By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
I had a 120-metre run from the panels down by the creek up to the tank on the ridge, and for six months I tried to get away with a 12 V pump and two different so-called heavy‑duty PWM controllers—a generic 30 A unit from a rural supply shop ($95) and later a supposedly better Epver Triron 40 A ($230). Both cooked inside a fortnight when the ambient temperature sat above 40°C day after day.
The wiring alone was 16 mm² copper, and voltage drop at 12 V across 120 metres with a pump pulling 13 A was enough that the controller tried to compensate by pushing current even harder during the midday heat, baking the MOSFETs. That destroyed three controllers in total before I stopped being stubborn. The fix was a 24 V system. I put two 12 V 250 W panels in series, swapped the pump to a 24 V helical‑rotor model (a Grundfos SQFlex 3 SQF‑2 cost $1750), and fitted a Morningstar ProStar MPPT 40 A controller at $345.
Running the same 120 metres at 24 V halved the current to about 6.5 A, which turned the voltage drop from a crippling 25% down to a manageable 6%. The controller now sits inside a ventilated steel box on a south‑facing wall and never touches its absorption cutoff even in central Queensland’s January heat. I didn’t lose another component. The only sane choice for pumping uphill here is 24 V, full stop.
— Dave. Measure twice, buy once.
ArticlesCalculatorsSite PlannerShopA solar pump controller that cooks itself on a January afternoon isn’t a nuisance — it’s a dry trough and a mob of cattle breaking a fence to find a dam. I’ve unbolted enough burnt-out power boards from the side of a bore to know the difference between a box that survives and a box that strands you. The job isn’t complicated: take the ragged DC off a string of panels, keep the voltage inside the pump’s sweet spot, and cut out fast if the bore runs dry.
When it works, a helical-rotor pump runs cool at 30–48 V and pulls maybe 5–8 A through a 6 mm² twin-core down the hole, delivering 1,200 litres an hour into a poly tank 300 metres away. Screw it up and the thrust bearing welds itself because the controller never backed off the current when the water level dropped. Common hardware gives you a feel for the cost. A basic PWM unit with low-water shutoff — something like a standard Shur
I still remember the Grundfos SQFlex controller I opened up at a property out past Blackall. Cockatoos had pecked through the conduit and a hard short had welded the input terminals into a single blob of copper. The owner had been without water for three days, and that $650 controller was now a paperweight.
A solar pump controller is just a handful of mosfets, a microcontroller and some protection circuits, but out here it sits between your bore and the four panels on a star picket rack, and if it fails you don’t just lose pressure—you lose stock water, showers, and the beer fridge heat-exchange if you’re pumping from a dam. Basic 30 A MPPT units with no soft-start start at $220; a proper sealed unit with dry-run protection and low-water cutoff runs $500 to $900 trade, before you add a float switch and a couple of DIN-rail breakers.
A 1.5 kW helical rotor pump paired with a Morningstar ProStar MPPT 40 A
A basic PWM solar regulator — the sort you find in a $30–$50 plastic box at the rural supply, often badged PowMr or unbranded — just connects the panel straight to the battery when it’s bulk charging. A nominal 200 W panel with a maximum power voltage around 20 V gets dragged down to 12.5–14.5 V the instant the clamps hit the battery. The voltage gap goes straight to heat, and on a 40°C day in western Queensland that heat cooks the board inside the shed.
Net result: 20–30% of the sunlight power that hits the panels never makes it to the pump. An MPPT controller runs a fast DC‑DC converter that lets the panel sit at its true maximum power point — typically 17–18 V for a 12 V nominal panel on a hot roof — and transforms the extra voltage into extra charging current. Same 200 W panel, same bore, 25–30% more litres in the tank by sundown.
I stock Victron SmartSolar 100/20 units out here; they run $180–$220 and log every watt‑hour so you can see exactly how much water you lost yesterday before the swap. On a stock‑water trough fed by a 24 V Shurflo 9300 diaphragm pump, going from a PWM ProsStar‑10 to a Tracer 2210BN MPPT lifted daily yield from roughly 1800 L to over 2300 L — no extra panel, no new cable.
The pump starts earlier in the morning and keeps running a few minutes later when cloud scuds past, all because the controller isn’t throwing away a quarter of the panel’s oomph.
Practical tip: Don't buy a PWM controller. Ever. If your pump is 12V or 24V, match the controller to that. A 12V pump with a 24V controller? You'll fry it. Simple.
Float switches stop the pump when the tank's full. No float switch? Pump runs dry. Dry pump = dead pump. Done. You'll be buying a new one. $500+ gone.
Practical tip: Install TWO float switches. No clever circuits, just a switch for “full” that stops the pump and one for “low” that starts it. That differential is your cheap insurance against a pump hammering on and off every time a cow slobbers in the trough. At the tank, cable-tie the low switch to the poly riser so the float hangs 150–200 mm above the bottom outlet. That keeps the pump from gulping sludge and stops it losing prime when the water gets shallow. Mount the full switch 100 mm below the overflow.
When the tank is full, the float rises to vertical, the internal microswitch opens, and the pump cuts out. No electronics needed—it’s a mechanical tilt switch in a moulded body. Bunnings sells basic ones for $25–$40. The float bodies are typically HDPE with a 5-metre H07RN-F cable hanging out the back. Get the stainless steel ones—they last. The plastic-bodied models with nylon pivot pins turn brittle after three or four Cape York wet seasons, then a $35 switch decides the bore pump should run all night.
