Getting the most out of solar – part 1

Last updated: 23 September 2021

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.

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.

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.

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).

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.

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