Battery backup for a garage door opener: one cycle is about 4 Wh, so buy for the start, not the hours

A garage door opener is the smallest motor load in the house and the one people size most wrongly. LiftMaster rates its 1/2 HP chain-drive opener at 6 A, which is 720 W while the door moves, and a full open-and-close takes about 22 seconds, so one cycle is about 4 Wh. Ten cycles a day is 44 Wh, which a 1 kWh unit carries for about three weeks. The battery you need is tiny; the inverter is not, because a motor starts hard. And if your opener has a battery compartment, the maker’s own pack is the answer and the rest of this page is for the refrigerator beside it.

Look for the battery compartment first

Since July 1, 2019, every residential opener sold or installed in California has had to include a battery backup, under SB 969, passed after the 2017 fires trapped people behind doors they could not lift. Chamberlain’s SB 969 notes list its models that carry one (the LiftMaster 8500W, WLED and 8550W, and Chamberlain’s B1381, B970 and C870), and plenty of openers sold elsewhere have the compartment too. It is behind the light lens on the motor unit, with a battery status light beside it.

If yours has one, buy the maker’s pack and nothing else on this page. Chamberlain’s page for its 12 V, 4.5 Ah backup batteries says a fully charged system supplies the motor for one to two days of normal operation during an outage, LiftMaster’s manual says the battery takes 24 hours to fully charge once the power returns, and Chamberlain puts its life at about one to two years. Test it once a season: unplug the opener and run the door.

What one cycle costs

Openers do not print watts. The product guide does: LiftMaster’s 81650 is rated 120 V, 6 A, which is 720 W with the motor running. How long it runs is on the same maker’s sheet for its 8550W: about 9 inches per second up and 6.5 down, so a 7-foot door opens in roughly 9 seconds and closes in 13. Twenty-two seconds at 720 W is 4.4 Wh, and the runtime table above turns that into days. Between cycles the opener waits at about 1 W, LiftMaster’s standby figure for the 8550W with the lights off, so the battery is spent on the door, not on the waiting.

The bar is the start, not the capacity

An AC opener is an induction motor, and an induction motor draws several times its running current for the first fraction of a second, per the running versus surge guide. No opener maker prints that figure. This site’s rule for an unpublished motor is a unit with at least 1,800 W of continuous output and 3,000 W of surge, and it is why the 288 Wh units that carry a router so well are absent here: they hold enough energy for a hundred cycles and cannot supply the first one. The 1 kWh class is the floor, and the two 1 kWh picks are the whole answer for a door on its own. The three larger units are on the page because a garage usually has a refrigerator or a freezer in it, and the door is then the smallest line on the list.

Connecting it, and the one thing not to do

Most openers plug into a receptacle on the garage ceiling. Unplug the cord, run a 15 A rated extension cord down to the unit on the floor, plug the opener into it, and the wall button and remotes work as before; the opener does not care that it was off for an hour, so there is no need to have the unit in place before the outage. An opener wired directly into a circuit, with no plug, is an electrician’s job to back up, through an inlet or a transfer switch like the one the transfer switch guide explains.

The thing not to do is treat the red release cord as the backup. It disconnects the door from the trolley so the door can be lifted by hand, and a balanced door lifts easily and stays put. A door that will not stay up, or that drops when released, has a spring problem, and a spring is a door technician’s part. Pull the cord with the door closed, never open.

The rest of the garage

Add the door to the sizing calculator and it becomes a rounding error next to whatever else is out there: a second refrigerator averages about 52 W around the clock by its label, per the refrigerator page, and a chest freezer about 25 W, per the freezer guide. Either one, not the door, decides the class.

What we left out

Pecron’s E1000LFP, an 1,800 W unit with a stated 3,000 W surge, would have ranked; both of its Amazon listings returned Page Not Found on the check date. Jackery’s Explorer 1500 v2 showed no buy box and two reviews, and its 1500 Ultra three reviews, so one sits as a mention and the other waits. Anker’s C800 Plus and Goal Zero’s Yeti 1000 Core are 1,200 W inverters, under the bar. The 1 kWh units ranked on the refrigerator and sump pages are at this site’s three-page limit, hence the two newer units here.