According to the brainy people in this world, here is the formula to heat water, provided that there is no heat loss during the process.
By using the formula Pt = (4.2 * L * T ) / 3600 you can calculate the time it takes to heat a specific quantity of water from one temperature to another temperature. Once you have calculated the power used to heat the water, as represented by Pt in the above equation, divide this number by the heater element rating to find out how long it would take to heat your water.
So if you want to calculate how long it would take to raise a 500litre tank's temperature by 15C with a 1kw heater -
pt = (4.2 x 500 x 15) / 3600
pt = 31500 / 3600
pt = 8.75 hours
as you have a 3kw heater divide 8.75 by 3 = 2.917 hours
(double that for a 1000 litre IBC tank, provided that it doesn't suffer any heat lose during these 3 hours - probably impossible to achieve.)