Your old gas water heater burns methane to make water and CO2. For an average family-of-four in Santa Clara county, that adds up to about 2,844 pounds of CO2 every year. But it's much worse than that. Because 2-3% of methane leaks, everywhere from drilling to your house piping, and because methane is 84 times as bad for climate as CO2, the real emissions are equivalent to 4,835 pounds per year.
What does 4,835 pounds mean? Well, it's the entire annual emissions of a small village of 18 people in Uganda, or 1/2 of someone from Sweden. It's the equivalent of driving your car 5,428 miles (404g/mile), or 20 miles a day for every working day of the year, or flying back and forth to Hawai'i 4 times. It's also over 20% of the emissions of the average Californian, so swapping out your water heater is a really big deal. You can save almost 33 tonnes of emissions over the heater's lifetime, or 3 years of your entire emissions, all in just one single act.
A heat pump water heater has minimal emissions. SVCE supplies almost 100% carbon-free electricity , so the running costs are low. You can also program your water heater to do most of the heating during the afternoon, when the grid is saturated with solar energy, or power it from your own solar panels, to be super-clean.
There are some emissions related to manufacture and disposal of water heaters, but they are pretty small compared with burning gas, and are similar for a gas-fired boiler that you would replace anyhow. The only difference is that most heat pumps have refrigerant that is also a potent greenhouse gas, so you need to make sure it is handled properly when you dispose of it, similar to disposing of a fridge. A small amount also leaks during use, estimated to be the equivalent of about 47 pounds of CO2 per year, or about 0.75% of the emissions that you are saving .
 Caveat: SVCE *buys* 100% clean power, so in the big picture, you are using clean energy but sometimes the actual electrons flowing to your house are from fossil fuels. You can make sure you're running on cleaner power by doing most of the heating mid-day, when solar power is in excess, and in the near future we can expect to have smart programs that will work with current heat pumps to use the cleanest possible electricity.
 Calculations based on a paper that estimates that the heater contains 0.75 kg of R134a refrigerant, which is 1433 times as potent as CO2, and has a 2% per year leakage rate.
Back to the Santa Clara County Guide to Climate-Friendly Water Heating.