Getting the most out of solar – part 2

Last updated: 27 September 2021

For the standard home, including or adding solar photovoltaic (PV) is a relatively cost effective way to reduce the home’s ongoing utility bills, whilst also benefiting the environment. But when combined with energy-efficient home design, the way you use solar PV changes – requiring a smaller system and achieving even better cost and emissions savings.

This is the second article in a two-part series on solar PV. In this article we explain how solar PV and energy-efficient design can work together to make things more cost effective and more efficient for both builder and home buyer.

For background on the benefits of solar PV and the types of things that solar PV can achieve in the home – including Zero Net Carbon (ZNC), Zero Net Energy (ZNE) and No Bills (NB) – read Understanding solar photovoltaic..

What you need to know

  • Upgrading a 6-star home design with standard appliances to 7-star NatHERS with an energy-efficient whole-of-home approach to design, can reduce the home’s energy use by up to 75%.
  • Combining an energy-efficient thermal envelope and whole-of-home design with solar PV creates a cost effective and desirable product for home buyers that is comfortable to live in, cheaper to maintain, and environmentally sustainable.
  • By reducing a home’s energy use, you also reduce the amount of solar PV required to offset energy and emissions.

The importance of energy efficiency

Installing extra solar panels on a roof does not always provide as big a benefit as homeowners expect. This is due to several factors, like local electricity network export caps of 5kW, the lower income on exports versus imports, or because the home itself has high energy requirements.

By coupling solar PV with a high performing thermal envelope and energy-efficient appliances you reduce the number of panels required, as the home consumes less energy. You also get a more comfortable home and cost savings that aren’t susceptible to price fluctuations, the same way solar PV savings are.

In addition to an enhanced thermal envelope, a whole-of-home approach to design will reduce a home’s overall annual energy use, cost, and greenhouse gas emissions with little additional construction cost. Read more about the positive impacts of a whole-of-home approach.

To better demonstrate the benefits of combining solar PV with energy-efficient design, we analysed 3 homes with various configurations, from a standard home to an all-electric energy efficient home.

The data demonstrates the impact of different designs on the amount of solar PV needed to achieve the status of a Zero Net Carbon Home (ZNC), Zero Net Energy Home (ZNE) and No Bills Home (NB).

You can read more about what these mean and the cost versus benefit of different solar PV outcomes in Understanding solar photovoltaic.

Data and analysis

We analysed 3 homes (a 2 bed, 3 bed and 6 bed) ranging in size from 115m2 to 295m2. Each home was assessed against multiple scenarios to achieve ZNC, ZNE and NB.

The modelling was based on the typical behaviours of a 4-person household. An assumed plug-in appliance load was also used. In all scenarios, solar PV sizing was specified assuming a north facing orientation with no shading.

A solar PV export cap of 10kW was assumed for the NB homes. However, some areas on the grid have an export cap of 5kW.

2 bedroom detached home, 115 m2

2 bedroom detached home A. Energy use [MJ] A. Solar PV size [kW] A. Annual energy bill [$] B. Energy use [MJ] B. Solar PV size [kW] B. Annual energy bill [$] C. Energy use [MJ] C. Solar PV size [kW] C. Annual energy bill [$]

Annual performance without solar

34,349

0

$2,526

39,378

0

$2,110

19,504

0

$1,765

ZNC

-109

7.2

$1,163

19,067

4.2

$1,281

-323

4.1

$838

ZNE*

-109

7.2

$1,163

-277

8.2

$677

-323

4.1

$838

NB**

-37,728

15.2

-$22

-22,474

12.8

-$7

-26,525

11

$19

NB with 10 kWh battery

-25,328

12.7

-$10

-14,971

11.5

-$53

-16,583

8

-$13

*The energy use indicated to achieve the ZNE shows the annual energy use of the home, not the solar PV system. A negative number indicates that on an annual basis the home produces more energy than it uses. This is because it is not possible to have half a solar PV panel.

**An export cap of 10kW had to be assumed as it was otherwise not possible to achieve a NB home with the available roof space. It should be noted that in some areas a 5kW export cap applies.

Analysis

  • The energy-efficient all-electric scenario uses 44% less energy than the biggest consumer – the Standard Electric – and requires the smallest solar PV system to achieve ZNC and ZNE status, providing significant capital and on-going cost savings.
  • Whilst achieving ZNC and ZNE in all-electric homes requires the same amount of solar, this differed in the mixed fuel home, where the ZNE home required almost double the system.
  • Substantial on-going cost savings can be achieved on an annual basis with a NB home; however, the size of the solar PV was extensive at 15 kW and would be prohibitive on most 2-bedroom homes, which tend to have limited roof space. To achieve NB status, this home would require both energy-efficient design measures and battery storage.
  • It should also be noted that NB homes are only a snapshot in time. Read more about the benefits and considerations for NB homes in Understanding solar photovoltaic.

