Backup runtime
How many hours your battery backup would really give you
Backup systems are sold on a runtime figure, and the figure is almost always prefixed by the words "up to". Those two words are carrying an enormous amount of weight. They assume a new battery, a low head, and a pump that spends most of the outage resting. Change any one of those and the real number moves a long way, usually downward, and always on the night you find out.
Runtime is not a property of a battery. It is a property of a battery, a pump, a head, and a storm — and the storm decides the duty cycle, which is what actually converts amp-hours into hours. This tool derives runtime from those, rather than repeating a marketing number back at you.
What your backup would really do
How long this backup keeps up
7.2 hours
Runs out before the outage ends.
Runtime, low to high 5.0 to 9.0 hours
Biggest swing factor Battery capacity at pump current. Amp-hour ratings are quoted at a 20-hour discharge; a sump pump pulls the pack far harder than that, and the delivered capacity is lower than the label by an amount that depends on the specific battery.
- This falls 2.8 hours short of the 10-hour outage you entered. Adding a second battery in parallel roughly doubles runtime; a larger backup pump does not, and often shortens it.
Not counted in this runtime
- Charger and controller draw, which continues during the outage and shortens runtime slightly.
- Cold-temperature capacity loss. A battery in an unheated basement in January delivers materially less than its rating.
- Any claim that the backup will start. A backup that has never been tested under load is an assumption, not a system.
- Whatever the primary pump would have done. This figure is for the backup alone, on battery, with no utility power.
The duty cycle is the whole trick
A backup pump moving 14 gallons a minute against inflow of 6 gallons a minute does not run continuously. It runs about 43 percent of the time and rests the remainder, which is why the battery lasts several times longer than its raw pumping hours would suggest. The raw pumping capacity here is 3.1 hours of actual motor time; the duty cycle stretches that into the 7.2 hours quoted above.
That relationship also explains the failure mode nobody expects. If inflow rises to meet the backup pump's capacity, the duty cycle goes to 100 percent, and runtime collapses to the raw pumping hours with no warning. Worse, if inflow exceeds the backup's flow at your head, the water level rises regardless of battery size — at that point you do not have a runtime problem, you have a capacity problem, and adding batteries does not touch it.
This is the argument for checking the backup pump against the same head figure as the primary. A backup rated at 1,000 gallons an hour at ten feet is not a 1,000-gallon-an-hour backup in a basement with fifteen feet of head, and the shortfall shows up as duty cycle long before it shows up as failure.
Why we do not trust the amp-hour label
Two separate deductions stand between a battery's printed capacity and the energy it will actually give a sump pump.
The first is depth of discharge. Deep-cycle lead-acid chemistry degrades rapidly when routinely taken below about half its capacity, so treating the full nameplate as available is a way of buying one long outage at the cost of the battery. We therefore count 50 percent as usable.
The second is rate. Amp-hour ratings are conventionally quoted at a twenty-hour discharge — a gentle trickle. A sump pump pulls 11 amps, which empties the pack far faster than that, and a battery discharged hard delivers materially less total energy than its slow-rate label. We do not model that second effect at all, which means the runtime above is, if anything, optimistic.
Your 4-year-old 100 amp-hour battery derates to 68 amp-hours at 8 percent per year, and then to 34 usable amp-hours after the discharge floor. The label said 100. That gap is the point of this section.
An untested backup is an assumption
The most common backup failure is not a flat battery. It is a system that was installed, never exercised, and quietly stopped being a system — a charger that failed years ago and never announced it, a float that corroded in place, a battery that went open-circuit and sat there looking installed.
Test it the way it will be used. Kill the power at the breaker, pour water into the basin, and watch the backup start, clear the pit, and shut off. Do that twice a year and note the date somewhere you will find it. If the backup has an alarm, confirm the alarm sounds when the water reaches it, because an alarm nobody has ever heard is not evidence of anything.
A load test on the battery itself is worth more than any age-based rule, including ours. Most auto parts stores will do one at no charge, and a battery that fails it has been failing it for a while.
Related tools
- Failure exposure — how deep a backup your own numbers actually justify.
- Pit inflow test — the inflow figure this page depends on.
- Head and discharge — the head your backup pump must also work against.