A typical US battery backup sump pump runs on a 12-volt deep-cycle battery, with Group 27 and roughly 75Ah to 120Ah covering most homes.
When the power cuts out mid-storm, the only thing standing between your basement and standing water is the battery in your backup pump — and picking the wrong one is the quiet reason many systems die in the first hour. The sizing question comes down to three numbers: voltage, amp-hours, and physical group size. Get those right and a fully charged deep-cycle battery typically delivers 7 to 8 hours of active pumping, per Consumer Reports.
Here’s the short version before the details: start with 12 volts, aim for Group 27 at 75Ah–120Ah, then verify your specific pump’s required group size, maximum Ah, and battery box dimensions before you buy anything.
What Voltage And Capacity Do You Actually Need?
Every residential backup sump pump in this category takes a 12-volt deep-cycle battery — not an automotive starter battery, which sulfates and rots far faster under repeated deep discharges. Capacity is where the real decision lives.
Consumer Reports notes that a 75Ah to 120Ah battery allows several hours of intermittent pumping, and that same range is the practical target for most US homes. For longer outages or higher-demand setups, some guidance pushes toward 90Ah–120Ah for extra margin.
Two things drive how long that battery actually lasts: the pump’s amp draw and the duty cycle (how often it kicks on). A sump pump cycling every few minutes drains far faster than one that runs occasionally during a moderate storm.
Do not size by horsepower alone. A 1/3 HP and a 1/2 HP pump can have wildly different runtime on the same battery depending on amp draw and cycling frequency.
Why Group Size Matters As Much As Amp-Hours
Group size is the physical footprint of the battery, and it can kill an otherwise perfect choice. A Group 31 battery with ideal amp-hours is worthless if it won’t fit your system’s battery box.
Little Giant’s SPBS Series backup systems, for example, specify a Size 27, 12-volt deep-cycle battery, and it’s sold separately. Other brands vary — some enclosures only accept Group 24, while others take Group 27, 29, or 31.
Check your backup system’s manual for two things before buying: the required battery group size and any maximum Ah rating the manufacturer specifies. Going over the max can void the controller or stress the charger.
Then measure the box.
| Battery Spec | Typical Range | What It Means For You |
|---|---|---|
| Voltage | 12V | Standard for all residential backup sump systems |
| Group Size | 24, 27, 29, 31 | Must match your battery box; Group 27 is most common |
| Capacity | 75Ah–120Ah | Several hours of intermittent pumping; 90Ah+ for longer outages |
| Type | Deep-cycle (AGM or sealed lead-acid) | Never use an automotive starter battery |
| Runtime | 7–8 hours active pumping | Fully charged battery, per Consumer Reports |
| Weight | ~67 lbs at 120Ah | Confirm your shelf or box can support it |
| Surge Coverage | 2kWh+ for multi-day outages | Startup surge can hit 1800–2500W |
What Most People Get Wrong When Sizing
The most common mistake is grabbing whatever deep-cycle battery is on sale without checking the pump’s stated requirement first. That’s how you end up with a battery that either doesn’t fit or overworks the charger.
Second mistake: ignoring startup surge. Sump pumps draw far more power at startup than while running — running wattage is often 600 to 900 watts, but surge can spike to 1800–2500 watts. If your backup runs through a power station rather than a straight battery, surge capacity matters as much as watt-hours.
Third: swapping in a marine deep-cycle when the system expects AGM, or vice versa. Most 12V deep-cycle batteries work, but sealed AGM and flooded lead-acid have different charging profiles.
For a straightforward range of batteries that fit standard backup systems, this roundup of backup batteries for sump pumps covers models tested for common group sizes.
One more note: many backup sump pumps don’t include the battery at all. Budget for it separately — .
Quick Sizing Checklist
Start by opening your backup pump’s manual and finding the required battery group size and any maximum Ah rating. If it says Group 27, stick with Group 27 — don’t try to squeeze a Group 31 in “for more power.”
Next, pick your capacity target: 75Ah for moderate storms and shorter outages, 90Ah–120Ah if you want margin for multi-day events or a higher-draw pump. Consumer Reports’ guidance puts a fully charged deep-cycle battery at roughly 7 to 8 hours of active pumping.
Finally, verify physical fit and weight. Measure your battery box interior and confirm your shelf can handle a 60–70 pound battery. A 120Ah unit at 67.5 pounds is not something to lift onto a flimsy shelf.
The Little Giant SPBS Series specification page is a good example of how manufacturers state requirements — Little Giant’s SPBS Series product specs list “Size 27, 12V deep cycle battery” directly in the battery field, and mark it not included.
FAQs
Can I use a regular car battery for my backup sump pump?
No. Automotive starter batteries are built for short, high-current bursts and their plates sulfate quickly under repeated deep discharges. A backup sump pump needs a deep-cycle battery designed to be drained and recharged many times. Using a car battery will shorten its life dramatically and may leave you without backup power when you need it most.
How long will a 75Ah battery run my sump pump during an outage?
Consumer Reports notes a fully charged deep-cycle battery typically provides 7 to 8 hours of active pumping time. That’s active runtime, not total outage coverage — a pump that only cycles occasionally can stretch a 75Ah battery across a longer storm. Higher-draw pumps and frequent cycling shorten that window considerably.
Do I need 90Ah or 120Ah instead of 75Ah?
It depends on your outage risk and pump draw. For typical US storms and intermittent pumping, 75Ah is often enough. If you face multi-day outages, have a higher-draw pump, or want a safety margin, 90Ah–120Ah is the safer target. Just confirm your system’s maximum Ah rating and battery box size before sizing up.
References & Sources
- Consumer Reports. “Sump Pumps Buying Guide.” Source for 12V deep-cycle battery type, 75Ah–120Ah range, and 7–8 hour active pumping runtime.
- Little Giant. “SPBS Series Battery Backup Systems.” Official product page confirming Size 27, 12V deep-cycle battery requirement, sold separately.
- Basement Defender. “12V Battery Backup Pump Batteries: What You Need To Know.” Guidance on battery group sizes, capacity ranges, and multi-day outage planning.
