I have a low power 1800W induction cooktop... sadly It took somewhere a little over 2 hours to get 8 gallons of 62F water to boil in it with the lid on. This is better than other brew kettles I have tried on it... I have some insulation coming to try and reduce the time needed... It may just be used as a normal kettle though since I have no 220V power in my condo :-/
I have learned that kettle design and quality makes a huge difference when using an induction range. I can bring 5-gallons of run-off to a boil with the lid off when using my American-made 24-quart Vollrath Tri-Ply 77620. I have to cover and insulate the kettle that I made from a Chinese-made 27-quart Vollrath Optio stockpot to get it to boil 4.5 gallons of run-off. Both kettles are classified as being induction in ready.
Sadly, eight gallons is too much to boil with any kind of speed with an induction range on a 120V/15A circuit. However, the circuit that feeds counter-high receptacles in most American kitchens is usually 20A. If you have access to your breaker box, I would check the rating on the kitchen circuit breaker (the circuits should be named in the box). if it's 20A, you can install a 2200W 120V/20A Blichmann BoilCoil, which will be almost 100% efficient.
I posted this math on another thread, but I will re-post it here with your numbers:
1 calorie = amount of energy required to raise a gram one degree Celsius
1 milliliter of water = 1 gram
1 Watt = 0.2388 calories per second
2200 Watts = 2200 x 0.2388 = ~525 calories per second
1 gallon of water = 3785 milliliters
Time to raise one gallon of water one degree Celsius = 3785 / 525 = 7.2 seconds
Time to raise eleven gallons of water one degree Celsius = 8 x 7.2 = 57.6 seconds
Tap liquor temp = 20C (68F)
Boiling temperature at sea level = 100C (212F)
Temp delta = 100 - 20 = 80C
Time to raise 8 gallons of liquor 80 degrees C = 80 x 57.6 / 60 = 76.8 minutes
Strike liquor volume = 4 gallons
Tap liquor temp = 20C (68F)
Strike liquor temp = 77C (171F)
Strike liquor Temp delta = 77 - 20 = 57C
Time to raise 4 gallons of liquor 57 degrees C = 57 x 7.2 x 4 / 60 = 27.3 minutes
Sparge liquor volume = 5 gallons
Sparge liquor temp = 77C (171F)
Sparge liquor Temp delta = 77 - 20 = 57C
Time to raise 5 gallons of liquor 57 degrees C = 57 x 7.2 x 5 / 60 = 34.2 minutes
Run-off voume
Run-off temp = 66C (151F)
Boil temp = 100C (212F)
Temp delta = 100 - 66 = 34C
Time to raise 8 gallons of run-off 34 degrees C = 34 x 7.2 x 8 / 60 = 32.64 minutes
If you continuous sparge, heating and collecting the run-off can be performed in parallel as soon that the run-off column height exceeds that of the BoilCoil.
One last thing: If you have an electric range, then you have a 240V circuit in your kitchen. Not only is this circuit 240V, it is usually capable of carrying 50A of current, which is more than enough to handle 8-gallons of run-off. A 4400W heating element will cut the times listed above in half.