Every 1 minute, 3L of water gets heated by 50K. With a specific heat capacity of 4200 J / kg / C and density of 1 g / mL, it takes 3 x 4200 x 50 = 630000J per minute.
With a rate of 4.0 x 10^4 J / g for the heater, we can get the rate of combustion with 630000 / 40000 = 15.75 g per minute.