Diesel was $2.84 a litre at a Ballarat servo on 31 October 2026. How to size a solar pump by head and flow, and when it needs a battery.
Most stock and transfer pumping can come off diesel. If the job is filling a tank or a trough during the day, a solar direct pump with no battery does it, because the tank is the storage. A battery, or a hybrid controller that blends in the grid or a generator, only earns its place when water has to move at night, on demand, or at a pressure a tank cannot give you. The way to decide is to work out the head, the daily volume and what your diesel is actually costing per hour.
What diesel costs per hour of pumping
Diesel at the Shell in Sebastopol, on the Ballarat side of our patch, was $2.839 a litre on 31 October 2026 according to the state's Servo Saver data, and its average since 3 August was $2.772. That price includes GST. A farm business registered for GST claims the GST back, which takes it to $2.581, and then claims the fuel tax credit. The ATO rate for diesel used off public roads, including in a generator or a pump engine, is 53.7 cents a litre for fuel bought from 3 August 2026. That leaves a net cost of about $2.04 a litre.
How many litres an hour depends on how hard the engine is working. Cummins publishes the fuel table for its C22 D5, a common 20 kVA diesel generator rated at 16 kW prime power:
- quarter load (4 kW): 2.5 litres an hour
- half load (8 kW): 3.2 litres an hour
- three quarter load (12 kW): 4.1 litres an hour
- full load (16 kW): 5.2 litres an hour
At three quarter load the set burns about 0.34 litres per kWh. At quarter load it burns 0.63 litres per kWh, nearly twice the fuel for the same work. A big generator running a 4 kW bore pump sits at quarter load, which is exactly where diesel power is dearest.
A worked example: a bore pump on a generator
Take a bore that has to lift 30 cubic metres a day against 60 metres of total head (the depth to water plus the height to the tank plus the pipe friction).
- Hydraulic power is flow times head times gravity. At 12 cubic metres an hour and 60 metres, that is 12 times 1,000 times 9.81 times 60, divided by 3.6 million, about 2 kW of water power.
- Pumps and motors lose energy on the way. Australian Pork's farm audits found centrifugal pumps that should run at 60 to 75 percent efficiency were often measured at 30 to 50 percent. At 50 percent overall, the motor draws about 4 kW.
- At 12 cubic metres an hour the 30 cubic metres takes two and a half hours, or about 10 kWh a day.
Run that from the 16 kW generator above at quarter load and it burns 2.5 litres an hour, so 6.25 litres a day. At $2.04 a litre that is about $12.75 a day, or about $4,650 a year if the pump runs every day of the year. Per unit of energy it is about $1.28 per kWh, before oil, filters, servicing and the drive out to start it.
For comparison, the Victorian Default Offer for a small business on Powercor's network from 1 July 2026 is 27.36 cents per kWh including GST on a flat tariff. Light load diesel costs several times that, and the grid is rarely at the bore anyway.
Solar direct pumping: the tank is the battery
A solar direct pump runs straight off the panels through a controller that slows the pump as the sun drops. There is no battery to buy or replace. Water goes up the hill while the sun is out, and gravity delivers it day and night.
That is how a free range piggery at Tooborac runs its stock water, in a case study from Agriculture Victoria's Energy Smart Farming program. Two trailer mounted solar pumps move between three dams and refill a header tank on the highest point of the farm by day, and gravity supplies troughs and a cooling spray around the clock. A solar powered level monitor texts the owners when the level moves outside set limits. They report the pumps have nearly eliminated the farm's petrol use.
Sizing a solar direct pump comes down to three numbers:
- Total head in metres, including friction in the delivery pipe.
- Daily volume in the worst month you need it, which for stock water in Central Victoria is often summer and for some systems is winter.
- Sun in that month. The European Commission's PVGIS model puts a north facing array at Trentham, tilted 30 degrees, at about 4.0 kWh per installed kW per day across the year, 5.25 in January and 2.43 in June.
In the example, 10 kWh a day in June needs a little over 4 kW of panels, and in January about 2 kW. Oversize the array so the pump starts earlier, and size the tank for two or three overcast days.
When a battery or a blend makes sense
A battery belongs in a pumping system when the water cannot wait for the sun:
- pressure systems for a house, a dairy or a wash down, where a tap opening at 9 pm has to get water
- night irrigation, or a pump that must run long hours in winter
- fire fighting, where the pump has to start in smoke when the panels are making little
- sites where the tank cannot be big enough or high enough
The alternative to a big battery is a hybrid pump controller. LORENTZ, the German solar pump maker, sells its PSk3 controller in 7 and 15 kW versions for three phase pumps. It runs on solar alone, blends in grid or generator power when demand needs it, or runs on the second source alone, and an optional SmartStart starts the generator automatically. The generator then runs only for the shortfall.
Agriculture Victoria's Ellinbank SmartFarm in Gippsland took the battery route for its dam pumping station: a 9.96 kW array of 24 panels, a 10 kW inverter and a 10 kWh zinc bromine flow battery.
Where a pump sits on a property that is already off grid, it becomes a load on the main system and we size it into the bank and the generator like any other.
What to measure before you call anyone
- depth to standing water and pumping water level in the bore, or the lift from the dam
- height from the pump to the top of the tank, and the length and diameter of the delivery pipe
- the volume you need in the hottest week and in midwinter
- the pump's nameplate (kW, voltage, single or three phase)
- your litres of diesel per week now
With those numbers the array size and the battery question answer themselves.
Questions
Can a solar pump run at night?
Not on its own. A solar direct pump stops when the sun goes, so a tank supplies water overnight. To pump at night you need a battery, the grid, or a generator blended in by a hybrid controller.
How many solar panels does a bore pump need?
Work out the daily kWh the pump draws (its kW times the hours it runs), then divide by the sun per kW in the worst month. In Trentham PVGIS gives about 2.4 kWh per kW per day in June and about 5.2 in January, so a pump drawing 10 kWh a day needs roughly 4 kW of panels for winter and 2 kW for summer.
Do I still get the fuel tax credit if I keep a diesel pump as backup?
Yes, for the litres a business uses off public roads, at the rate on the day the fuel was bought: 53.7 cents a litre from 3 August 2026.
If you send us your head, volume and current diesel use, we will come back with a pump and array size, or start with the sizing tool if the pump is part of a whole off grid property. Our off grid sizing guide explains how we plan for winter.
Sources
- ATO, fuel tax credit rates for business, 1 July 2026 to 30 June 2027
- Shell Sebastopol diesel price, Servo Saver data via fuel prices EU (read 31 October 2026)
- Cummins C22 D5 generator set data sheet DS340-CPGK
- Engineering ToolBox, pump power calculation
- Australian Pork Limited, Understanding energy in pig production (May 2018), pump efficiency section
- PVGIS, European Commission Joint Research Centre (Trentham, 30 degrees, north)
- Energy Smart Farming, McIvor free range piggery case study (Agriculture Victoria)
- Energy Smart Farming, Ellinbank SmartFarm solar and battery dam pump fact sheet (January 2022)
- LORENTZ PSk3 hybrid solar pumping controller data sheet
- Essential Services Commission, Victorian Default Offer 2026 to 27
Written by the Trentham Energy team.
