How to Size Solar for a Home With an EV, Heat Pump & Battery

The first number you need is your household's actual electricity consumption. Look at your electricity bills and find your average daily usage in kWh. For example: Current household consumption = 15kWh/day. That gives you a starting point — but it is only the starting point.

The Australian Government says Australian homes can use roughly 11–23kWh per day on average, but individual homes vary substantially depending on appliances, household size, heating and cooling, hot water and lifestyle. It also recommends looking at seasonal usage because solar generation and electricity consumption can change significantly between summer and winter.

An EV can significantly change the electricity profile of a home. The calculation is straightforward:

Using the approved example: household electricity 15kWh/day, EV driving 40km/day, EV efficiency 18kWh/100km.

The EV therefore adds approximately 7.2kWh/day. The household's theoretical daily demand becomes:

This does not mean you automatically need a solar system that produces exactly 22.2kWh every day. Solar generation varies with season, weather, roof orientation, shading, panel temperature, system design, inverter capacity and location. The calculation simply shows why an EV needs to be included when designing the system.

Heat-pump hot water adds another electrical load, but it needs to be considered differently from a simple resistance electric water heater. A heat pump uses electricity to move heat rather than simply generating heat electrically. The actual electricity consumption depends on household hot-water demand, tank size, heat-pump efficiency, outdoor temperature, water temperature, incoming water temperature, operating schedule, booster operation and household usage patterns.

This is why simply adding a generic number for "heat-pump consumption" to a solar calculation can be misleading. Instead, the system should be designed around the home's actual hot-water requirements and expected operating pattern.

Current Solar Victoria Tank-Sizing Guide

Solar Victoria also recommends considering around 75 litres of stored hot water per person, while recognising that actual household consumption varies. Importantly, bigger is not automatically better. An oversized heat-pump system can cost more and may operate less efficiently or cycle more frequently.

Now let's put the pieces together.

Now add the electricity required by the heat pump. Instead of inventing a fixed number, the correct approach is to use the proposed heat pump's expected operating requirements and the household's hot-water profile. For example, if the system design modelling indicates that the heat pump requires an additional 3kWh/day on average, the planning figure becomes:

That is an illustrative calculation, not a claim about what every heat pump consumes. The same principle applies to air conditioning, pool pumps, induction cooking, electric heating, dryers, home offices, additional refrigeration and future appliances. The point is to build a realistic future electricity profile.

Melbourne households can have very different electricity demand depending on how they heat and cool their homes. Air conditioning can become a significant additional load during periods of extreme weather.

A household may look like a 15kWh/day home when assessed using an annual average but behave very differently during a hot summer week. For a home with solar, EV, heat-pump hot water, battery and reverse-cycle air conditioning, the system should be designed around the combined electrical demand rather than the original electricity bill alone.

This is one of the most important points in solar sizing. If your home uses 25kWh/day, it does not automatically mean you need a 25kW solar system. Solar systems are rated in kW, while electricity consumption is measured in kWh.

A 10kW solar system can produce significantly more electricity on a good summer day than on a cloudy winter day. The Australian Government notes that solar generation is affected by climate, sunshine, roof orientation, shading and other site-specific conditions. This is why solar sizing should use system modelling rather than a simple "one kW of solar equals X kWh per day" rule.

Samsun Energy offers residential system options including 6.6kW, 10.56kW and 13.2kW configurations. But the correct size depends on the property.

A solar calculator may suggest a large system, but your roof still has the final say. The Australian Government notes that available sunny roof area is often a limiting factor for residential solar systems. A typical residential panel is around 1.7m², although panel dimensions vary.

The installer needs to assess north-facing roof space, east-facing roof space, west-facing roof space, roof pitch, shading, chimneys, vents, skylights, trees, neighbouring buildings, roof condition and panel layout. North-facing panels can produce strong overall generation, while east and west orientations can be useful for spreading generation across the day. For a household with an EV and heat pump, that generation profile matters. A west-facing section may, for example, produce useful electricity later in the day when household demand is increasing.

