Solar and wind are intermittent sources -- the sun sets and wind speeds vary -- so batteries are needed to store energy generated at peak times for use when generation drops. Lead-acid batteries are cheaper upfront but have a shorter usable lifespan, typically rated for only a few hundred charge cycles, and should generally not be discharged below about 50% to avoid damaging them. Lithium-ion batteries cost more initially but last for several thousand cycles and can safely be discharged much more deeply, which is why they are increasingly preferred in solar home systems and mini-grids despite the higher upfront price. Battery capacity is measured in kilowatt-hours (kWh), and 'depth of discharge' describes what percentage of that capacity can safely be used before recharging.