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

A 100ah battery is often the preferred choice to store solar energy. It is not too big and not too small either, just right for many situations. But how long can you use a 100ah battery on an appliance that needs 100 watts? The answer is not that difficult to find out once you learn the missing variables in this equation.

A 12V 100ah battery can run a 100W appliance for 20 hours at a draw of 5 amps/hour. Due to inverter inefficiency and energy losses, drawing more than 5 amps an hour reduces the ah capacity rapidly, but drawing less slows the discharge rate.

How Long Will a 100ah Battery Run a 100A Appliance?

You can use any appliance or device that requires 100W in our calculation that follows. This formula also assumes the use of lead acid batteries and AGM, but you can replace it with a lithium battery, which has a better discharge rate. A better discharge rate means the battery will last longer as there is more power available. The numbers assume a power inverter with an 85% efficiency rating. If the efficiency is higher, the battery life will be longer.

As the calculation shows, the lifespan of a 100ah battery depends on how much amp hours are drawn. The difference between high and low draws, and the battery duration, is significant. The life cycle of gel, AGM and other batteries will vary according to the manufacturing design. The Absolute 100ah deep cycle AGM battery for instance, is designed to run longer than the average lead acid battery. Self-discharge is lower so it runs longer each cycle and it is leak proof too.

100AH Battery For 100W Appliance Calculation

To keep it simple, you use more battery power with a draw greater than 5 amps an hour. You use less battery power with a lower draw.

Battery size is measured in amp hours (ah) and volts (12V, 24V etc.), while appliance power consumption is measured in watts (W). In this case, 100W means the appliance needs 100 watts an hour. So how many watts is 100ah? The conversion formula is amp hours x volts = watts. So our 12V 100ah = 1200W.

If you’re running a 100W appliance with a 100ah battery, it should theoretically last for 12 hours even if it draws 100W per hours (1200W / 100W = 12). But it does not work out that way for two reasons, inverter inefficiency and Peukert’s Law. Inverter inefficiency results in energy loss during current conversion. The energy loss can vary, but most are in the 15% range. Peukert’s formula meanwhile, states that battery capacity goes down with higher draws.

When you draw a big load, the ah capacity is reduced significantly. But if you draw small loads, the discharge rate gets smaller. So a 100ah battery running a 100W appliance will last longer if the draw is under 5 amps/hour. It really depends on how many amps the appliance will use per hour.

A battery with a 100ah rating assumes the usage will be 5 amps per hour, which means the battery will last 20 hours. Almost all deep cycle batteries use the 20 hour rate. But it doesn’t mean the battery will last 100 hours if you draw just one amp per hour, or 50 hours if you draw 2 amps. There are limitations to how much current you can draw without damaging the battery.

Now let’s get back to Peukert’s Law, where the higher the current being drawn, the lower the ah rating goes. The opposite is true. The lower the current, the higher the ah rating goes. So instead of a 100 hour lifespan for 1 amp an hour draw, you will probably get 50 amps for 75 minutes before the battery is discharged completely.

How Many Hours Will a 100ah Battery Last?

A 100ah battery will last 9.7 hours with an 85% efficient inverter. But we have to include the effects of Peukert’s Law, so the number will be from 6.5 to 9.7 hours depending on how many amps the appliance draws.

This assume the appliance will run for an hour. The battery will last longer if the draw load will only be a few minutes. You can run a 10A device for a maximum of 10 hours. But that same 100ah battery will last more than 100 days if the device runs just 5 minutes a day.

Here is another example. You can run a 12V, 60W portable fridge on a 100ah battery for 20 hours (60 / 12 = 5). But an energy efficient fridge can operate only 15% of the day. It draws just .75ah / hour so the battery can last for days.

The inverter inefficiency determines how long a battery will last before losing all power. To figure out how long our 12V, 100ah battery will last, follow the steps below. You can use the same formula for any battery volt and ah rating. Just replace the numbers with your own. This is for a 12V 100ah battery and an 85% efficiency rated inverter.

100W / 12V = 8.3 amps

8.3 / .85 = 9.7

As we pointed out above, we have to account for Peukert’s Law, so the battery might only last 6 hours. An energy efficient appliance will make the battery last longer. For instance, a 100W LED TV will use less power than LCD, OLED or older systems like plasma and CRT. That is why it is important that you use energy efficient appliances. It will really make a difference when you are counting every watt.

The inverter must be the right size to run any appliance. If you are drawing 100W, the inverter capacity must be at least 125W. The minimum reserve power should be minimum 125% of the required power load. You can always get a bigger inverter, but do not settle for anything smaller because the appliance will not run.

How Long Can You Use a 100AH Battery?

Flooded lead acid (FLA) batteries have a maximum 50% depth of discharge, while AGM and gel is between 50%-70% depending on the manufacturer. Unlike lithium, lead acid batteries should not be fully discharged. Once the level drops below 50% there won’t be enough power to run the appliance. This is true whether it is 100W or any rating.

Not only will the appliance fail to run, but recharging below 50% reduces the life cycle of a lead acid battery. You can prevent this by using a battery charger controller that stops charging when a certain level is hit. This is a great tool to keep the battery in good condition. Another option is to buy another battery and connect to the appliance to provide more power.

Because you can only half the capacity of FLAs, you have to double the capacity if you need to run 100W at the maximum draw. A 12V 100ah battery with a 50% DOD can run a 100W appliance for 10 hours, not 20. If you want to reach that 20 hour mark you have to buy a 200ah battery or use two 100ah batteries.

This is not a problem with lithium ion batteries because you can discharge it 85%-100%. With a 100ah lithium ion you will get 100 amp hours, or close to it. The only disadvantage is the cost as lithium is more expensive. If you want to try lithium, we recommend the Battle Born LiFePO4, which has ten times the power of a typical FLA battery.

Can I Connect Two 100ah Batteries?

Yes you can. Doing so will increase the amp hours and run devices for longer periods. There are two ways to connect batteries, in parallel or in series. A parallel connection is ideal because it boosts the amp hour rating but the voltage is at the same level. In a series, the voltage doubles but the ah rating does not change.

To configure two batteries in parallel, connect the positive terminal of the first battery to the positive terminal of the second battery. Do the same for the negative terminals of the batteries. Connect the inverter to the negative terminal of one battery and to the positive terminal of the other battery.

Only new batteries for parallel or series connections. Do not mix old and new batteries because the system will reduce the capacity of the new to match with the old. An old battery might also cause other problems and damage other components.

Conclusion

So figuring out how long a 100ah battery will run a 100W appliance is not as straightforward as it seems. While there are a lot of numbers involved, it is always good idea to enhance your knowledge of how solar power works.