Solar panel to DC heating element to pre-heat water before on demand water heater
Sadly not much. 100W would not effectively heat a 40 gallon water heater, let alone an 80 gallon. Also, remember that the panel likely won't really put off 100W unless your element has the exact right resistance for your specific panel.

You can do some simple calculations if you're curious how much heat that will produce... 
If you use 100 Watts for 4 hours, that is 400 Wh (Watts * hours = Wh)
It takes approximately 2.4 Wh to heat 1 gallon of water 1 degree Fahrenheit.

So 400 Wh per day could heat 10 gallons of water 16.67 degrees. (The equation is [(Wh) ÷ 2.4 ÷ (#Gallons) = (change in degrees). So in other words, if your water comes into the tank at 70° (pretty warm) and you use 10 gallons per day, the theoretical temperature it could produce would be 86° (not very impressive is it?). This will somewhat reduce your draw on the on-demand but not by much. 

So doing a little math the other way, if your water came into the tank at 50° and you wanted it to heat up to 120°, that is a 70° increase. If I used -say- 30 Gallons per day and I wanted to heat it up 70°... 
The formula is (Wh)=(Change in Degrees)*(#Gallons)*2.4
So that would be [Wh = 70° * 30 Gal * 2.4] which comes to 5040 Wh. Spread that across 4 hours and you can see that you could do it with 1260 Watts.

I understand that your goal isn't to heat your water exclusively with the solar but the numbers show how to do the basic math. For example, if I really do use 30 Gallons per day and I heat it  70° above its original temperature, my total need would be around 5040 Wh so if I offset it by 400 Wh (100W for 4hrs) then I would be offsetting apporximately 5/63, (or 7.9%) of my usage

Please note that this doesn't account for a myriad of factors that could affect it, such as heat-loss, inefficiency, or the derating of your solar panel... These numbers should some idea but don't expect them to be exact. Please let me know if this was helpfull or if there is anything I could explane better by leaving me a comment.

Thank-you,
Maxfield Solar