How big of an inverter do you need to run a sump pump?

Determining the right size inverter for a sump pump is important to ensure it can provide enough power to keep the sump pump running during a power outage. The size of inverter you need depends on the power requirements of the sump pump, how long you need it to run on battery power, and any additional loads you want to connect.

How much power does a sump pump use?

The power consumption of a sump pump is measured in watts or amps. A typical 1/3 HP sump pump draws around 5-7 amps while operating. Some key factors determine how much power a specific sump pump will draw:

• Horsepower rating – Higher HP pumps use more power
• Voltage – 120V AC pumps draw more amps than 240V models
• Water lift height – Pumps pushing water higher use more power
• Pump efficiency – More efficient pumps may use less power

Check the sump pump nameplate or manual to find its rated amperage draw under load. This is the minimum size inverter you need for running the pump itself. Any additional loads will increase the inverter size requirement.

Estimate run time needed

The run time needed from the battery bank is a major factor in proper inverter sizing. Sump pumps usually cycle on and off intermittently. But in a power outage you need the battery bank to run the pump continuously for hours or even days.

Estimate the longest duration you expect to need from the pump during an outage. Some key factors include:

• Power outage history in your area
• Seasonal weather patterns
• Battery bank size
• How critical the application is

For a residential sump pump needed during seasonal storms, an inverter with 6-12 hours of run time may be sufficient if outages average less than a day. For a commercial application where uptime is critical, sizing for multiple days may be needed.

Calculate inverter capacity

Once you know the pump power draw and estimated run time needed, you can calculate the required inverter and battery capacity using this formula:

Inverter Size (Watts) = Pump power draw (Watts) x Desired run time (Hours) + Safety factor

A safety factor of 25% helps account for:

• Aging batteries with reduced capacity
• Inverter inefficiency (~5-10% power loss)
• Additional loads like controls and alarms

For example:

• Sump pump power draw: 1000W
• Desired run time: 24 hours
• 25% safety factor

Inverter size = 1000W x 24 hrs + 25% = 30,000W (30KW)

To get the minimum amperage rating, divide the watts by voltage:

• 30,000W / 120V = 250A
• 30,000W / 240V = 125A

So for this example you would need an inverter with at least a 30KW capacity and 125A 240V or 250A 120V output.

Matching the inverter voltage

The inverter voltage must match the voltage of the pump being powered. Most residential sump pumps are 120V AC. Some larger pumps may be 240V. Choose an inverter with output voltage that matches the pump.

Resistive loads like sump pumps can typically utilize a pure sine wave or modified sine wave inverter. Pure sine wave models provide cleaner power but cost more. Modified sine wave is usually adequate for pumps and other simple inductive loads.

AC coupled vs battery based inverters

Sump pump inverters are available in two main configurations:

• AC coupled – These systems use a battery charger and external batteries. The inverter draws from the batteries. More complex but allows greater flexibility in battery bank sizing.
• Battery based – The inverter and batteries are integrated in a single enclosure. Simpler plug-and-play setup but battery capacity is fixed.

AC coupled inverters give you the advantage of custom sizing the battery bank to achieve your desired run time. But battery-based models are simpler to install as a retrofit to an existing pumping system.

Putting it all together in a backup power system

A complete sump pump backup power system consists of several components:

• Sump pump – Primary pumping device
• Inverter – Converts battery power to AC
• Battery bank – Energy storage to power the pump during an outage
• Battery charger – Recharges the batteries from AC when available
• Transfer switch – Automatically swaps over to battery power when the mains electricity goes out
• Enclosure – Protects the system from the elements

Properly sizing each component ensures the system can deliver reliable power to the pump when needed. Professional installation is also recommended to configure the system for seamless automatic operation when primary power fails.

Backup power is crucial for sump pumps to prevent flooding during outages. Correctly sizing an inverter using the pump power draw, run time needed, and safety factor will ensure your system can handle the load and provide peace of mind.

Frequently Asked Questions

How do I figure out wattage for a sump pump?

Check the sump pump manual or nameplate for the amperage or wattage rating. If only amps are listed, multiply by voltage to get watts. For example, a 5 amp, 120 volt pump draws 600 watts (5 x 120 = 600 watts).

What size inverter do I need to run a 1/3 hp sump pump?

A typical 1/3 hp, 120V sump pump will draw around 500-600 watts. Size your inverter using the pump wattage, desired run time, and a 25% safety factor. For 8 hours of run time, an 800 watt inverter (600 x 8 + 25% = 800) would be sufficient.

Can I run a sump pump off a car battery?

Technically you can, but a car battery is not ideal for running a sump pump. The small capacity would only give you 1-2 hours of runtime. Use deep cycle batteries designed for deep, continuous discharge to get through extended outages.

What is the best battery for a sump pump?

Flooded lead-acid and AGM batteries are common choices for sump pumps. Look for deep cycle batteries with 100Ah capacity or greater. Golf cart batteries provide cost-effective backup power if properly maintained.

How long will a sump pump run on a fully charged battery?

Run time depends on the pump power draw, battery capacity, and age/health of the batteries. A good rule of thumb is to size your battery bank to provide 8-12 hours of run time. Properly maintaining the batteries will maximize the backup time.

Do I need a special inverter for a sump pump?

You need an inverter capable of delivering the wattage required by the pump. The output voltage should match the pump voltage. A modified sine wave inverter is adequate for powering a sump pump in most cases.

What happens if the inverter is too small?

An undersized inverter could overheat, shut down, or blow a fuse during extended pump operation. Make sure to size your inverter 25% larger than the pump rating to allow a safety margin.

Where is the best place to install a sump pump inverter?

Locate the inverter as close to the batteries as possible to minimize voltage drop. The inverter and batteries should be protected from water and extreme temperatures. An attached garage or basement are typical installation spots.

Can I run multiple pumps off a single inverter?

You can run multiple sump pumps from one larger inverter provided it is sized to handle the combined power draw of all pumps running simultaneously.

What size wire should I use for my inverter?

For safety and efficiency, choose an appropriate wire gauge based on the inverter amperage rating and wiring distance. Consult an inverter wiring guide or electrician for recommendations.

Conclusion

Properly sizing an inverter for a sump pump ensures reliable operation during power outages to prevent basement flooding and water damage. Determine the pump power draw in watts and the runtime needed from the batteries. Use the pump wattage, desired run time, and a 25% safety factor to calculate minimum inverter capacity. Choose an inverter with suitable voltage to match the pump. Use deep cycle batteries and proper maintenance for maximum backup time on battery power. With a properly designed system, you can rest assured your sump pump will keep working even when the lights go out.