3 bedroom and study detached home, 202 m2

3 bedroom and study detached home A. Energy use [MJ] A. Solar PV size [kW] A. Annual energy bill [$] B. Energy use [MJ] B. Solar PV size [kW] B. Annual energy bill [$] C. Energy use [MJ] C. Solar PV size [kW] C. Annual energy bill [$]

Annual performance without solar

38,979

0

$2,852

48,464

0

$2,353

21,967

0

$1,938

ZNC

-120

7.2

$1,264

24,473

4.7

$1,389

-493

4.4

$896

ZNE

-120

7.2

$1,264

-29

9.5

$624

-493

4.4

$896

NB

-40,458

16.6

-$9

-20,675

13.7

-$13

-29,079

10

-$29

NB with 10 kWh battery

-29,680

14

-$47

-11,392

12

-$32

-17,830

8.1

-$13

Analysis

  • Although this home is 75% bigger than the 2 bedroom home, none of its energy or solar PV needs are substantially greater.
  • In the energy efficient all-electric scenario, the solar PV size to obtain ZNC or ZNE with an efficient thermal envelope and efficient appliances is still below 5kW and that cost savings are close to $2,000 per year when compared to the most expensive annual energy user.
  • From an on-going cost perspective, the cheapest home to run, other than NB scenarios, is the Standard mixed fuel with ZNE. However, this also requires a 9.5kW solar PV system, which is twice the size of the system needed for the energy-efficient ZNE home. This is a cost increase of more than $6,000 for the solar PV system.
  • Since some efficiency upgrades can be made at little or no cost, it makes sense to consider the energy efficiency option over the less efficient mixed fuel home. Read more about the positive impacts of a whole-of-home approach.

6 bedroom detached home, 295 m2

6 bedroom detached home A. Energy use [MJ] A. Solar PV size [kW] A. Annual energy bill [$] B. Energy use [MJ] B. Solar PV size [kW] B. Annual energy bill [$] C. Energy use [MJ] C. Solar PV size [kW] C. Annual energy bill [$]

No solar WoH

46,020

0

$3,347

69,205

0

$2,815

24,114

0

$2,089

ZNC

-1,084

11.5

$1,406

37,771

6.5

$1,595

-66

5

$966

ZNE

-1,084

11.5

$1,406

-120

14.5

$420

-66

5

$966

NB

-46,038

22

-$22

15,339

19

-$50

-31,500

11.5

-$55

NB with 10 kWh battery

-33,862

18

-$14

-4,256

16

-$53

-20,314

9.5

-$34

Analysis

  • This 6 bedroom, two storey home demonstrates why it might be difficult for a builder to achieve outcomes like ZNC, ZNE and NB without including energy-efficiency features in the design.
  • Although it is 2.5 times bigger than the 2 bedroom home, the energy-efficient scenario for this home still achieves ZNC and ZNE status with a 5kW solar PV system. This also optimises financial gains as it is within the size limit for the 5 kW export cap that exists in most electricity network areas.
  • When it comes to construction costs and energy-efficiency, this larger home provides excellent value for money. The price increase results in the smallest percentage of construction cost increase, across those compared. This makes it even more attractive to home buyers as the smaller percentage of construction cost increase is offset with increased on-going cost savings from energy efficiency and solar PV. See Zero Net Carbon Homes for more information.

Results

By building homes that include enhanced energy efficiency measures, builders can install moderate sized solar PV systems to boost the cost and energy outcomes of the home. Builders can also have homes that achieve ZNC and ZNE status, whilst still ensuring the solar PV system fits on available roof space and doesn’t exceed export restrictions, that in turn affect the income from the feed in.

Zero Net Carbon and Zero Net Energy

ZNC is very achievable in all situations but in the absence of energy-efficient design, ZNC is not necessarily an appealing product to home buyers.

When it comes to ZNE, the analysis showed that achieving a ZNE and ZNC requires the same amount of solar in all-electric scenarios.

To achieve ZNE in mixed fuel homes, almost double the amount of solar PV was needed when compared to achieving ZNC; making ZNC the better option for mixed fuel homes. This is because gas has a lower emission intensity than electricity in Victoria at present; however, the gas homes use a lot more energy at the end use point than their electric counterparts.

No Bills

A combination of constraints make achieving NB status difficult or, in some instances, impossible. These include:

  • grid export capacity limits (limiting the amount of solar feed-in-tariff that can be obtained)
  • available roof space
  • home buyer budgets.

Additionally, NB homes can lead to dissatisfied customers, who may behave differently to the modelled energy use such as time of use and assumed plug in load versus actual.

The following additions can make NB status more achievable:

  1. Include energy efficiency measures.
  2. Include batteries.
  3. Identify where there might be export limitations in the electricity network.
  4. Enable the homeowner to use as much of their own generated solar energy at the time of generation as possible rather than import the energy.

Where to next