Getting the most out of solar – part 1

Last updated: 2 August 2023

Solar photovoltaic (PV) can achieve different things in homes. Factors like the energy efficiency of the home can impact how a solar PV system performs and how much solar PV is required.

When talking about what solar PV can achieve, terms such as zero net carbon (ZNC), zero net energy (ZNE) and no bills (NB) are sometimes used, and often confused.

This article is the first in a two-part series. In this article, we draw on modelling data to explain what these terms ZNC, ZNE and NB mean from a construction, cost, sustainability and liveability perspective.

In article two, we expand on these terms and explain how you can get these most out of your solar PV with energy-efficient home design.

Modelling assumptions

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.

What you need to know

Solar PV systems on new homes currently qualify for Commonwealth support through the Small-scale technology certificates scheme.
Combining energy-efficient home design with solar PV provides the best overall outcome for homebuyers. Together, these best balance cost savings – reduced energy bills and lower capital cost, with liveability outcomes – such as comfort and sustainability outcomes like reduced greenhouse gas emissions.
The approach to energy efficiency and the size of your solar PV system changes depending on if you are trying to achieve a ZNC, ZNE or NB home. These also result in different benefits for the homeowner.
Overall, a ZNC home is easier to achieve than a ZNE or NB home. Although, in all electric homes, ZNC and ZNE are achieved with the same size solar PV system.
Energy exports and imports are subject to different tariffs. Therefore, for homes with solar PV, paying close attention to time of energy use can save homeowners money.
In all-electric homes, offsetting emissions with solar PV energy generation is locked in for the life of the solar PV (provided the homeowner uses the home as the modelling predicts). This is because every kWh exported offsets every kWh imported, as the greenhouse gas emission factor of the offset is the same as the import.
In a mixed fuel home, exports are subject to different import and export emission factors, which will change at a different rate. For example, as the electricity grid transitions to net zero emissions, the solar PV export will no longer offset the gas energy import (as it may not become zero emission in the same time).
NB homes – which have little to no bills – are not guaranteed to remain so. This is due to several factors such as changes to feed-in tariffs, time of day energy use and behavioural changes in the energy use of the home’s occupants.
A 5kW export cap in certain areas can reduce the financial benefits of a large solar PV system. Therefore, the focus should be on reducing the need for a large solar PV system through energy efficiency measures, batteries and time of energy use (optimising self-consumption).
Updates to the National Construction Code are currently underway. This includes proposed changes to the minimum energy efficiency standards. This process is developed through a national process led by the Australian Building Codes Board.

Comparing homes with different solar PV

Below is an overview and breakdown of different types of homes and varying solar PV outcomes. These home types are based on common scenarios currently seen or discussed within the Victorian market.

Home type	Solar PV size	Details	Home buyer profile
Standard home with solar	Up to 3kW	A 6-star NatHERS rated home with standard appliances and a solar PV system on the roof up to 3kW	Customers who like renewable energy or are interested in reducing their bills, carbon footprint or energy use, but may not have the capital to purchase the other options.
No solar energy-efficient home	No solar PV	A home with a minimum 7-star NatHERS rating, efficient appliances, and no solar PV system	Anyone who wants an energy-efficient home and to reduce their bills, carbon footprint or energy use. It also suits people who want to focus on liveability with a more comfortable home.
Zero Net Carbon (ZNC) Home	From 4–5kW for efficient homes and 4.2–11.5kW for standard homes	A home that produces enough renewable energy to offset the emissions produced during the operation of the home	Any new home buyer who is: looking for an affordable way to save money on their annual bill environmentally conscious.
Zero Net Energy (ZNE) Home	From 4–5kW for efficient homes and from 7–15kW in standard homes	A home that produces enough renewable energy to offset the annual energy use of the operation of the home	Any new home buyer who: has more available funds than the ZNC is looking for an affordable way to save money on their annual bill wants to produce their own energy.
No Bills (NB) Home	From 10–12kW for the efficient electric homes and from 13–22kW for the standard home	A home that produces no net energy bills in its operation	Any new home buyer who wants to have: no or low energy bills a higher level of control over their day-to-day expenses, but is not deterred by the upfront capital costs.
No Bills (NB) battery home	From 8–18kW	A home that produces no net energy bills in its operation and stores energy for the home to use when needed	Any new home buyer who wants: no or low energy bills a higher level of control over their day to day expenses but is not deterred by the upfront capital costs to use their own renewable energy rather than importing from the grid.

