We have a lot of numbers and estimates on this guide. Here's the background as to how we came up with them. Everyone's house is different, so we worked hard to pick the most typical situations for each case.
Emissions from your water heater include the CO2 emitted from burning methane, as well as the direct effect of methane leaks. But since CO2 is heavier than methane (1 mole methane = 16g; 1 mole CO2 = 44g), then 1g of methane makes 2.75g of CO2.
An estimated 2.3% of methane leaks during transit, and methane is 84 times worse than CO2 for global warming, so that leakage from that 1g of methane is the equivalent of 2.3/100*84 = 1.832g of CO2, added to the 2.75g produced when burning it..
Residential gas usage for San Mateo County was 118 million therms in 2021, when the population was 737,888 (Google), so we each used on average 160 therms of gas. 49% of residential gas usage is for water heating (2022 Sustainable San Mateo Indicators report, p24), so we each use about 78 therms a year for water heating.
A family of four uses about 312 therms/year for water heating (78 x 4). EPA.gov says that burning 1 therm of gas releases 0.0053 metric tons CO2, so 312 therms leads to 1.65 tonnes or 3,646 pounds of CO2, . Methane leakage adds an additional 70% emissions, to a total of 6,200 pounds of CO2 equivalent.
Peninsula Clean Energy has several rates, but the default and the most common is called E-TOU-C, which has summer and winter rates, and costs extra in the hours 4-9 pm. If we assume that you program your water heater to avoid those 5 hours, and that your hot water usage is 50% less in the summer months (vacation, warmer days, less energy to keep the water hot), then your average cost is 13c per kWh. PG&E charges another 19.4c to deliver that electricity, so your total cost is about 32.4 c/kWh.
PG&E charges a baseline and a Tier 2 rate for gas. Cutting your water heating will mostly cut into the more expensive Tier 2, so we if average them out 2:1 (twice as much Tier 2 as Tier 1) and add on their PPPS surcharge, we get $2.39 per therm ($2.32 if we just to a straight average, so not much of a difference). The hot water cost for the typical family-of-four is then $746 per year.
The Efficiency Maine calculator allows a comparison between costs of different heating methods. If we use the default energy efficiencies, and the prices we just calculated, then 98 gallons per day gives us an annual cost of $744 for gas (matching the $746 calculated above) and $566 for a heat pump water heater, a difference of $170/year.
If you use less hot water, the costs and savings go down linearly, so if you live on your own, you will use about a quarter as much hot water, and save about a quarter as much, $35, year, but you will always save money at current rates.
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Back to the San Mateo County Guide to Climate-Friendly Water Heating.