How Long Will a 200ah Battery Run an Appliance That Requires 400W?

RVs and off grid homes need a battery bank for two reasons, to serve as storage for extra solar energy and to be an additional power source. 200ah batteries are widely used, but how long will it last with a 400 watt appliance load? Is it enough or do you need more?

A 12V 200ah battery can run a 400 watt appliance load for 6 hours, while a 24V battery will last for 12 hours. If the battery has a 50% depth of discharge, the runtime will be 3 hours for a 12 volt system and 6 hours if the battery is 24 volts.

Is 200ah Enough For a 400 Watt Appliance Load?

A 200ah battery can run a 400 watt load, no question. Whether it is enough depends on how long you need that power. To better understand the capacity we need to look at the numbers.

A 12V 200ah battery has 2400 watts. Theoretically you can run a 400 watt appliance for 6 hours:

2400 watts / 400 = 6

If you have a 24V battery like the Lynx Lithium Ion, the runtime is doubled:

4800 watts / 400 = 12

This calculation presumes the battery will be fully discharged. If you have a lead acid battery the depth discharge should be 50%. A 12V battery will have 1200 watts usable instead of 2400, and 2400 watts instead of 4800.

For 12V 200ah:

1200 watts / 400 = 3

For 24V 200ah:

2400 watts / 400 = 6

Other batteries like AGM have a 70% discharge while lithium can be 90% to 100%, so adjust your calculations depending on the depth discharge.

So to answer the question, yes a 200ah battery is enough for a 400 watt load. But the discharge rate affects how long the battery can power the load. Also note that these numbers assume you are running the load at a full 400 watts.

How Long Will a 200ah Battery Really Last?

Calculating the runtime for a 200ah battery will not give you an exact answer, only estimates. Because of how batteries work the discharge rate increases when more power amps are drawn.

Suppose you have a lithium battery with a 100% DOD. You have a 200W laptop, a 12V freezer and some lights. If you run all these, the battery can run it for 5 to 6 hours.

Remember that more amps drawn, the faster the battery capacity drops. A 400 watt load is equal to 33.3 amps an hour. While the math says 33.3 x 6 = 199.8, in reality it will probably be just under 6 hours.

However if the draw is lower, that 200ah battery capacity will hold up longer. If you have a 400 watt load but with variable usage, the runtime will be longer.

Sample Runtime Calculations

A food blender requires 300 to 400 watts an hour. Run that on a 200ah and the battery will drain at the same time as the example given earlier. But you do not run blenders for 5 or 6 hours. You only use it for a few minutes at a time.

So you can have two 200ah batteries with a 400 watt load with different runtimes. The 200ah battery that runs the load continuously will run out in 6 hours maximum. The battery that 400 watt load with variable usage will last longer.

This running time will be different again if the battery has a lower discharge rate. If you have a 50% DOD battery and running appliances with variable usage like a blender, expect the results will be different.

The simplest way to figure this out is to list all the appliances you will run, and for how long. It is easy if you will power them on continuously. If they will be used intermittently, calculate the wattage used per minutes.

For instance if you are going to use a 400 watt blender for 15 minutes an hour, that is 100 watts. Repeat the same process for the other appliances to find out how many watts will be used. Once you have the watt hours, you can divide the watts by the volts to get the amps.

How Many Watts Do I Need to Charge a 200ah Battery?

It takes 600 watts of solar power to recharge a 200ah battery. Two 300 watt solar panels can produce up to 3000 watts a day, enough to charge the battery.

To run a full 400 watt load, the battery should be fully charged. Besides two 300 watt PV modules, you can also use 250 watt solar panels or 300 or 100 watts. We chose 300 watts because of its popularity and practicality. However you can use other solar panels if you like.

The important thing to remember is the sunlight available. A 600 watt solar array can produce 3000 watts provided there is 5 to 6 hours of sun available. If the weather is overcast the output will be lower and charging time will take longer. If you charge the battery with electricity it will be faster.

If you have a deep cycle battery only half the capacity should be used. For a 200ah battery that means you have to recharge it when capacity drops to 1200 watts. in that case a single 300 watt solar panel can charge the battery to a 100%.

Solar Panel Charging Efficiency

This is where efficiency comes into play. Solar panels have an efficiency rating, usually around 21% to 23% for the best ones. This refers to how much sunlight can be converted into solar power. This does not refer to their output. Most panels have an output rating of 85% to 100%, so a 300 watt panel will give you 285 to 300 watts.

But these numbers assume the sun is shining. If the weather is unfavorable, give the panels more time to recharge the battery . During summer the charge time should not be a problem, but for other seasons you have to expect it to take a bit longer.

Battery Age and Quality

Batteries will also take longer to charge as they get older. A new battery charges faster and discharges slowly. As it gets worn out, the opposite happens.

If you notice your battery is taking longer to charge and loses power more quickly, it is probably due to age and wear and tear. This can also happen if you discharge a deep cycle battery to 0% and recharge it up to 100%.

Ideally deep cycle batteries should be recharged at 50% and topped off at 85%. However this is not always possible, but try to follow this as much as possible. But even if you do this the time will come when the battery will show signs of age, and it has to be replaced. if the battery is new, you can do some troubleshooting.

What Type of Battery Should I Buy?

You have two options, lead acid or lithium, then you have to decide whether to get a 12V or 24V. What you get depends on the load you want to run.

With regards to the type, lithium batteries like the offers a longer life cycle, no maintenance and higher discharge. These batteries also cost more. Lead acid batteries come in different forms like FLA, SLA, gel and the Renogy AGM Deep Cycle Battery, which we prefer . Most of them have a 50% DOD but others are at 70%.

As for the voltage, 24V can store more watts than 12V. But 12V batteries can run the 400 watt load required here. If you want to run the load for longer periods, a 24V battery is the way to go. If not a 12V will be just fine.

The battery you choose will also depend on the solar panels and charge controller you have. If you have a PWM charge controller, your solar panels and battery have to match. If the solar panel is 36V or 46V, an MPPT charge controller is better because it allows the panels to produce the highest possible output.

Conclusion

200 amp hours is plentiful and should be fine for most RVers and campers. But if you use a lot of appliances it may be insufficient. So the key is to plan ahead so you can estimate if the battery is enough or you need more.