Water heating

How we built this number

Nothing above is a headline figure we were handed. It is derived, step by step, from a stated consumption assumption and a stated efficiency, both of which are published with their sources. If you disagree with an input, you can see exactly which one to argue with.

The arithmetic

1. 257 therms/yr burned
2. x 29.307 kWh/therm = 7532 kWh of gas energy
3. x 0.55 delivered efficiency = 4143 kWh of heat actually delivered
4. / 2.3 seasonal COP = 1801 kWh/yr of electricity

Fuel used today

257 therms

Cost of that fuel

$715

Electricity instead

1,801 kWh

Cost on E-ELEC

$557

Annual difference

$159

The consumption assumption

Assumes an average San Francisco single-family home burning 257 therms a year on hot water, through a gas tank about 10 years old: 0.60 EF on the label, delivering about 55% of that fuel energy as hot water once standby and flue losses are taken off. The age is our assumption about a typical replacement-age tank, and the derating from 0.60 to 0.55 is our engineering judgement rather than a measured figure. Hot water is the largest gas end use in most San Francisco homes — bigger than heating.

The rates we priced it at

Both sides of this comparison are priced at published rates, effective 2026-06-01: a PG&E residential fuel rate on the old equipment, and PG&E's E-ELEC electricity rate on the new. The exact figures, what they include, where they came from and when they were last checked are all on the methodology page. Rates move, and when they do, every number on this site moves with them.

The time-of-use caveat

E-ELEC is a time-of-use rate, not a flat price. Assumes the tank heats off-peak. Heat pump water heaters hold 50-80 gallons, so they can be scheduled to reheat overnight and coast through the 4-9pm peak — but only if the schedule is actually set up that way.

This matters more than most people expect. The same appliance on the same rate plan can cost substantially more or less depending purely on the hour it runs, which is why we state the assumption rather than quietly averaging it away.

Why it costs less to run

This is the upgrade most people overlook, and in San Francisco it is usually the biggest one. Because winters here are mild, hot water — not heating — is the largest gas load in a typical home. A heat pump water heater moves heat from the surrounding air into the tank instead of burning gas under it, doing the same job for well under half the energy.

What it is like to live with

A heat pump water heater holds a large tank of hot water and recovers steadily, so running out mid-shower is less likely than with a small gas tank. It is not silent — there is a compressor, and it belongs in a garage or utility space rather than next to a bedroom wall. It also dehumidifies and slightly cools the space it sits in, which is welcome in a damp garage.

What it changes beyond the bill

Water heating runs every day of the year, so switching it off gas removes a steady, year-round combustion load rather than a seasonal one. It also removes a gas flue and the carbon monoxide risk that comes with it.