Home battery arbitrage works because the individual is too small to move the market. But what happens when 100,000 Irish homes all charge at 2AM and discharge at 5PM?
Here's the thing about temporal arbitrage that the LessWrong post gets right in theory but understates in practice: the opportunity exists precisely because nobody is exploiting it yet.
Right now, Irish wholesale electricity prices follow a predictable pattern: cheap overnight (wind + low demand), expensive in the evening (high demand + gas marginal). A home battery buys the spread. But if enough batteries start charging at 2AM, that extra demand raises overnight prices. And if enough batteries discharge at 5PM, that extra supply lowers evening prices. The spread compresses.
Ireland's electricity system clears about 5,000 MW of peak demand and 83 GWh per day. A single home battery (10 kWh, 5 kW) is noise — 0.0001% of daily consumption. Even 100,000 homes with batteries (roughly 6% of Irish households) would aggregate to 1 GWh / 500 MW — about 1.2% of daily consumption and 10% of peak demand.
That's not nothing, but it's not catastrophic either. For context, the grid-scale BESS pipeline in Ireland is 10 GW across 155 projects. If even a fraction of that gets built, it dwarfs the home battery fleet. Grid-scale batteries are the ones that will compress spreads, not residential ones.
We can estimate the price impact using the SEM bid-ask curve data. From our analysis of March 2025 – March 2026 auction data, a 50 MW demand shift moves the clearing price by a median of EUR 0.80/MWh. Scaling linearly (a simplification, but reasonable for small shifts):
At 100,000 homes, the fleet adds 500 MW of coordinated demand during off-peak and 500 MW of supply during peak. This would shift prices by roughly EUR 8/MWh — about 10% of the typical wholesale spread. Noticeable, but not enough to kill the arbitrage.
At 500,000 homes (2,500 MW / 5 GWh), you're looking at a EUR 40/MWh price shift — which would eliminate roughly half the wholesale spread. At this scale, home battery arbitrage starts eating its own tail.
Ireland has about 1.7 million households. As of mid-2025, roughly 155,000 have solar panels, and a much smaller fraction have batteries. Even optimistic projections suggest 300-500k solar+battery homes by 2030. At 300k homes with an average 10 kWh battery, the fleet would be 3 GWh — meaningful but not spread-destroying.
More importantly, the spread isn't static. As Ireland adds more renewables (the Climate Action Plan targets 80% renewable electricity by 2030), wholesale price volatility increases. More wind means cheaper troughs; more gas-marginal peaks means expensive evenings. The spread may grow faster than batteries can compress it.
For an individual homeowner considering a battery today, fleet-scale spread compression is not a reason to hesitate. The economics are driven by your tariff spread (which is fixed by your supplier contract) and your system cost (which is driven by your installer quote), not by what your neighbours are doing. The wholesale spread that feeds dynamic tariffs will compress somewhat as battery penetration grows, but we're years away from the point where it matters at residential scale.
The real threat to home battery economics isn't spread compression — it's tariff changes. If suppliers eliminate EV night rates or raise off-peak prices (which they have commercial incentive to do as battery uptake grows), the arbitrage window narrows regardless of wholesale dynamics. Lock in a good tariff plan now.