To make sure you're not testing too early, assume the sperm had very long lives (5 days is considered a very long sperm life) and that conception occurred then. Then allow 14 days for the embryo to implant and develop enough hCG to show up on a pregnancy test (96% of pregnant women will test positive by then.)
A gas water heater will recover in half the time of an electric unit. A 40-gallon gas water heater can recover in about an hour. Recovery for a 40-gallon electric water heater takes about two hours.
Cost to heat water a 40-gallon tank with gas. 1  The federal standard for gas water heaters is only 55.6-59.4% efficiency (higher for smaller tanks), and actual models are rarely much better, except for Energy Star models which are at least 67% efficient. ( 2 DoE 2010, p. 9).