The specific heat capacity of water is 4.184 joules/gram/deg, so to raise 5 litres of water from 20 to 80 degC, would take 1255200 joules (4.184 * 5000 * (80-20)), or 0.349 KWh. Now if you've only got 120W, that means it would take 2.91 hours (0.349 / 0.12) assuming you have zero heat loss.

However, unless you've got insulated containers, I'd doubt you'd ever get to 80degC.