The stainless version uses a glass-reed switch sealed in a stainless stem and a stainless float collar. Same price bracket, three times the lifespan in hard water. Wire the floats in series with the pump‑run circuit using a weatherproof junction box on a tank stand post. No screw connectors full of mud—use gel‑filled crimps or a small IP65 enclosure with cable glands. The low‑level float is normally closed when hanging down (tank low, pump on), and opens when the water lifts it to about 45 degrees.
The high‑level float is normally closed when dangling (tank not full) and opens when it goes vertical. Both must be closed for the pump to run. If a lizard crawls into the float shroud and jams it, the pump just stops—you’ll notice a dry trough, not a drained bore and a burnt winding. Check the floats once a year when you’re doing the anode. Spin them, clean the slime off the pivot, make sure the cable hasn’t rubbed through on a tin edge. A five-minute look saves a Saturday chasing trench collapses.
Dry run happens when the pump runs without water (bore runs dry, tank empty). It overheats and destroys the motor. Most controllers don't have this built-in. If yours doesn't, you're gambling with your pump.
A pump running dry will cook its impeller and shaft seal in under two minutes. In a bore or a dam-fed setup the water level drops, the pump gulps air, and the friction heat wrecks it before you notice the tank isn’t filling. That’s why every pump controller worth fitting has a way to detect no flow and cut power. Check the specs printed on the side or in the manual — if you see “dry run protection” or “low flow protection” listed, the brains are already inside.
Victron and Renogy solar pump controllers include it as standard, so the feature is baked in from the factory. If your controller lacks it, fit a standalone dry run sensor. These are simple paddle flow switches or adjustable pressure switches with a low-pressure cutoff, wired in series with the pump circuit. In the paddock, I keep a couple of RainFlo inline flow switches in the ute — they run $35 to $70 depending on pipe size and switch rating, and they just screw into 25 mm poly with two BSP threads.
When water stops moving, the paddle drops and the microswitch opens. No flow, no power, no new pump. Don't skip it.
A 100W panel is enough for a shallow spear-point pump lifting water 5 or 10 metres, the kind of setup that keeps a trough damp in the wet season. For a 50m bore, 300W+ of panel is the minimum. Anything less and a helical rotor pump won’t hit startup torque until midday, then it gurgles froth for an hour before the low-voltage cutout kicks in. You end up pumping mostly air, which does nothing for the tanks.
Three hundred watts typically means two 150W panels in series on a single-axis ground mount — around $400 for the glass, another $150 for galvanised Unistrut and tek screws. A decent MPPT pump controller like a PS2-150 or an SQE-02 will sit between $500 and $800, and a 0.37 kW helical rotor pump adds $900–$1,400 depending on brand. Over-size the array beyond the pump’s rated input and you’re just cooking the controller’s caps for no extra water — spend the leftover budget on a float switch and a bypass valve instead.
Bores deeper than 80 metres often need 400–600W to lift a usable stream all day, but a 50m bore on 300W+ done right will deliver 1,200–1,800 litres by sundown in summer at 28° South. Anyone selling you a kit without first calculating total dynamic head and the pump curve is guessing. Pay a local sparky who can read a water meter and a multimeter — the $200 fee is cheaper than a dry trough in October.
Forget the Chinese knockoffs. Get Australian stock. No delays, no warranty headaches.
Renogy 10A MPPT Controller (12V/24V, includes dry run protection) $275 at Bunnings (online) or Off Grid Supplies. Best for small bores. Don't buy the 5A version—it's a joke for anything serious.
Victron SmartSolar MPPT 150/35 (24V, full dry run + float switch support) $680 at Solar Wholesale or Powerhouse. Overkill for small setups? Maybe. But it's the only one that won't fail when the sun's at 45°C. Worth it.
Stainless Steel Float Switches (2-pack, for tank/bore) $35 at Bunnings or Energy Save. Don't use plastic. They crack in 6 months. Stainless is $5 more but lasts 10 years.
MPPT controller: $250-$650. Float switches: $35. Dry run protection: built-in or $35. Total: $300-$700. New pump: $1,000-$2,500. Math is obvious. Don't be the bloke who buys a $300 controller and a $1,500 pump because he skipped the float switch.
Worth a watch: The ULTIMATE GUIDE to Off-Grid Solar Water Systems: Simple & Cheap to Fancy & Complex · Tiny Shiny Home
You can, but you'll be throwing away most of your sunlight. A proper MPPT controller matches the pump's needs to what the panels are actually putting out, so you get more flow for the same sun. Without one, your pump either runs poorly or cooks itself, and a dead pump is an expensive lesson.
A float switch sits in your tank and tells the pump when to stop once the water's high enough. Without one, the pump keeps pushing water until it has nowhere to go, which burns it out fast. Pair it with dry run protection and you've got a setup that looks after itself while you're not there.
Most of the time it's running dry — bore drops, tank empties, pump keeps grinding. That's what I call the silent pump killer, because you don't notice until the motor's cooked. A float switch at the tank and dry run protection at the bore will stop it happening again.
Depends on what's involved. Swapping a controller on the low-voltage DC side is something a careful DIYer can handle. The moment you're touching 240V mains, a licensed sparkie isn't optional — it's the law, and your insurance won't cover a stuff-up. If you're not sure which side of the fence you're on, ask before you start pulling wires.
While many off-grid projects are achievable as DIY, certain situations require licensed professionals:
Always check local regulations and obtain necessary permits before commencing work.