Drive south from Bunbury on the Coalfields Highway and you hit Collie before you’ve finished your coffee: a town of maybe 9,000 people that has run Western Australia’s coal fleet for the better part of sixty years. What’s going up on the outskirts now, next to switchyards that used to only ever move power one way, is a pair of battery projects big enough that AEMO’s own Western Australian planning documents treat them as load-bearing for the whole South West Interconnected System. The Collie battery build is not a side project. It’s the thing standing between the SWIS and a reliability gap once Muja and Collie power stations finish closing out this decade.
Let’s be careful with the numbers here, because this is exactly the kind of project where megawatts and megawatt-hours get swapped around in press coverage until nobody’s sure what’s actually being compared. Synergy’s Collie battery, built with Wärtsilä as the technology partner, is a 500 MW / 2,000 MWh system – four hours of storage at full output. Neoen’s Collie battery, a separate project a short drive away, is 219 MW with roughly 877 MWh of capacity, again around four hours’ duration. Add them together and you get something north of 700 MW of instantaneous power sitting in the SWIS, discharging for the better part of a working day if called on hard. That’s a genuinely large slab of capacity for a grid that peaks at around 4,000 MW in summer.
What’s actually built and what’s still coming #
Synergy’s project has been staged. The first portion of the Collie battery energy storage system came online through 2024 and 2025, with Synergy’s own project updates confirming progressively larger blocks reaching commercial operation. By late 2025 the bulk of the 500 MW capacity was dispatching into the Wholesale Electricity Market, making it one of the largest standalone batteries anywhere in the country, let alone in an isolated grid with no interconnector to lean on.
Neoen’s Collie battery has followed a similar staged path, sitting alongside the company’s existing Western Australian footprint. Neoen is no stranger to big storage: the Hornsdale Power Reserve in South Australia put the company on the map for battery scale, and I’ve written before about how that pioneering position shaped the rest of its Australian portfolio (see Neoen Australia: the battery pioneer now building wind at scale). The WA project is smaller than Synergy’s but still material next to the total SWIS generation stack, which runs to a bit over 6,000 MW of registered capacity across all fuel types.
Why the SWIS needed this more urgently than the NEM did #
Western Australia’s grid doesn’t get the attention the National Electricity Market does, mostly because it’s a third of the size and genuinely cut off: no interconnector, no NEM backup, nothing. I’ve gone through this in more detail in an earlier piece on WA’s SWIS grid: the island nobody can send power to, and the point stands here too: when Collie and Muja retire, there’s no neighbouring state to borrow capacity from during a heatwave peak. Everything the SWIS needs, the SWIS has to build.
The WA government’s South West Interconnected System plans have set retirement dates for the coal fleet – Muja’s remaining units and the Collie power station closing out progressively through the back half of this decade, part of the state’s broader move to exit coal generation by 2030. That’s a tighter runway than most of the NEM states are working with. NSW, for comparison, has been managing a staggered coal exit against a Renewable Energy Zone build-out that I’ve covered as a widening gap in its own right (see NSW’s coal exit and the REZ rollout: a widening gap). WA doesn’t have the luxury of staggering across multiple REZs feeding one market. It’s one grid, one government owner in Synergy, and a shorter clock.
The capacity question versus the energy question #
Here’s where I’d push back gently on some of the breathless framing around these batteries “replacing” coal. They don’t, not directly, and it’s worth being precise about why.
Collie power station alone has historically supplied something in the order of 300 MW of continuous, round-the-clock output: coal plants run at high capacity factors, often above 70-80 per cent annually, because that’s the economics of a plant with high fixed costs and comparatively low marginal cost once it’s running. A four-hour battery, no matter how large its headline MW figure, delivers energy in a completely different shape: a sharp, short burst matched to the peak, then it’s flat until someone recharges it, typically from solar in the middle of the day.
So the honest comparison isn’t MW for MW. It’s asking whether 700-plus MW of four-hour storage, charged mostly from WA’s considerable and still-growing rooftop and utility solar fleet, can cover the evening peak that coal used to carry continuously. For peak-shaving and the evening ramp, the hours after the sun drops and demand is still high, batteries do this job well, arguably better than coal ever did, because they respond in seconds rather than requiring hours of notice to ramp. For the 2am trough on a still, cloudy winter week, batteries alone don’t solve it, and WA still leans on gas peaking plants and existing gas baseload for that gap. I’ve laid out this comparison in more detail in gas peakers versus big batteries: who actually wins the firming job, and the conclusion holds for Collie too. It’s a complementary technology stack, not a straight swap.
