Two thousand megawatt-hours. That’s roughly what Genex Power’s Kidston pumped hydro scheme, built into an old gold mine near Georgetown in north Queensland, is designed to deliver at 250 megawatts of output, running flat out for eight hours. Compare that with the Victorian Big Battery near Geelong, rated at 300 megawatts but only 450 megawatt-hours of storage — a two-and-a-bit hour tank at full output. Same grid, same job description on paper. Wildly different shape underneath.
That difference is the whole story of pumped hydro versus batteries for firming, and it’s a story most coverage gets muddled because it talks about megawatts as if that number alone tells you what a project does. It doesn’t. Power tells you how hard something can push. Energy tells you how long it can keep pushing. Firming the grid needs both, and the two technologies split that job very differently.
Two ways to store energy, one very different shape #
A big battery is essentially a warehouse of lithium cells wired into inverters, sited wherever there’s grid capacity and land, built in twelve to eighteen months once approvals are through. A pumped hydro scheme is two reservoirs at different elevations, a tunnel or penstock, and reversible pump-turbines — Kidston uses the old open-cut pit as the lower reservoir and a new upper dam on the ridge above it. Water goes up when there’s cheap solar in the middle of the day, comes down through the turbines when the grid needs it in the evening peak.
The physics dictates the economics. Batteries are compact, fast to build, fast to respond — sub-second, genuinely useful for frequency control as much as for shifting energy. Pumped hydro needs civil works, years of approvals, and a site with the right topography. But once it’s built, the marginal cost of adding storage duration is mostly civil engineering — a bigger reservoir — rather than more cells. That’s why pumped hydro schemes tend to be quoted in many hours or days of storage, while grid-scale batteries are still mostly built for two to four hours.
The number everyone gets sloppy with #
Let’s be careful with that number, because this is where a lot of public commentary goes wrong. A battery announced as “300 megawatts” tells you almost nothing about how much energy it holds unless the duration is also stated. AEMO’s own planning documents, including the Integrated System Plan, are explicit about this: they model storage in megawatt and megawatt-hour pairs precisely because the NEM’s problem isn’t a shortage of instantaneous capacity, it’s a shortage of capacity that can sustain output through a still, cloudy evening.
| Project | Power (MW) | Energy (MWh) | Duration at full output |
|---|---|---|---|
| Kidston Pumped Storage (Qld) | 250 | ~2,000 | ~8 hours |
| Victorian Big Battery (Geelong) | 300 | 450 | ~1.5 hours |
| Hornsdale Power Reserve (SA, original build) | 150 | ~194 | ~1.3 hours |
Read across that table and the pattern is obvious. Batteries lead on power-to-cost ratio and speed of deployment; pumped hydro leads on sustained duration. Neither number is better in the abstract. It depends entirely on what gap in the market you’re trying to fill, and that’s the bit that gets lost when a press release just says “megawatts.”
Why duration decides the winner #
Most of the grid’s firming problem right now isn’t a five-minute gap, it’s a four-to-eight-hour gap — the evening period after rooftop solar drops off and before overnight demand tapers, sometimes stretching across a whole low-wind week. AEMO’s dispatch engine settles the market every five minutes, as I’ve written about before, but the reliability question that keeps planners up at night is measured in hours and days, not dispatch intervals.
A fleet of two-hour batteries can smooth the sharp edges of that evening peak and do frequency response brilliantly. What it can’t do on its own is ride through a genuinely long dry spell of wind and solar, the kind south-east Australia gets a few times a year. That’s the job pumped hydro, and to a lesser extent long-duration battery chemistries now entering the market, are actually built for. Snowy 2.0 is the biggest bet on this in the country, and it’s worth reading how that project’s cost and timeline blowouts have played out before assuming any pumped hydro scheme is a straightforward build.
The build-time and money trade-off #
Here’s the part project developers don’t love dwelling on. A battery of a few hundred megawatt-hours can go from financial close to energised in under two years, largely because the supply chain is mature and the civil works are modest. Pumped hydro is a different animal: geotechnical studies, water licences, often a decade from concept to commissioning. Queensland’s own pumped hydro ambitions under its state energy plan reflect that timeline, banking on projects that won’t deliver firming capacity until well into the 2030s.
Capital cost per megawatt-hour tends to favour pumped hydro at long duration and batteries at short duration, but that’s a generalisation that shifts constantly as battery cell prices keep falling. The Clean Energy Finance Corporation and ARENA have both backed projects on both sides of this ledger, which tells you the answer isn’t ideological, it’s project-by-project.
What the Capacity Investment Scheme is actually paying for #
The Commonwealth’s Capacity Investment Scheme, which I’ve covered in detail elsewhere on this site, doesn’t discriminate by technology. It runs tenders for dispatchable capacity and lets pumped hydro, batteries and other firming technologies compete on price and reliability contribution. That’s sensible in theory. In practice it means the scheme’s contracted mix so far leans heavily towards batteries simply because they’re faster to bid, faster to build and easier to price. Whether that’s picking winners by default rather than by design is a question I’ve put to the scheme’s early results before, and I don’t think it’s been settled.
Where I land on this, for what it’s worth #
My honestly contrarian take: I think the market has over-rotated towards short-duration batteries because they’re the easy yes — quick approvals, quick construction, quick revenue from frequency and arbitrage markets. That’s a rational developer response to the incentives on offer, but it leaves the genuinely hard multi-day firming problem still mostly unsolved, propped up by gas peakers and whatever coal capacity hasn’t shut yet. Eraring’s life extension into 2027 is a symptom of that gap, not a coincidence.
I’ll admit I used to think batteries alone could close that gap if you just built enough of them. The maths doesn’t quite work that way — stacking four-hour batteries to cover an eight-hour shortfall is possible, but you’re paying for a lot of idle steel and lithium most of the year to cover a handful of tight weeks. Pumped hydro’s reservoirs sit there doing the same job more cheaply once built, they’re just brutally slow and expensive to get started.
Two technologies, two different races #
I ride long climbs on weekends when the roster allows it, and the analogy that keeps coming back to me is pacing. A battery is the sprint effort — explosive, precise, over in minutes, and it needs to recover before it can do it again. Pumped hydro is the long climb, paced for the distance, slower off the mark but built to keep delivering for the whole ascent. The NEM needs both riders in the bunch. It just needs planners, and readers, to stop assuming a headline megawatt number tells you which one you’re looking at.
Projects like Kidston, Snowy 2.0 and the various batteries now scattered across the grid from Geelong to the Hunter aren’t competing for the same job so much as splitting a job that used to be done almost entirely by coal and gas. Whether the split ends up in the right proportion is still, genuinely, an open question — and one AEMO’s next Integrated System Plan update should sharpen considerably.
— Anjali Rao, Grid & Storage Correspondent
Photo by Kiran Naidu on Unsplash