That's an interesting question, but I think it's a bit moot; we don't have any leeway in the form of amount that's safe to emit. Anything we can save, we should save.
>Groundwater 47°F, Heater set to 110°F: 63° rise x 8.33 Btu x 40 gallons = 20,992 Btus
>Groundwater 47°F, Heater set to 120°F: 73° rise x 8.33 Btu x 40 gallons = 24,324 Btus
>Groundwater 47°F, Heater set to 140°F: 93° rise x 8.33 Btu x 40 gallons = 30,988 Btus
Average of 25,435 Btus.
0.000293 kWh/Btu for electric so something like 0.19 kWh per gallon for electric.
Looks like 1lb of coal makes about 1kWh (wow!)[1].
1 pound of carbon combines with 2.667 pounds of oxygen to produce 3.667 pounds of carbon dioxide.
So roughly 0.7 pounds or 0.32 kilograms of CO2 per gallon of water in an electric water heater?
The average American shower uses 17.2 gallons [2], so if you showered daily and only used hot water your CO2 from showers alone would very roughly be something like 2000kg annually.
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