Learning rate (Swanson's law)
The percentage cost reduction of a technology for every doubling of cumulative production; for solar modules, Swanson’s law puts it near 20%.
The learning rate describes the empirical relationship, first noted by T. P. Wright for aircraft in 1936, that unit costs fall by a roughly constant fraction each time cumulative production doubles. For solar PV it is known as Swanson's law, after SunPower founder Richard Swanson, who observed module prices falling about 20% per doubling of shipments.
C is unit cost, x cumulative production and b the experience exponent. An LR of 20% corresponds to b ≈ 0.32.
Observed learning rates (Way et al. 2022, Joule; IRENA; BNEF):
- Solar PV modules: about 20 to 24% over four decades; module prices fell from roughly 76 USD/W in 1977 (2013 dollars) to around 0.10 USD/W in China by 2024.
- Lithium-ion battery packs: about 18 to 20%; average pack prices fell from over 1,000 USD/kWh in 2010 to 115 USD/kWh in 2024 (BNEF).
- Onshore wind: about 10 to 15% for turbine costs, higher for LCOE when capacity factor gains are included.
- Nuclear: flat or negative in the US and France, reflecting large one-off projects and regulatory change.
Learning curves underpin cost projections in IEA, IRENA and IPCC scenarios. Way et al. found that experience-curve forecasts outperformed most historical expert projections, which repeatedly underestimated solar cost declines.
Related terms
All terms →- CAPEX
Capital expenditure: the upfront investment to build an energy asset, usually quoted per kW of capacity for power plants or per kWh for storage.
- LCOE (Levelized Cost of Electricity)
The constant price per MWh that recovers all lifetime costs of a power plant, discounted to present value, divided by its discounted lifetime output.
- Grid parity
The point at which electricity from a new source, usually solar or wind, costs the same as or less than power bought from the grid.
- Feed-in tariff
A policy that guarantees renewable generators a fixed price per kWh exported, usually for 15 to 20 years, independent of market prices.
- Solar cell
A semiconductor device, usually a doped silicon wafer with metal contacts, that converts sunlight directly into direct-current electricity via the photovoltaic effect.
- Lithium-ion battery
A rechargeable battery in which lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode; the dominant chemistry for EVs and grid storage.