How Long Will a 400 Watt Inverter Last?

As an Amazon Associate, this site earns commissions from qualifying purchases. For more details, click here.

A 400 watt inverter is a practical solution for small AC appliances and devices. It costs less than a 1000W inverter for sure, but how long can a 400 watt inverter realistically last? Is it enough for your power requirements?

A 400 watt inverter will last about 3 hours on a 12V 100ah battery, with the exact runtime dependent on the efficiency rating. An inverter with an 85% efficiency rating will last around 2.5 hours.

How to Calculate Inverter Conversion Efficiency

An inverter converts DC power to AC. During the process, some energy is lost. This is expressed in percentage and referred to as inverter conversion efficiency.

Low quality inverters have an efficiency rating of 80% or less. Modern quality inverters are 85% to 95% efficient. The higher the rating, the less power is wasted. The Voltcube Power Inverter is a great example of a high efficiency system that will last a long time.

If a 400W inverter is 95% efficient, only 5% energy is lost and the system can run almost at full power. The lower the rating, the shorter the inverter runtime.

A 100W TV running on a 400W 95% efficient inverter only wastes around 20 watts per hour. If the inverter is only 85% efficient, that number increases to 70 watts.

If you run the TV for four hours the total wasted energy can reach 280 watts. That is enough to run another TV or a laptop.

All inverters consume some power even when in standby mode. But with a high efficiency unit you can minimize the energy loss.

This also means that a large inverter consumes more power when running a low load. So for small appliances and devices it is better to use a 400W inverter.

The bottom line is you should get the most efficient inverter you can afford. Doing so will save not just energy but also make your solar system more effective in the long run.

What Will a 400 Watt Inverter Run?

Any appliance or device under 400 watts can be loaded. The runtime will be determined by the power source.

The guide below is for a 400W inverter with a 95% efficiency rate. Note that the runtimes are estimates. Fro more info on what a 400 watt inverter can run, click here.

  • Laptop 2 hours
  • Electric blanket 3.5 hours
  • Ceiling fan 25 hours
  • Waterbed heater 1 hour
  • 25W light bulb 15.5 hours
  • Radio 7.5 hours
  • Energy efficient refrigerator 3 hours
  • Slow cooker 2.5 hours
  • TV 3.5-4 hours
  • Blu-ray 15 hours
  • Drill 1 hour
  • Sewing machine 3.5 hours
  • Blender 1 hour

The appliance wattage are estimates only. Check the label or manual for the exact number.

Some appliances have variable wattage use. A blender might be rated at 400 watts, but you only run it for a few minutes.

There are many factors that might affect inverter performance, especially if it is running on batteries. An old battery doesn’t hold charge very well and will deplete faster.

A pure sine wave inverter offers better performance than modified sine. Less energy is lost and appliances run smoother. There is less noise too. But modified sine wave is more affordable.

Running and Surge Watts Explained

When calculating how long an inverter can last, you have to account for the running and surge watts.

An energy efficient refrigerator might need 100 watts to run but 200 watts or more to start up. This is also the case with some power tools, so keep that in mind if you want to run power tools on solar power.

The surge watts is only a second so your inverter should be fine. But the surge watts must be under 800 watts, with this inverter size at least. The rule of thumb is the surge capacity is double, so a 400W inverter has a surge limit of 800 watts.

A furnace fan for instance, might use 300 watts as it runs. But most of them need 1000 watts to start up. In this case the inverter will not be enough.

Whether it is a pure sine or modified sine, this rule applies. You should always check the specs before installing an inverter but generally you can assume the surge capacity is twice the running watts.

How Many Batteries Do I Need For a 400 Watt Inverter?

The answer depends on three factors; how much power you need, for how long and the the battery depth of discharge.

The formula is simple:

Total runtime in watts / battery volts = battery size

If you want to load 350 watts on the inverter and run it for two hours, you need at least a 12V 60ah battery.

350 watts x 2 = 700

700 / 12 = 58.3

You need at least 60 usable battery amps. Now if you don’t want to fully discharge the battery, get a 100 or 120ah deep cycle battery.

A 120ah 12V battery can run a 350W inverter load for two hours before capacity drops to 50%. You can run it for another two hours but that will completely discharge the battery.

AGM batteries are not meant to be fully discharged. If you want a higher discharge consider buying a lithium battery. They are more expensive but you do get more use per charge. A couple of 100ah batteries should be enough for most tasks. Our favorite is the Weize 100ah Deep Cycle Battery because it charges quickly and can handle a lot of power.

The calculations here are estimates only. Aside from the discharge you also have to factor the inverter efficiency. The following section explains how this affects the running time of any power load.

How Long Will a 12V Battery Last with a 400 Watt Inverter?

It depends on the inverter efficiency and how far you are willing to discharge the battery. We have covered the discharge rate already, so let us look now at how efficiency affects battery life.

The lower the inverter efficiency, the more battery power is used. Here is an example.

Suppose you have an 85% efficient inverter and run a 200 watt laptop. If the inverter was 100% efficient the laptop will use 16 amps an hour, but with an 85% efficiency rating, the inverter consumes 19.5 amps.

Here is the math:

200 / .85 = 235 watts

235 watts / 12volts = 19.5 amps

The inverter will use 235 watts an hour. The laptop itself consumes 200 watts with 35 watts going to waste.

With a running time of two hours that is an additional 70 watts or 5.8 amps pulled from the battery (watts / volts = amps).

In other words, the inverter draws more amps than the laptop needs. More amps drawn means faster battery discharge. This is why you need a high rated inverter to save energy and conserve battery life.

Your inverter is 85% efficient. it has a 300W load running on a 12V 100ah battery. How long will the battery last?

300 / .85 = 352.9

The inverter consumes 352 watts or 29.4 amps an hour.

352.9 / 12 = 29.4 amps

So at about 30 amps an hour, the battery will last about 3.3 hours.

What if the inverter is 95% efficient?

300 / .95 = 315.7

315.7 / 12 = 26.3 amps

The inverter only draws 26 amps an hour. The same load will last almost four hours. This example shows how much of a difference efficiency makes for an inverter.

Tips For Using a 400W Inverter

  • Know its capacity and limitations. List all the devices you will use. Calculate if the inverter will be sufficient. Note how many you will use at the same time.
  • Decide what battery bank to get. You can go with AGM, gel or lithium. The capacity depends on how long you intend to run your appliances.
  • Solar panels. Are you going to use solar panels with your inverter? Doing so will reduce the your dependence on the batteries. You can also charge the batteries with solar power.
  • Expansion and upgrade. Do you think you will stick with 400 watts for the foreseeable future? If there is any chance you might need more power, it is better to get a 500W or 1000W inverter now.
  • Choose quality. Always get the highest quality inverter you can buy. It doesn’t have to be pure sine as there are good modified sine wave inverters. Just make certain it is from a reputable manufacturer.
  • Follow the instructions. The manual will tell you what batteries can be used, capacity etc. Follow the installation and safety guidelines.


While large inverters are becoming more affordable, there is still a market for 400W units. If you only use a small range of appliances, this should be more than enough.