Intermittent renewable electricity generation is increasing in many power systems. Long-duration electricity storage (LDES) could complement this development to decrease curtailment and improve system cost-effectiveness. We investigate levelized costs of storage (LCOS) and investment profitability for prospective LDES technologies. Using granular German power market data, we optimize each technologies’ bidding schedule. We find that under current costs, only pumped hydro is profitable while also achieving the lowest LCOS by far. Assuming technological progress to maturity and improved financing conditions, iron redox flow batteries outperform and gravity storage converges to the pumped hydro benchmark of 47-54 EUR/MWh LCOS and 7-15% return on equity. Yet, meeting investor expectations is sensitive to cost and price volatility assumptions. Furthermore, additional revenue from a higher duration generally does not compensate for associated higher costs. Subsidies of 73-83 EUR/kWh or mechanisms enabling further revenue streams might be necessary to mobilize private investment into promising LDES.