How Long Will a 100ah Battery Run an Appliance That Requires 400W?

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With prices going down, more and more people are buying 100ah batteries for their off grid systems, RVs or to use as a power backup at home. But how effective is this battery when it comes to running 400W appliances? Is it going to run these devices fine or will it lack power?

A 400W appliance draws 33.3 amps per hour, so it will run for about 3 hours on a 12V 100ah battery. Divide the appliance wattage by the battery voltage and you will get the amps drawn by the appliance.

How to Calculate 100ah Battery Capacity for 400W

To find out how many amps an appliance uses, use the formula:

Watts / Battery volts = amps

If you have a 400W desktop computer and a 12V 100ah battery, divide the watts by the voltage. The result is 33.3 amps.

Your computer draws 33.3 amps per hour so theoretically you can run it for about 3 hours on a 100ah battery. If your computer is 200W, it will run for 6 hours. One that we can recommend is the Absolute 12V 100ah Battery which lasts longer than a typical AG, and needs less maintenance.

However this will drain the battery down to 0%. That is not advisable for lead acid batteries because it will shorten the life cycle. Ideally you should recharge lead acid batteries like AGM and gel when it is halfway empty. This helps preserve the battery life for the long term. Being only able to use half the capacity at a time is frustrating, but that’s just how many solar batteries work.

Because a 100ah battery has a 50% depth of discharge, only 50ah is available for use. Instead of powering a 400W appliance for 3 hours, the battery can only run it for 1.5 hours. If you want to use the appliance for 3 hours, buy two 100ah batteries or a 200ah battery.

This applies only for lead acid batteries (FLA, gel and AGM). Lithium ion batteries such as the Miady 100ah have a 75% discharge rate or better so you should be able to operate that appliance for another hour or so. In fact there are some manufacturers who claim lithium ion batteries can be run all the way to zero.

Unless the product instructions specifically state you can drain a lithium battery to 0%, don’t. While lithium batteries are better than lead acid, completely draining the battery might be detrimental in the long term.

But if the manufacturer says you can completely discharge the battery, then you will get 100ah or lose to it. Due to how batteries discharge, only about 85%-95% may be usable even if it is full. But that should be enough to keep the 400W appliance or appliances running for 3 hours.

Battery vs. AC Power Comparison

You can use the same formula above with AC power. Most appliances in the US run on 120V, so use that instead of 12V.

400W / 120V = 3.3 amps

The same 400W appliance that draws 33.3 amps from a 12V battery will draw 3.3 amps from a 120V power source. The higher the volts, the lower the amps drawn.

A lot of batteries use 12V, but there are 24V, 36V and higher volts available. Higher voltage batteries cost more but you will get more usage from your appliance.

You can also increase the voltage by connecting batteries. Join two 12V 100ah batteries in a series and the voltage goes up to 24V. So there are many ways to use a 100ah battery and boost voltage if necessary.

Does it make a difference if you run a single 400W appliance or two 200W appliances? No it should not. You can connect as many appliances as you want on a 100ah battery as long as the total is below 100ah (or below 50ah depending on the discharge rate).

The only time this will matter is if one of the appliances you are using has a high surge watt requirement. Two appliances combining for 400W will run for 3 hours on a 100ah battery. But if one of those has a heavy surge watt requirement, it will deplete the battery and there might not be enough to run the appliances.

Battery Discharging Rate

Battery discharge rate is usually 5 amps for 20 hours. Theoretically if an appliance draws 10 amps, the discharge rate drops to 10 hours and so on.

All this is theoretical because in real life, the discharge rate will vary depending on the battery type, temperature, heat generation and other factors. The higher the amps drawn, the fewer hours the battery can supply, so it won’t be strictly 10 = 10 hours, but around 9 or so. For convenience it is easier to calculate using the 5 amps per 20 hour formula.

The discharge rate also depends on the battery type. Lithium batteries are more energy efficient than lead acid, so if you want to get maximum usage per amp hour, lithium may be worth the extra price. But even with lithium you should not expect a strict 1 amp = 1 hour rule. It will still be 5 amps = 20 hours and as the capacity goes down, so does the available power.

What Appliances Can You Run on a 100ah Battery?

You can run any number of appliances as long as the combined amp draw does not exceed the battery capacity. If you have a lead acid battery, the capacity is 50ah.

Here is a chart showing typical appliance amp usage and their watt equivalents, These are only meant to give you a rough idea of how many watts and amps will be consumed. You should check the appliances individually for more precise details.

ApplianceAmps Watts
Ceiling Fan 0.820-100
Coffee Maker9-10480-1000
Desktop Computer3-7200-700
Portable Heater7-12850-1450
Refrigerator3-7150-300 running (1200W surge watts)
Washing Machine 12.51500
Water Heater16-214000-5000
Window Air Conditioner6-12720-1500

Always check the surge watts when calculating 400W appliance requirements. A blender runs on 400W but its starting wattage is 850W or 70ah. With a 50% depth of discharge, it is beyond the capacity of a 100ah battery. The same with any appliance or device that has a hug starting wattage. A refrigerator uses around 6 to 7 amps , but it needs 1200W to 1500W during startup so a 100ah is insufficient.

If the surge watts exceeds 50ah, a 100ah lead acid battery cannot run it, even if the running watts is less than 100ah or 50ah. The surge watts lasts only a few seconds but without it the appliance won’t run. This applies not just to batteries but inverters and solar generators too.

Note also that while most US appliances use 120V, large ones like washing machines may use 240V. Check the appliance product manual for details.

You can get more from your battery with an energy efficient appliance. The difference can be nearly double, especially with large appliances. So if you haven’t upgraded or replaced your old appliances yet, now is the time. Simply put, it does not make financial sense to run inefficient appliances on solar batteries because they will consume too much power.

What Type of 100ah is Best For 400W Appliances?

A 100ah lithium ion battery has more usable amps than a 100ah lead acid battery because of a better discharge rate. If you need the full 100ah from your battery, get two 100ah lead acid batteries or one lithium ion.

The drawback with lithium as we pointed out is the cost. If you have to use 100ah batteries frequently then get the best battery type you can buy. But if you will only use it occasionally during RV camping for instance, lead acid will be fine.

Determine how long you need to use those 400W of power. If it is only for a few minutes then 50ah will be enough. If you need for more than an hour, either get 100ah lithium or another source like a portable solar generator. You can also connect a solar panel to the battery and keep it charged up.

With enough sunlight the panels can keep the battery fully charged so there is enough to use for evening. However you need a powerful solar panel to match the 400W power requirement. If you are in an RV you can easily install a solar panel and have a full solar system in place.


To wrap it up, a 100ah battery can power 400W appliances, but the runtime depends on the battery voltage and the depth of discharge. So it depends on the appliance and more so on the battery type and capacity. If you decide to use a 100ah battery, make sure you know its discharge rate and how long you want to run the appliance or appliances.