A worked example, because the numbers get thrown around loosely #
Take Synergy’s 500 MW / 2,000 MWh asset on its own. At full discharge, it can supply 500 MW for four hours before it’s empty. That’s the simple division, 2,000 MWh divided by 500 MW. If WA’s evening peak runs for roughly three hours of genuinely tight demand, the battery can carry a meaningful chunk of that window on its own, assuming it started the evening fully charged.
But fully charged is doing a lot of work in that sentence. If the battery discharged partially the previous evening and wasn’t topped up by midday solar – say a run of overcast days – its available energy at 6pm could be well under its rated capacity. This is the bit that gets glossed over in headline MW numbers: a battery’s contribution to reliability depends on its state of charge at the moment it’s needed, which depends on weather and dispatch decisions in the hours before. AEMO’s capacity adequacy assessments for the SWIS factor this in explicitly, derating battery contribution rather than counting the full nameplate figure, which is the right way to do it even if it makes the headline numbers look less impressive.
Who’s paying, and the ownership split that matters #
Synergy is the state-owned generator and retailer, so its Collie battery sits on the government’s own balance sheet, financed through Western Australia’s broader energy transformation funding rather than a market-driven investment case alone. That’s a genuinely different risk model to Neoen’s project, which is a private developer betting its own capital (and project debt) on the WA market delivering adequate returns through the Wholesale Electricity Market’s capacity mechanism and energy arbitrage.
It’s a pattern I keep coming back to across the country – public money underwriting the big strategic pieces, private capital picking up the rest where the returns pencil out on their own. Queensland’s doing something structurally similar with its pumped hydro bets, which I went through in Queensland’s energy plan rests on pumped hydro and public money. WA’s version is smaller in dollar terms but arguably cleaner, because Synergy already owned the sites and the grid connection infrastructure from the coal plants it’s retiring, which strips a meaningful chunk of cost and delay out of the build compared with a greenfield project chasing a new connection point.
What could still go wrong #
Battery projects at this scale carry risks that are boring compared to coal plant failures but no less real: supply chain timing for battery cells and inverters, grid connection studies, and the commissioning process that, as anyone who’s watched a big BESS come online knows, rarely runs exactly to the first announced schedule. Neither Synergy nor Neoen has reported major public setbacks on Collie specifically, but the staged commissioning both projects have gone through is itself a signal that moving from first energisation to full nameplate dispatch takes longer in practice than a press release timeline suggests.
There’s also a fire safety and siting question that gets less attention than it should. Big lithium-ion batteries sit under Australian Standard AS/NZS 5139 for electrical installations, and WA’s Department of Fire and Emergency Services has had input into siting and emergency response planning for projects of this scale, as it has for comparable batteries on the east coast. None of this is exotic by 2026. There’s a reasonable base of operating experience across the NEM and now the SWIS, but it’s not nothing either, and it’s part of why commissioning timelines stretch.
The bit that actually matters for Collie the town #
There’s a human dimension worth not skating past. Collie has run on coal jobs for generations, and the transition plan for the town, canvassed in WA government and union discussions around worker transition funding, depends partly on these battery projects and associated construction employing locally, at least during the build phase. Battery operations, once running, employ a small fraction of the workforce a coal station needed. That’s not a criticism of the technology; it’s just the arithmetic of a plant that doesn’t need to be fed, watered and maintained the way a combustion plant does. I’ve covered a similar transition dynamic in Victoria’s Latrobe Valley, where the Yallourn closure has forced a comparable reckoning (see Yallourn’s 2028 closure: the battery and the workers racing the clock), and Collie is running the same experiment a few years ahead of schedule in a much smaller, more isolated grid.
My own read, for what it’s worth: WA has actually sequenced this better than some of the east coast’s coal exits, mostly because it had no choice. There’s no interconnector to blame for a shortfall, so the state government moved early and owns the biggest piece of the replacement capacity outright. Whether 700-plus MW of four-hour batteries plus existing gas is enough once the last coal unit at Muja shuts will come down to how well AEMO’s demand forecasts for the SWIS hold up against a state that’s still adding rooftop solar faster than almost anywhere else in the country. For the full picture, AEMO’s published Electricity Statement of Opportunities covers the WA market specifically and is worth reading directly rather than relying on press summaries of it.
I’ll be watching the state-based capacity mechanism auction results over the next couple of cycles. That’s where we’ll see whether the market, not just the government, thinks this mix is actually enough.
– Anjali Rao, Grid & Storage Correspondent
Image: Thennicke (CC BY-SA 4.0) via Wikimedia Commons