How Long Will a 12V Battery Last with a 1500 Watt Inverter?

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Whether it is for a house or RV, a 1500 watt inverter can get the job done as far as converting DC devices to AC and running them. In a typical solar power system, the inverter runs from a battery bank. But how long will a 12V battery last with this inverter if it runs a full load?

A 1500 watt inverter is going to last about 75 to 80 minutes on a 12V 150ah battery with a full load. How long the inverter lasts depends on how much load it carries, the battery capacity and the inverter conversion efficiency.

How to Calculate 1500 Watt Inverter Runtime

To figure out how long a 1500 watt inverter like the NDDI Direct is going to run, we need four things. A battery, how much load the inverter will carry and for how long. Lastly we need to consider how efficient the system is.

Suppose you want to run a 1300 watt load for 2 hours. Batteries come in different voltages but the most commonly used is 12V. So how many 12V batteries do you need?

First we have to convert watts into amps, which is how battery sizes are measured.

Inverter watt load / battery voltage = required battery size

We need to run the 1300W load for two hours, so:

1300 x 2 = 2600

Now we find out what battery size we need:

2600 / 12 = 216

You need 216 battery amps to run a 1300 watt load for two hours.

There is no 216ah battery, so get a 250ah battery or larger. Our pick is the Mighty Max 250ah 12V SLA for its efficiency.

To get an even more accurate estimate, you should account for the inverter efficiency.

How Many Amps Does a 1500 Watt Inverter Draw?

No matter what the inverter size is, they all do the same thing. That is convert DC power to AC. Solar power is direct current, so to run any AC device the current must be converted into AC.

Some power is lost at the conversion, and this determines the inverter conversion efficiency. Back then an 85% efficiency rating was considered the standard. Nowadays it is 90% or higher.

The higher the efficiency rating, the fewer amps are pulled from the battery bank. The more efficient the inverter is, the longer the battery will last.

Imagine you have two 1500W inverters, one is 85% efficient and the other is 95%. Both will run a 1400W load on a 12V 300ah battery bank.

How long will the battery last on an 85% efficient inverter?

1400 / .85 = 1647

An 85% efficient inverter with a 1400W load pulls 1647 watts an hour, not 1400. That is 3294 watts for two hours.

A 12V 300ah battery is equal to 3600 watts / 274ah, so the inverter is going to last just over two hours.

What if the inverter has a 95% effective conversion rate?

1400 / .95 = 1473

In this case the inverter uses 1473 watts an hour or 2946 watts / 245ah for two hours.

The 95% rated inverter draws less power from the battery, allowing the load to run longer. This is especially important depending on what battery type you use.

Which Battery Should I Use on a 1500 Watt Inverter?

There are basically three types, flooded lead acid (FLA), sealed lead acid (SLA) and lithium. AGM and gel are SLA batteries and among the most widely used at homes and RVs.

There are many differences between these batteries. But the most important in relation to inverter runtime is the depth of discharge.

FLA and SLA batteries have to be recharged when capacity drops to 50%. With lithium you can fully discharge it.

This makes a huge difference when figuring out how long your 1500W inverter can hold up and if you are worried about battery overcharge.

A 12V 250ah battery can run a 1300W load for two hours, but that is on a full discharge. If you want to recharge at 50%, you will need a 500ah battery bank.

If you don’t want to get an additional battery, get a 24V unit (if compatible with your inverter) or a lithium battery. A 24V battery holds twice as much watt power as a 12V though it costs more.

A lithium battery can be used up to 100%, sometimes 90% depending on the manufacturer. But it will definitely last longer than an SLA or FLA. If you do not mind paying the extra cost, lithium is an ideal solution.

It really comes down to what you need and your long term plans. Generally lithium batteries are used with large inverters, 3000 watts and up. A 1500W inverter is for small appliances, and in many cases a gel or AGM battery bank will do.

What Will a 1500 Watt Inverter Run?

An inverter is only limited by its capacity, or just below it depending on the efficiency. You can load up to 1500 watts, but in reality it is best to set the limit at 1400.

Inverters should have reserve power in case of a sudden energy spike. If the inverter is maxed out at 1500 watts, a spike could damage the system and whatever is loaded on it.

With reserve power, a rush of power will not be a problem. This should also be done with solar panels, batteries and charge controllers. Never run these systems to the limit.

In terms of load, these inverters can run any as long as it is less than 1500 watts. You can load a laptop, TV, blender, a small microwave and most kitchen appliances. We have an appliance watt guide if you want to check it out.

Some things to keep in mind:

  • 1500 watts is the running time, but it does not account for the efficiency rating. Check the rating of your inverter and use the calculations given here to find its runtime.
  • Appliance wattage are per hour. A microwave rated at 800 watts means it is going to consume that much if it runs for an hour. If you just use it to reheat food for a couple of minutes, power usage will be much lower.
  • Some appliances are designed for pure sine wave inverters only. Others work fine with modified sine. If you are going to buy modified sine wave, check with the manufacturer if it is compatible with your appliances.
  • An inverter can only run appliances for as long as there is power. Whether it comes from a battery bank, generator or the grid, there must be a power source.
  • It is better to overestimate the power you need than underestimate it. The larger the battery bank, the longer you can run whatever load you need.

Running and Surge Watt Capacity Explained

Inverter capacity is measured by its running and surge watt capacity. Surge or start up watts is the power needed to load an appliance, device or motor. Running watts is power consumed once the load has started.

If an inverter is rated 1500 watts for example, it refers to the running watts the system can handle. Its surge watts is usually twice that, so that would be 3000 watts in this case.

Surge watts only last for a few moments, and that is also how long the inverter can supply it. A 3000 watt surge for a second or two is no problem for the system.

An Energy Star refrigerator might consume 150 watts as it runs. But to get the motor started, the inverter battery must provide 1000 watts or more.

This is only an example, and the surge watts for your fridge might be different. But a lot of appliances like refrigerators, AC systems, furnaces, blenders etc. have surge watt requirements.

The surge power should only be used for starting up powerful appliances and not to run appliances. A 1500W inverter can supply a 3000W surge watts, but it cannot continuously run a 3000W appliance. if you need that much power, it is time to upgrade to a larger inverter.


If you are going to run a 1500 watt inverter, make sure that the battery is the right size. The correct size depends on your personal usage so keep that in mind before you set up an inverter.