We have all heard the phrase 'this house is such and such star rated' but what does that actually mean in terms of space heating use? The National Home Energy Rating System or NatHERS gives your home a star rating from 1 to 10 stars where 1 star homes are glorified tents and 10 star homes are what is called a passive house (or passivhaus as these type of houses were pioneered in Germany) as they are passively heated by the sun rather than needing a powered heating system.
To illustrate how the NatHERS star system gives you your home heating requirements, let us look at a typical Melbourne home that is 240 m2 in size, where 140 m2 of the house needs to be actively heated (i.e. excluding bathrooms, laundry, spare room etc). Let us also assume that like the typical house in Melbourne it has a star rating of 2 stars (a glorified tent)
To work out the heating requirements for this home we look up the NatHERS website http://www.nathers.gov.au/ and determine that Melbourne is in climate zone type 21. The table for climate zone type 21 is as follows (note figures are usually given in MJ/m2 the kWh/m2 figures have been added for simplicity)
Heating required per m2 for Climate Zone 21 (Melbourne) by NatHERS star rating.
|
Star Rating |
kWh heating energy required per m2 per annum |
kWh heating energy required for heating 140 m2 per annum |
|
1 |
155 |
21,739 |
|
1.5 |
128 |
17,967 |
|
2 |
107 |
14,933 |
|
2.5 |
89 |
12,483 |
|
3 |
75 |
10,539 |
|
3.5 |
64 |
8,944 |
|
4 |
55 |
7,700 |
|
4.5 |
48 |
6,650 |
|
5 |
41 |
5,794 |
|
5.5 |
36 |
5,094 |
|
6 |
32 |
4,433 |
|
6.5 |
27 |
3,811 |
|
7 |
23 |
3,228 |
|
7.5 |
19 |
2,644 |
|
8 |
15 |
2,100 |
|
8.5 |
11 |
1,517 |
|
9 |
7 |
972 |
|
9.5 |
4 |
506 |
|
10 |
1 |
78 |
Source:
Using this table we can then simply take the figure for kWh heating required per m2 and multiply it by 140 (of heated home area) to get the column on the right hand side, which shows how much total energy per year is required to heat this home given various NatHERS star ratings.
So given this is a 2 star house we can see that it takes 14,933 kWh to heat this home every year (more if you wanted to heat the entire house). Note: that drops to just 78 kWh per year if you brought this house up to NatHERS 10 star rating, which may or may not be economically viable.
In short the NatHERS gives home owners a tool to work out the size of heater they need for their home, given it star rating. This is very useful in working out how best to reduce your energy bills, in some cases improving the NatHERS star rating will be the right move, in other situations installing a space heating heat pump (aka a reverse cycle air conditioner) will yield a better return in terms of energy savings per dollar spent.
Either way it is highly recommended that you look at your home's NatHERS star rating when working out where to spend your money on energy bill reduction.
