We have a lot of numbers and estimates on this guide. Here's 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 1g of methane produces 2.75g of CO2 directly, and and additional 2.3/100*84 = 1.83g of CO2 equivalent from methane leakage.
Residential gas usage for Santa Clara County was 236 million therms in 2021, when the population was 1.886 million (Google), so we each used on average 125 therms of gas. 49% of residential gas usage is for water heating (2022 Sustainable San Mateo Indicators report, p24), so we each use about 61 therms a year for water heating.
A family of four uses about 244 therms/year for water heating (61 x 4). EPA.gov says that burning 1 therm of gas releases 0.0053 metric tons CO2, so 244 therms leads to 1.29 tonnes or 2,844 pounds of CO2. Methane leakage adds an additional 70% emissions, to a total of 4,835 pounds of CO2 equivalent.
SVCE 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 you program your water heater to avoid those 5 hours, and 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 13.8c per kWh. PG&E charges another 21.5c to deliver that electricity, so your total cost is about 35.3c/kWh (this calculation ignores baseline allowance, which would make electricity cheaper, but is fully used by most customers).
PG&E charges a baseline rate (Tier 1) 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 (these numbers were calculated before the huge increase in gas prices in late 2022, so the recent situation is even worse for gas).
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 $616 for a heat pump water heater, a difference of $130/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, but you will always save money at current rates.
Natural gas prices are consistently expected to increase much faster than electricity or overall inflation. For instance, a CPUC report from 2021 (Table 27) predicts electricity rates to increase by 3.2% per year for the next 10 years and natural gas to increase by 6%. That means that in 10 years time, gas prices will increase 30% more than electricity prices, so anyone replacing a gas appliance with gas will be locked into an every-more-expensive form of heating (these are just predictions, so unlikely to be highly accurate, but they do represent the most likely future).
In addition to these predictions, it's likely that gas will become even more expensive as more people electrify, and the cost of maintaining the gas network falls on fewer and fewer customers. A 2023 report suggests that this may add 30-60% or more to the price of gas.
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Back to the Santa Clara County Guide to Climate-Friendly Water Heating.