An electric vehicle (EV) charging beneath a solar-equipped roof can appear to be running for free. There is no petrol purchase, and the solar electricity does not create a grid-energy charge. But that electricity may otherwise have earned the household a feed-in credit. Where export is the alternative, using it in the car means giving up that income.
That does not make solar charging a bad deal. It changes the question: how much does using the surplus save compared with the charging option the household would otherwise choose?
Three ways to supply the same 100 km
Consider a hypothetical EV requiring 18 kWh from the household AC electricity supply to travel 100 kilometres. This assumption already includes EV charging losses and is held constant across the comparison. It is not a measured vehicle result or an Australian fleet average.
Assume ordinary grid electricity costs 30c/kWh, an available off-peak rate is 10c/kWh, and surplus solar would earn a 5c/kWh feed-in tariff. All rates are illustrative, not current retailer offers. Import prices include GST; export credits are used as stated. No home battery is involved.
The solar figure is not a retailer charge. It is the $0.90 export credit the household gives up. Under these assumptions, solar charging saves $4.50 per 100 km compared with ordinary grid charging after accounting for forgone exports, but only $0.90 compared with off-peak charging.
These are marginal energy comparisons, not total ownership costs. They exclude solar and charger purchase, installation, maintenance, financing, daily supply charges and any change in demand charges. A decision to buy equipment needs a separate assessment.
Suppose half the required energy comes directly from surplus solar and half is bought off-peak. The calculation becomes 9 kWh × $0.05 plus 9 kWh × $0.10: $1.35 per 100 km. Of that, $0.90 is a grid-energy charge and $0.45 is forgone export income.
Compared with supplying all 18 kWh off-peak for $1.80, the benefit is $0.45 – not the $4.05 difference against ordinary grid charging. Both comparisons are mathematically valid, but only the household’s realistic alternative tells it what changing behaviour would save.
The available solar is what remains after other household demand. In a simplified example with no home battery, 5 kW of solar generation and 2 kW of other household load leave 3 kW for the car. A charger drawing 7 kW would still need 4 kW from the grid. The relevant grid rate is the one applying at that time.
The vehicle must also be connected while that surplus is available. Morning exports do not become a one-for-one credit for charging after sunset. A solar-aware charging system can help match charging with excess generation, but an ordinary timer alone does not guarantee solar-only charging.
Use the export credit that would actually apply to those kWh. Time-of-use and tiered feed-in tariffs can make a headline rate misleading. This comparison assumes self-consumption reduces paid exports; older gross-metered arrangements require a different calculation.
Holding energy needs and all other charges equal, a hypothetical 10c/kWh export credit and 10c/kWh off-peak import price produce the same $1.80 energy cost per 100 km. At a 15c export credit, using that solar for the car gives up $2.70. Exporting it and charging later for $1.80 would be $0.90 better on this narrow comparison – provided those rates and the later charging opportunity are genuinely available.
Some solar output would otherwise be curtailed because it cannot be exported. Capturing that would-be-curtailed energy can have zero forgone export income, provided there is no other valuable use for it. That applies only to the affected energy, not every kilowatt-hour generated by the system.
A home battery adds another alternative. Diverting solar into the car may leave less stored energy to avoid expensive household imports later. Charging through a battery also introduces storage losses and battery wear. The direct-solar figure above should not be presented as a stored-solar cost.
Before changing plans, compare the complete bill: import rates and charging windows, daily supply charges, any demand charges, export credits and eligibility conditions. IPART cautions that a higher feed-in tariff can come with higher prices elsewhere in the plan.
Use electricity bills, charger records and solar monitoring to establish when the car is home, how much energy it needs and how much surplus is available. A longer period of interval data is more useful than one sunny day; generation and household use vary across seasons.
Cheap charging is not the same as a solar-system payback result. Existing equipment and a new installation are different decisions, and any additional hardware cost must be weighed against the extra savings it enables.
The useful test is not “Did the electricity come from the roof?” It is “What would that electricity otherwise have earned or saved, and what would charging the car cost instead?”
For the assumptions here, surplus solar wins – but the size of the benefit depends on the alternative.
Author: Oliver S, Evital, a vehicle directory and research hub focused on EVs in the Australian market
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
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