Standard home with solar

A Standard home with solar is a 6-star NatHERS rated home with standard appliances (including 5 W/m² lighting), and up to 3kW of solar. This could be either mixed fuel or an all-electric home.

Solar PV size of up to 3kW is assumed as that keeps it below the ZNC, ZNE and NB range.

Key benefits
- The homeowner can still realise savings up to $670 on the solar PV alone and can reduce their annual energy demand off the grid and their carbon footprint.
- The installation also requires limited roof space.
- The expected expense is less than $3,000 with a short payback period of just over 4 years.
Considerations
- Energy costs may increase and/or feed-in tariffs decrease, either of which will result in reduced cost savings and a longer payback period than originally planned when the solar PV system was bought.

No solar energy-efficient home

A No solar energy-efficient home achieves a 7-star NatHERS rating or above and includes energy-efficient appliances.

While you can have both energy-efficient mixed-fuel and all-electric homes, the home analysed in this scenario was a 7-star NatHERS rated electric with highly efficient appliance choices and light power density of 2W/m². There is no solar PV on the roof.

Key benefits
- Annual energy cost savings of up to 38% depending on the size of the home, which translated to between $761 and $1,258 in homes ranging in size from 115 m² to 294 m².
- Annual energy savings between 50–75%.
- Greenhouse gas emission reduction by 43–48%.
- Efficiency measures will have ongoing energy cost savings that are locked in or will represent even greater value if there are increases to electricity costs. Further, the savings are not affected by changes to the feed-in tariff for solar PV.
- The home is typically more comfortable as it remains warmer in winter and cooler in summer, compared with a standard 6-star home
- The cost of building energy-efficient homes is set to fall over time as supply increase and legislation changes.
Considerations
- While it requires less energy to run, this home does not produce its own energy and, therefore, doesn’t maximise on the on-going cost and environmental savings of a solar PV system.
- Although these types of homes generally had good overall cost, liveability and sustainability outcomes, they are more expensive to construct than the Standard Solar Home, with costs starting around $6,000 (for a 115 m² home). The full cost of the construction varies depending on the size of the home and features added.
- Retrofitting energy-efficient measures, such as insulation in walls and ceilings or high-performance windows, is more expensive compared to installing solar PV. Prioritising energy efficiency upfront is the most cost-effective avenue if the homeowner is having to decide between an efficient home or a BAU home with solar.

Zero Net Carbon Home

A ZNC home produces enough renewable energy to offset the homes operational emissions on an annual basis. The amount of solar PV required to achieve ZNC typically ranges between 4–5kW for an energy efficient home, and between 4.2–11.5kW for less efficient homes.

Key benefits
- Annual energy cost savings from 40–71% or just over $900 to more than $2,300
- Annual energy savings of 100% for the all-electric homes. Note: this is not the same as no energy drawn from the grid, but rather that the home generates as much energy as it uses.
- Annual energy savings between 40–50% for mixed fuel homes.
- 100% greenhouse gas emissions reduction throughout its lifetime if it is fully electric (unless changes are made to the home).
- 100% greenhouse gas emission reduction in a mixed fuel home at the point in time it is assessed. Over time, this home will start to generate emissions because greenhouse gas intensity for the gas use will likely remain the same and the greenhouse gas offset by the solar PV will reduce in line with the reduction of greenhouse gas intensity of the electricity grid, which will become net zero by 2050.
- Most of these homes are net cash flow positive for homeowners from the first year of operation with the current financial factors (loan cost and energy costs, feed-in tariffs etc). For more information, read the ZNC Homes case studies.
Considerations
- For homes ranging in size from 115 m² and 295 m², the addition of changes to achieve ZNC accounts for a cost increase of between 3.8 - 6.7 per cent of a standard home construction cost. Generally, the bigger the home the lower the percentage in cost.
- On average, the addition of solar PV to a standard home will cost between $5,000 to $13,800. An additional cost of between $10,000–$20,000 is required to add energy efficiency upgrades.

