There’s probably nothing more nerve-wracking during a power outage than to watch your sump overflow and flood your basement.
If you’ve got a deep cycle battery on hand, or if you were looking to plan ahead, you’re probably wondering just how far that brick of lead and acid can take you.
A deep-cycle battery can run a sump pump motor continuously for 45-80 minutes depending on the size of the pump. If the pump is only turning on a couple of times per hour you can easily get 1 or more days of use out of the battery.
In the below examples, I looked at 5 different deep-cycle battery set-ups and with so many makes and models of pumps I used a range of sump pumps from 600 to 1,000 watts and in increments of 50.
There were 3 considerations to be made when I came up with my numbers regarding the batteries, the inverter, and Peukert’s Law.
If you’re just looking for a quick chart of the results to get an idea, feel free to click here.
Deep-Cycle Battery for a Sump Pump
Combined with a properly sized inverter and cables, a 12-volt deep-cycle battery should be able to power your sump pump for a little while.
I took a look at some common sizes and types, and made my examples using a 100 amp-hour (AH) AGM, an 80AH AGM, a 105AH Flooded an a 90AH Flooded battery. I also took a look at what two 215AH GC2 6-volt golf cart batteries could do when hooked up in series.
All of the AH ratings are at the 20 hour rating.
I’m only using lead-acid batteries in my examples, but the AGM’s are maintenance-free as opposed to the flooded types. I figure this may be of interest if you’re not too interested in doing quarterly battery maintenance.
Inverter and Sump Pump Energy Requirements
Having a properly sized inverter is critical to operating your sump pump with a deep-cycle battery. If you size your inverter too small, it doesn’t matter how large your battery is.
How many watts is a sump pump?
Sump pumps typically have between 600 and 1,000 running watts with 1/3 horsepower pumps on the lower end and 1/2 horsepower pumps on the high end. They also have a start-up requirement that is usually 2-3 times the running watts that will last for a second or so.
The inverter you choose will need to be able to adequately power the running watts and handle the initial start-up surge.
What size inverter do I need for a sump pump?
A 1,500-watt inverter with a surge rating of 2,000+ watts is a solid choice for powering most 1/3 and 1/2 horsepower sump pumps that you’ll find in most households. It is enough to cover the 600 and 1000 running watts of the sump pump along with the start-up watts which can be 2-3 times as much as the running watts.
For some of the 1/3 horsepower sump pumps, you might be able to get away with a 1,000-watt inverter.
Still, though, I would stay north of 1,000 watts for sure and try to make sure your peak watts will be high enough.
It is best to consult the manufacturer’s sticker on your pump and even contact the manufacturer to get an idea of exactly what watts are required.
If you’re looking to make this a dedicated system, I would go with a pure sine wave inverter which will deliver cleaner and more reliable electricity to the pump. If you’re just buying an inverter to have on hand and use with your pump here and there, you will probably get away with a modified sine wave inverter.
The final thing to note when using an inverter is that they are about 85% efficient, or 15% inefficient when switching the DC power from your battery to AC power for your pump.
In my findings, I factored this in. So, if a pump was rated at 600 watts did the following math: 600 watts/12-volts(battery) = 50 amps ==>50 amps/0.85 efficiency factor = 58.8 amps being drawn from the battery to overcome the loss in efficiency with the inverter and still deliver 50 required amps to the pump.
3. Peukert’s Law
I made sure to include Peukert’s Law in my findings as well and I assigned a Peukert Constant of 1.1 to the AGM batteries and 1.2 to the flooded ones.
Peukert’s Law basically states that the higher the draw you put on a battery relative to its AH rating, the lower the overall AH capacity will be.
For example, a 100AH battery can give out 1 amp for 100 hours, and probably 2 amps for 50. But, if you were to give out 50 amps, you wouldn’t get 2 hours. You’d likely only get just over an hour.
The higher the draw, the lower the amp hours of the battery. Conversely, the lower the draw (relative to the rate at which it was tested – typically the 20-hour rate), the higher the amp hours.
