Thanks for the watts v temperature equation, 100 watts is the smallest immersion heating that I have found for 24 volts, so that should give 60 deg.C per hour roughly, at 4 amps/hour.
I'm not sure you've quite got this. The rating of your 100W heater is the power that it will dissipate when rated voltage, in this case 24V, is applied to it - provided that the power source is capable of supplying the necessary current, in this case 4.17A (A, note, not A/hr). A 24V 100W heater is a passive device with an (almost) constant resistance of 5.76 ohms. You have a variable power source , with a potential output dependent on wind strength. If you connect a fixed resistance of 5.76 ohms to it at a time when it is unable to generate 100W then the voltage will be dragged down accordingly, and your heater will deliver less than 100W.
Maybe your 60 degC per hour is just shorthand, but I thought I had explained (perhaps badly) that a given amount of energy can provide a given PRODUCT of temperature rise and water mass.