Zero Net Energy Home

A ZNE home produces enough renewable energy to offset the homes operational energy on an annual basis. The home can be either an energy-efficient home (requiring 4–5kW of solar PV) or a standard home (requiring 7–15kW of solar PV).

Key benefits
- Annual energy cost savings vary between 40–85%.
- 100% greenhouse gas emission reduction for the all-electric ZNE home throughout its lifetime.
- 200% greenhouse gas emission reduction in mixed fuel homes, however this status will change, refer above.
- Annual energy savings of 100% Note: this is not the same as no energy drawn from the grid, but rather that the home generates as much energy as it uses.
Considerations:
- For all-electric homes, the construction cost of ZNE home and ZNC home is the same.
- A mixed fuel ZNE home requires a larger solar PV system (up to twice the size) compared to a mixed-fuel ZNC home. This can cost between $8,400 to $17,400.
- For home buyers that want to avoid grid connection, a battery will be required.

The No Bills Home and No Bills Battery Home

The NB home produces no net energy bills in its operation. In some instances, where a battery is installed, the home also stores energy for use as needed.

For homes that do not include energy efficiency features, the solar PV size will range from 11.5kW to 22kW. By including energy efficiency features, the solar PV size range is reduced to between 10–11.5kW.

Key benefits
- Annual energy cost savings of 100%. This is at a point in time only and depends on user behaviour.
- Annual energy savings vary between 122% and 235%.
- Reduces greenhouse gas emissions by up to 300% in mixed fuel homes and up to 235% in all electric homes.
Considerations
- Inclusion of energy efficiency features, batteries, and solutions for time of use issues, are very important considerations when looking to achieve a NB home.
- It is not likely that a home will retain the status of no bills in the longer term as the feed in tariff may change with time. Also, the behaviours of people living within the home, such as
- Read more about the NB home that was trialled recently.
- Costs associated to reaching NB status range between $17,000–$30,000 for the efficient all-electric home, however, larger homes can cost even more. This does not include the cost of a battery which typically ranges from $15,000–$17,000.
- As the required systems are so large, it can be very difficult to fit the required panels on the roof of a home.
- Based on modelling that SV completed on the housing sizes mentioned above, it is not possible to fit the solar PV on the roof for any of the standard homes. With energy-efficient homes, solar PV could fit on the roof only with the inclusion of a battery, which reduces the number of panels required.
- Some areas on the grid have an export limit of 5kW, and it should be noted that solar PV export limits are determined by the Distributed Network Service Provider.

In summary

When assessing what you would like to achieve from a solar PV system, it is important to ensure that your aims are realistically achievable. For example, that you have enough room on the roof for the panels required or that you’re not exceeding grid constraints, such as the 5kW export limit.

With that in mind, solar PV can contribute to lower energy bills and improve the overall sustainability of a home.

Including a mix of solar PV and energy-efficient features in a new home achieves the best overall benefits for home buyers. This combination also creates homes that are future proof, efficient and resilient.

Read more on how energy-efficient design and solar PV work together.

Where to next

Read the next solar PV article.
Read about Commonwealth support for solar PV systems in new homes.
Visit 7 Star Homes for information on rebates for energy-efficient homes through Sustainability Victoria.
Visit Solar Victoria for more information.