This is going to offend the coffee purists even more - in fact a way over the top offense that probably is a travesty:
I heat up instant coffee on a burner of the RV's propane cooktop stove. I even use a titanium cup to drink it out of that can itself be directly placed right on the burner.
I wipe out the cup with a paper towel so it's ready for the next time. No mess to clean up, no noise, no solar, no inverter, and very little electricity period (.... except for the spark from the pizzo-electric burner igniter). :B