STRINGS.CODEX.POWER.PARAGRAPH_3
Power is measure in Joules when stored in a Battery. Power produced and consumed by buildings is measured in Watts, which are equal to Joules (consumed or produced) per second.
A Battery that stored 5000 Joules could power a building that consumed 240 Watts for about 20 seconds. A generator which produces 480 Watts could power two buildings which consume 240 Watts for as long as it was running.