# Flywheels and small mechanical stores: Correlation Prices it moves with, and why Source page: https://www.materialinsights.org/energy/e_flywheels/correlation/ Flywheels store energy in a fast-spinning rotor, giving quick bursts of power for grid frequency control, data centres and trains. Cost depends on the rotor material, magnetic bearings, the motor-generator and power electronics. No public price Flywheels and small mechanical stores has no public monthly price series: Small niche market; prices quoted per project. #### What its price moves with Commodity [St Steel](https://www.materialinsights.org/commodities/steel/)Rotor material Many flywheels use heavy steel rotors that spin relatively slowly. Steel is cheap and well understood, so steel rotor designs trade lower energy density for low cost and easy manufacture. Not enough overlapping price history to measure. Chemical [ACN Acrylonitrile](https://www.materialinsights.org/chemicals/acrylonitrile/)Carbon fibre precursor Fast-spinning rotors use carbon fibre composites, made mostly from polyacrylonitrile fibre, which comes from acrylonitrile. Carbon fibre is costly, and its price drives the cost of the high-speed flywheel designs that store most energy. Not enough overlapping price history to measure. Commodity [REE Rare earths](https://www.materialinsights.org/commodities/rare_earths/)Motor magnets Motor-generators in flywheels often use neodymium magnets for compact, efficient drives. Rare earth prices and Chinese export rules therefore affect their cost and supply security, and some makers use magnet-free motors to avoid the risk. No public price for rare earths, so the link is explained in words only. Energy [Mot Electric motors and drives](https://www.materialinsights.org/energy/e_motors/)Power conversion A motor spins the rotor up and becomes a generator when energy is withdrawn. Motor efficiency and losses at high speed shape how long a flywheel can hold its charge before it runs down. Not enough overlapping price history to measure. Energy [PE Transformers and power electronics](https://www.materialinsights.org/energy/e_power_electronics/)Grid connection Inverters convert the variable frequency output of the spinning rotor into grid power, and they are a large share of system cost. Cheaper power semiconductors therefore lower flywheel system costs directly. Not enough overlapping price history to measure. Energy [SC Capacitors and supercapacitors](https://www.materialinsights.org/energy/e_supercaps/)Competitor Supercapacitors compete with flywheels for short, frequent bursts of power, for example in trams, cranes and grid frequency services. Batteries are entering the same niche as their cycle life improves. Not enough overlapping price history to measure. How to read these numbers **Correlation of monthly changes** (the number in the coloured pill) compares how much each price moved, month by month, since 2006 or the first month both series exist. It runs from −1 to +1: near +1 the two usually rise and fall in the same months, near 0 there is no link, and a negative number means they tend to move in opposite directions. Above 0.7 is strong, 0.4 to 0.7 moderate, 0.2 to 0.4 weak. **12-month changes** compare year-on-year moves, which shows whether two prices share the same cycles even when the month-to-month noise differs. **Price function** is a straight-line fit of the two prices on logarithmic scales: price ≈ k × (partner price)^β. β is how much this price moves, in per cent, for a 1% move in the partner over the whole period, and R² is the share of the variation the fit explains. Two prices that both drift up with inflation can fit well without any real link, so read it together with the monthly correlation. **Lead or lag** checks whether the monthly correlation is clearly higher when one price is shifted by 1 to 6 months. Correlation is not cause. The reasons on each card explain the economic link; the numbers show how strongly it has shown up in prices. Proxy series are US producer price indexes or related prices and can blur the real link. Data: World Bank Commodity Price Data (Pink Sheet), October 2026; US Bureau of Labor Statistics producer price indexes, IMF primary commodity prices and US EIA prices via FRED, Federal Reserve Bank of St. Louis, retrieved November 2026. Calculations by this site. ## Other sections - [Summary](https://www.materialinsights.org/energy/e_flywheels/index.md) - [Overview](https://www.materialinsights.org/energy/e_flywheels/overview/index.md) - [Science and technology](https://www.materialinsights.org/energy/e_flywheels/sg-sci/index.md) - [Engineering and performance](https://www.materialinsights.org/energy/e_flywheels/sg-eng/index.md) - [Economics](https://www.materialinsights.org/energy/e_flywheels/sg-econ/index.md) - [Impact and additionality](https://www.materialinsights.org/energy/e_flywheels/sg-impact/index.md) - [What to track](https://www.materialinsights.org/energy/e_flywheels/tracking/index.md) - [Relationships](https://www.materialinsights.org/energy/e_flywheels/links/index.md)