Clean Energy SF: San Francisco, fog then sun, est. 2026

Vol. 1 - No. 14 - Thursday, September 24, 2026 - Free to readers

Grid ยท Analysis

Muni's electric buses and the yards that have to charge them

Trolleybuses already run on the wire. Battery buses need the depot to hold enough power overnight, and in San Francisco that is the hard part.

Filed October 2, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. Two ways to run a bus on electricity

A trolleybus takes its power straight off the wire, continuously, with only a small onboard battery for steering around a stalled truck or a parade. That is why it climbs the Fillmore and Castro grades so well and needs no time plugged in: the wire is the charger. The catch is that stringing new overhead wire down a street is slow, expensive, and often contested by the neighbors who have to look at it, so Muni has not been adding much of it.

The battery bus solves the wire problem and creates a new one. It carries its own energy and can run any route without a cent of overhead line, which is exactly why San Francisco wants it for the diesel-hybrid routes. In exchange, the constraint moves from the street to the yard. SFMTA has been running battery buses in pilots and has pointed at a zero-emission fleet around 2035, while California's Innovative Clean Transit rule (the state regulation that pushes transit agencies to buy only zero-emission buses) aims the whole state's fleets at zero-emission by about 2040.

II. The depot math

One fast depot charger might draw on the order of 150 kilowatts. A yard charging dozens of buses at once, then, needs several megawatts of electrical service, roughly the draw of a small neighborhood arriving all at midnight. Spread along a route, a trolleybus's demand is diffuse and the wire handles it. Concentrated at a single yard overnight, battery charging is one heavy, predictable load that the local grid has to be built up to carry.

That buildup means new transformers and switchgear (the heavy equipment that routes and protects high-power electrical connections), and sometimes a larger feed from the utility. Those upgrades take time, often years from the first request to the day the yard is energized, and the honest answer on timing is that it depends: on the yard's existing service, on where the SFPUC and PG&E hand the power off to each other, and on whether the buses all charge at once or are staggered through the night.

III. Potrero, and who has to say yes

Potrero Yard, a bus facility first built roughly a century ago near 17th and Bryant, is being rebuilt under the Potrero Yard Modernization Project, a new maintenance and storage building designed for a zero-emission fleet with hundreds of homes planned above it. How much charging that yard can support, and how quickly, is one of the quiet engineering choices that will decide which routes go battery first. If the yard's power comes in late, the buses wait for it.

No single agency can deliver this alone. SFMTA owns the buses and the yards; the SFPUC delivers Muni's Hetch Hetchy power, a real advantage because that hydroelectricity is public and carbon-free at the source; PG&E owns much of the local distribution the yards connect through; and CARB (the California Air Resources Board) sets the zero-emission deadlines. When those four are not lined up on the same schedule, the slowest one sets the pace for your route.

IV. Worth watching this month

1. SFMTA board meetings for construction and budget updates on the Potrero Yard Modernization Project, which is the single item most likely to move the whole timeline.

2. CARB's Innovative Clean Transit reporting, where a large agency like SFMTA has to show a growing share of zero-emission bus purchases on the way to a fully zero-emission fleet by about 2040.

3. Any SFPUC or Hetch Hetchy Power notice about capacity or rates for Muni's growing charging load, usually routine but worth a glance.

4. PG&E service-upgrade and interconnection timelines for fleet charging at the city yards, which rarely make news but quietly set the pace.

5. SFMTA announcements of which diesel-hybrid routes convert to battery buses next, the clearest public sign that the depot work is paying off.

Frequently asked questions

Are the wire trolleybuses going away?

No. Trolleybuses are already zero-emission at the tailpipe and handle San Francisco's hills well, so the plan is to keep them. The transition is mainly about the diesel-hybrid buses.

Why not just string overhead wire on every route?

Overhead wire is expensive to build, slow to permit, and contested on some streets. Battery buses let Muni electrify routes without hanging new wire everywhere.

Does Muni's electricity come from PG&E?

Muni runs largely on Hetch Hetchy hydroelectric power delivered through the SFPUC, the city's own utility. PG&E still owns parts of the local distribution network that the yards connect through.

When will Muni's buses all be electric?

SFMTA has pointed at roughly 2035 for a zero-emission fleet, and state rules require full zero-emission bus fleets by about 2040. Dates like these can slip, so treat them as goals rather than promises.

References