Since sump pumps are massive energy hogs (600-1,000 watts) relative to the capacity of the battery (1,200 watt-hours = 12-volts * 100AH), I felt it was important to include Peukert’s Law so we don’t overestimate the results that we’re likely to get in reality.
How Long Can a Deep-Cycle Battery Power a Sump Pump
|100 AH AGM
|80 AH AGM
|105 AH Flooded
|90 AH Flooded
|215 AH GC2 (2x6v Golf Cart Batteries)
|600 Watt Sump Pump
|650 Watt Sump Pump
|700 Watt Sump Pump
|750 Watt Sump Pump
|800 Watt Sump Pump
|850 Watt Sump Pump
|900 Watt Sump Pump
|950 Watt Sump Pump
|1,000 Watt Sump Pump
Showing how I got my numbers:
I’ll go over one example but spare you from the rest as it would be a boring read.
Let’s take the 105AH flooded battery and see how long it can power an 800-watt sump pump.
The equation we will be using to factor in Peukert’s Law is: Battery Time in Hours = rated hours of battery(AH of battery / (total amp draw * rated hours of battery))^Peukert Constant
So, 800-watts of sump pump / 12 volts (for the battery) = 66.7 amps.
66.7 amps / 0.85 inverter efficiency = 78.5 total amps being drawn from the battery.
Battery AH were at the 20 hour rate. I’m using 1.2 as the Peukert Constant for a flooded battery.
20 (105AH/(78.5 * 20))^1.2 = 0.78 hours, or 47 minutes.
What do the Results Mean?
At first glance, you might be a little let down by the results. I certainly was.
However, you have to remember that a sump pump likely won’t be running nonstop, so you could probably stretch these numbers out to the better part of a day or more.
For example, the 100AH AGM battery powering a 600-watt sump pump gets 1.33 hours of use (1 hour and 20 minutes) until fully discharged. Remember, this is when running the motor constantly.
If your pump only kicks on once every 15 minutes for 1 minute, then you’re looking at using only 4 minutes per hour.
1.33 hours of use * 60 minutes per hour = 79.8 minutes ==> 79.8 minutes / 4 minutes per hour = 19.95 hours of operational use.
So don’t discount a battery backup system as a viable option. It’s just going to depend on how often and how long your pump typically runs on a busy day.
Is a Battery Backup Sump Pump Worth It?
Short of an automatic whole home backup generator, a battery backup sump pump is worth it for one very key reason: it will keep your basement from flooding when you aren’t home.
Sure, conventional generators are probably more cost-effective and more versatile, but you have to be home to use them. The battery backup sump pump gives you a few hours to the better part of the day to make it home from work to get your house in order.
Now, if you know the water rates in your home and you know that when it rains the sump pumps will work continuously, then you might want to reconsider a battery backup from a deep-cycle battery and look into a standby generator since your time will be limited.
Generator for Sump Pump
Personally, I would secure a generator before investing in a dedicated battery backup sump pump or building one myself. I think that the versatility of an adequately-sized generator exceeds the benefits of a battery backup.
You could power your fridge, electronics, and tools during a blackout with a generator.
Sure, battery banks have their place, and I personally love them at night when I don’t want to leave a generator running at night unattended, but securing a generator is probably the wisest move.
An inverter generator with at least 2000 running watts would likely be sufficient, or any conventional generator at that size or greater.
I wouldn’t skimp on watts if I were getting a generator for the first time. Once you get into the market, it’s not that much more expensive to just size up — especially conventional generators.
5 kilowatts (5000 watts) is a good starting point for a conventional generator and you’ll be able to do most things around your home, just not necessarily at the same time.
Can I use a marine battery for my sump pump?
You can certainly use a marine battery to power your sump pump and will likely get around 40 to 70 minutes of motor use with 100AH and depending on watt requirements of the pump. A deep-cycle marine battery should be used rather than a dual starting/deep-cycle battery for optimal results and overall battery health.
How long will a marine battery run a sump pump?
A 100AH deep-cycle marine battery will continuously power an 800-watt sump pump motor for about 45 minutes until it’s fully discharged. The effective time will be greater since your sump pump will likely not be running nonstop. If it runs one minute at a time every 15 minutes you would get you about 11 hours of use.