Flywheel Energy Storage at Household and Village Scale: A Reference Design, Its Self-Discharge, and the Duties It Can and Cannot Serve

First-principles design and honest limits test of flywheel energy storage for a household overnight load of 2.00 kWh delivered over 10 hours. SIMULATED, NOT MEASURED; nothing was built or hardware-tested. A hoop-wound carbon-fibre rim of 71.7 kg spinning at 17,905 rpm stores 2.977 kWh and delivers 2.009 kWh through its working speed range, but standing losses of 42 W drain 1.41 percent of stored energy per hour, reaching half energy in 35.4 hours and empty in about 71 hours. Levelised cost at one cycle per day is $1.084 per delivered kWh against $0.376 for lithium iron phosphate and $1.058 for lead-acid. At 10 or more full-equivalent cycles per day the flywheel falls to $0.108 per delivered kWh and wins. The paper covers the material-strength ceiling from hoop stress, containment for a 10.72 MJ burst case, cycle life, and the niches (frequent cycling, power smoothing, short bridging) where flywheels genuinely win and the multi-hour storage duty they cannot serve.