This question is just as important as how far you drive. Suppose your EV uses 7.2kWh/day. There are several ways to supply that electricity.

A battery does not generate electricity. It stores electricity that has already been generated. That means the solar system still needs enough generation to: run the home, operate flexible daytime loads, charge the battery when useful, and export surplus electricity where appropriate.

Then later: Battery → Home, and Battery → EV where the system supports it. The objective is not necessarily to fill the battery every day. The objective is to make useful use of the solar energy the home generates.

This is where many solar sizing discussions go wrong. Imagine a Melbourne household currently has gas hot water, a petrol car, limited air conditioning, no battery, and electricity consumption of 15kWh/day. The household installs a solar system based only on that number. Two years later they add an EV, heat-pump hot water, more air conditioning and induction cooking.

A technically suitable solar system still needs to be acceptable to the local electricity network. Victoria has network limits on solar inverter capacity and/or the amount of electricity that can be exported. Solar Victoria notes that a standard residential single-phase connection may generally accommodate systems up to 5kW without the same level of additional process, while larger systems can require additional paperwork or network approval. Export limits can also affect the amount of electricity you can send to the grid.

This is particularly important if you're considering 10kW or 13.2kW solar + battery + EV charger + heat pump. The installer should check phase configuration, inverter capacity, distributor requirements, export limit, emergency backstop requirements, switchboard capacity, existing electrical infrastructure, proposed EV charger and future electrical loads. Do not assume that because a solar package is advertised as 10kW or 13.2kW, it will automatically be approved for every Melbourne property.

Your home's electrical connection can affect the design. Many existing Melbourne homes are single-phase, while some properties have three-phase electrical connections. Three-phase power can be useful for properties with higher electrical loads, but it does not automatically mean you need a three-phase solar system.

The design depends on existing connection, inverter configuration, EV charger, switchboard, solar array, battery, export requirements and distributor rules. If you're adding several high-power electrical loads at once, the switchboard and electrical supply should be reviewed before equipment is selected.

Let's return to the example from the approved brief.

Now consider heat-pump hot water, air conditioning, future appliances, battery charging and seasonal differences. The resulting design demand could be materially higher. But this still does not directly tell you whether you need 6.6kW, 10kW or 13.2kW solar. For that, you need to model: electricity demand + solar generation + roof + orientation + shading + tariff + export limits + future loads. That is why the final solar size should come from system design rather than a simple multiplication formula.

For Melbourne homeowners, these are useful starting points for discussion, not universal recommendations.

These are not promises that a particular system will cover a household's annual electricity consumption. The final recommendation should be based on the property's actual energy data and site conditions.

At Samsun Energy, the objective should be to design around the property rather than simply sell a package size.

A solar system size on its own cannot tell you exactly how much money you will save. The result depends on electricity consumption, electricity tariff, feed-in tariff, solar generation, self-consumption, battery utilisation, EV charging behaviour, heat-pump operation, weather, export limits, equipment performance and installation cost.

If your Melbourne home has an EV, heat-pump hot water and a battery, solar sizing should start with your future electricity profile, not just today's electricity bill.

Then the solar design needs to consider future demand, roof space, orientation, shading, solar generation, battery, inverter, export limits and electricity tariff. A 6.6kW system may be enough for one home. A 10.56kW system may make more sense for another. A 13.2kW system may be worth modelling for a highly electrified household.

• Solar Victoria — Solar Panel (PV) Buyers Guide
• Solar Victoria — Solar Assessment Calculator
• Solar Victoria — Solar Panels and Energy Efficient Hot Water Systems
• Solar Victoria — Hot Water Rebate
• Solar Victoria — 2026-27 Solar PV Requirements
• Clean Energy Regulator — Solar Batteries
• Clean Energy Regulator — Changes to Rebate for Solar Batteries from 1 May
• DCCEEW — Cheaper Home Batteries Program
• Victorian Government — Midday Power Saver
• Victorian Energy Compare