lesson Energy saving light bulbs consume less kilowatts per hour

The basic units of energy are Watts or Joules or Calories. When we think about appliances, we tend to use Watts or Joules. Just as when we looked at energy for the body we looked at kilojoules, we can also consider a kilowatt. A kilowatt is simply $1000$1000 watts, just as a kilojoule is $1000$1000 joules.

Energy within the home is generally measured in kilowatt hours (kWh). This measures how many watts an appliance uses in one hour while running. So a full kettle on average may use $0.04$0.04 kWh to boil water. A washing machine could use $0.945$0.945 kWh per use.

A single kWh is equivalent to $3.6\times10^6$3.6×106 Joules, You can see how it could sometimes be more useful to think of the amount of energy used by a washing machine in as $0.945$0.945 kWh rather than $3402000$3402000 J.

#### Worked Examples

##### question 1

Use the fact that $1$1 kWh = $3.6\times10^6$3.6×106 J to calculate how many joules $104.23$104.23 kWh is equal to.

### The price of using energy

The number of kWh you use is often quoted on electricity bills, so you can see how much you are being charged.

The cost of running appliances can be assessed before using them by seeing their star rating. Appliances with higher star ratings are cheaper to run, but may be more expensive at first to purchase.

##### QUESTION 2

A family is comparing the price and energy consumption of dishwashers in a store. They expect that over a year they will use the dishwasher for $320$320 loads. They find a $2$2-star rated dishwasher for $\$320$$320 that uses 1.91.9 kWh per load, and compare this to an energy saving 44-star rated dishwasher which costs \460$$460 and uses $1.1$1.1 kWh per load.

Use the fact that $1$1 kWh costs $17$17 cents to answer the following questions.

1. Over a one year period, how much will they expect to save in electricity costs using the $4$4-star dishwasher rather than the $2$2-star dishwasher? Round your answer to the nearest cent.

2. Will the electricity saving in the first year offset the difference in purchase price of the dishwashers?

Yes

A

No

B

Yes

A

No

B

### Outcomes

#### GM6-2

Apply the relationships between units in the metric system, including the units for measuring different attributes and derived measures

#### 91030

Apply measurement in solving problems