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I drove a HPV, with no heater and lots of heavy breathing. I found that it was enough to keep a flow of outside air to the inside of the windshield - no heating needed. You can also use anti-fog treatments to keep any condensation flat and transparent.
If you want to carry some heat, the best thing is a phase-change material such as Glauber's salt which will put out an even temperature for quite a while, and put out far more heat in the useful range than a jug of water.
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