U=I•R
P=U•I
1 Watt over a time of 1 second ist equal to 1 Joule
1 Joule will heat 1g gramm of water 0,239 Kelvin
the heat capacity of water is 4,186 kJ/(kg K)
the heat capacity of H2SO4 is 1,386 kJ/(kg K)
So it will take about 3 times as much energy to warm up H2O compared to H2SO4 (for H2SO4 ~0,72 Kelvin per gramm per Joule)
If we use the cell (1 kg H2SO4) at let us say 40 Watts, it will get 0,0288 K warmer every second, 1,728 K every minute, 51,84 K in a half hour.
this is without calculating the loss of warmth, just as if we would use a thermal isolated container
Quite seldom I have to calculate anything else but water and I wasn't sure, if I could find out, but when I do the plausibility check and compare this result to what I have experienced, it seems to be just fine.
One advice: everyone who wants to basically understand his cell, should have a look on Ohm's law! Just as Geo already said: The Ampere is always given by the setting of voltage and resistance. The resistance is given by the electrode areal, distance and composition of electrolyte.