As global power systems transition towards higher shares of variable renewable energy, ensuring a reliable, continuous electricity supply has become both a systemic integration challenge and an increasingly viable plant-level design objective. This paper introduces the firm levelised cost of electricity (F-LCOE) framework to assess the economics of hybrid power plants — combining utility-scale solar PV or onshore wind with co-located battery energy storage — as a transparent, replicable benchmark for comparing firm renewable power with conventional dispatchable generation.

Using a linear optimisation model applied to IRENA's global database of hundreds of utility-scale projects, we determine the least-cost combination of generation overbuild and storage capacity required to deliver a constant power output across reliability targets ranging from 90% to 99%. Our findings show that firm renewable costs have fallen sharply — from above USD 100 per MWh in 2020 to USD 54-82 per MWh at high-quality locations in 2025 — and that in leading markets such as China, hybrid systems already deliver firm electricity well below the cost of new coal and gas-fired generation across a wide range of reliability targets. Further cost reductions of 30-40% are projected by 2030-2035 as solar, wind, and battery storage costs continue to decline.

A sensitivity analysis reveals that resource quality and temporal generation patterns — particularly the depth and duration of prolonged low-generation events — are the primary determinants of firming costs, outweighing the influence of capital expenditure and reliability targets. Short-term variability plays a secondary role; it is the persistence of supply shortfalls, not their frequency, that drives storage and overbuild requirements. Technological diversification — combining solar PV and wind within a hybrid configuration — significantly reduces firming costs relative to single-technology configurations by exploiting the natural complementarity between the two generation profiles and reducing the need for large storage volumes.

The paper concludes that hybrid power plants represent an increasingly cost-competitive and commercially mature solution for firm, dispatchable renewable power in diverse geographical and market contexts — and that, at a time when fossil fuel dependence is once again exposing economies to acute price and supply risks, the energy security case for firm renewables is as compelling as the economic one.