Adding a cottage, granny flat or worker's house to a rural block raises a big power question: share the main system or build a separate one? Compare the trade-offs, cable runs, metering options and how to size for two households. Powering a second dwelling on a rural block comes down to one early decision: extend the main system to the new building, or give the second dwelling its own. A shared system usually suits dwellings that are close together, with loads the main system can absorb and one household happy to manage it. A separate system usually wins when the buildings are far apart, the occupants are independent, or the main system is already close to its limits. Cable distance, metering and combined sizing are what tip the balance.
Shared or separate: the core trade-offs
Neither option is automatically cheaper. A shared system avoids buying a second inverter, battery and generator, but a long sub-main cable and a bigger battery can eat into that saving. Use this comparison as a starting point:
| Factor | Shared system | Separate system |
|---|---|---|
| Equipment | One inverter, battery bank and generator, sized larger | Two smaller sets of equipment |
| Cabling | Sub-main run between the dwellings | Short runs at each building |
| Independence | One household's use affects the other's supply | Each household manages its own energy |
| Faults and servicing | One fault can affect both homes | A fault affects one home |
| Splitting costs | Needs sub-metering or monitoring | Built in |
| Future changes | Harder to separate if the property is divided or sold | Each system can stay with its building |
The same logic applies to sheds and workshops, which the shed and workshop guide discusses when weighing a standalone system against connecting to the house.
Cable runs and trenching
Distance is often the deciding factor. The longer the run, the more voltage is lost along the cable, and the wiring rules limit how much drop is acceptable. Long runs therefore need heavier conductors, and cable plus trenching costs rise quickly with each extra stretch of paddock.
Points to plan with your electrician:
- Send AC, not battery DC. Power should travel between buildings as a 230 V or three-phase sub-main from the switchboard. Running low-voltage battery DC over long distances is impractical because of the currents involved.
- Choose the route carefully. Keep trenches clear of future driveways, fence lines, septic trenches, tanks and large tree roots.
- Meet the depth and marking rules. Underground cables need the correct depth, conduit and warning tape. Any excavation should be coordinated with the licensed electrician so it meets those requirements.
- Lay a spare conduit. A spare conduit or data cable in the same trench costs little now and makes monitoring, internet or future upgrades far easier.
- Record the route. Mark the trench on a site plan so nobody digs through it later.
- Place a shared generator thoughtfully. If one generator will serve both dwellings, position it where noise and exhaust do not reach either building's bedrooms.
Metering and splitting costs
On a grid-connected block there are two broad approaches. The first keeps one grid connection and adds a private sub-meter on the second dwelling's sub-main, so the bill and any solar benefit can be split fairly. The second gives the second dwelling its own connection, meter and retail account. That requires an application to the local network operator, and in NSW the connection work is carried out by a Level 2 Accredited Service Provider. Cost depends on distance and local network capacity. Keep in mind that a network export limit usually applies per connection, so two solar systems behind one connection share that limit. If you plan to charge a tenant for electricity, get advice on the rules for on-selling energy first.
On an off-grid block there are no retail bills, but there are real running costs: generator fuel, battery wear and servicing. A monitoring system with separate measurement for each dwelling shows who uses what, which helps when agreeing contributions and highlights faults early. If you are still deciding whether the second dwelling should connect to the network at all, the off-grid vs grid-connected guide sets out the comparison.
Sizing a system for two households
Two households rarely use power at exactly the same moment, but evening peaks overlap: two kitchens, two hot water systems and two sets of lights and heating often run at once. A sound sizing process looks like this:
- Estimate each dwelling's daily winter energy use separately, using real appliance ratings.
- Identify overlapping peaks, such as both kitchens cooking at 6 pm, to set the inverter's continuous and surge ratings.
- Add the overnight use of both dwellings to size the battery, then multiply by the days of autonomy you want.
- Size the array for winter output and confirm there is enough roof or ground space.
- Decide the generator strategy. One generator charging a shared battery is simpler than two.
- Allow for change. A dwelling used by family today may house a tenant or farm worker with different habits later.
As a worked example, a main house using 12 kWh a day and a cottage using 5 kWh a day need about 17 kWh a day combined. Two days of autonomy would call for roughly 34 kWh of usable storage before any generator support. Load control helps here: shifting hot water heating and other flexible loads into daylight hours can reduce the battery required. Typical price ranges for systems of different sizes are in the off-grid cost guide.
Agree who controls the system, too. In a shared off-grid setup, one household running a dryer or heater through a cloudy evening can pull the battery down for both homes. A clear agreement, a monitoring display in each dwelling and automatic control of high-draw appliances prevent most disagreements before they start.
If a battery is part of either design, the federal Cheaper Home Batteries Program applies to batteries installed with solar, and its per-kWh value is tiered: roughly $272 per kWh for the first 14 kWh, $163 per kWh from 14 to 28 kWh and $41 per kWh from 28 to 50 kWh. Values step down every 1 January and 1 July, so confirm current figures with the Clean Energy Regulator before signing.
Next steps
If you are adding a second dwelling to your block, request a free assessment from Blue Energy Solar to compare a shared and a separate design using your actual distances and loads. The Energy Monitoring Dashboard (from $590) and the Off-Grid Solar System (from $24,900) are listed on the energy market. Prices are indicative and are confirmed after a site assessment.
Frequently asked questions
Can a granny flat use the main house's solar without adding a battery?
Yes, if the granny flat is supplied from the main switchboard through a sub-main, daytime solar output will serve whichever loads are running in either building. There is no extra generation, so the same panels are simply shared, and evening use still comes from the grid. A licensed electrician needs to confirm the switchboard, sub-main and any export limit suit the combined load.
What happens to a shared system if the property is later subdivided?
Subdivision usually means each lot needs its own supply, so a shared system may have to be separated, relocated or duplicated. Cables crossing a new boundary can also need easements. If subdivision is a realistic prospect, a separate system for each dwelling from the start is usually simpler. Get planning and legal advice alongside the electrical design before committing to either arrangement.
Can the second dwelling be off-grid while the main house stays connected?
Yes. A remote cottage can have its own off-grid system while the main house keeps its grid connection, and this is often cheaper than extending supply a long distance. The two systems must remain electrically separate unless an accredited designer engineers a compliant connection between them. Compare the off-grid system cost with a quote for extending supply before deciding.
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