How Long Will a 400Ah Battery Last? + Calculator

in short, 12v 400ah battery with a 50% DoD limit will last between 20 hours (running a 110-watt AC appliance) to 1 hour (running a 2100-watt AC appliance).

There are many factors that will determine the backup time or how long will your 400Ah battery will last on load.

  • Battery Type
  • Output AC/DC load
  • Inverter efficiency
  • Battery life cycle

Which I’m gonna explain this in simple words for you in this post with the help of examples and charts, so stay tuned!

In the end, you’ll have a complete idea about how you can calculate the run time of any size battery in the future.

if you’re in hurry here’s a formula for you that you can use to calculate the backup time of your battery. or use our ” Battery Runtime CALCULATOR ” (given below)

Battery Runtime = Output Load + 15% (85% inverter efficiency) / Battery capacity in Watts

But there are many things to consider before making the calculation. So keep reading for more accurate calculations.

What does 400Ah mean in Watts?

Converting amp-hours (Ah) of your battery in watts will make the calculation stuff easy for you.

Because battery capacity is measured in amp-hours but most of our appliances are rated in watts

Watts = Ah * battery volts 
400*12 = 4800 watts or 4.8kWh

400Ah battery is equal to 4800 watts. But you may not be able to use all the 4800 watts depending on your battery which I’m gonna cover in a moment

So if you have a 24V battery then make the calculations accordingly. And also further in this post, I’ll take a 12v 400Ah battery as an example

Related post: Amps To Watts Calculator: How Many Watts In A 12-Volt Battery?

Video – Battery amp-hour, & watt-hour

watch this video to understand the basic of batteries (capacity in Ah, watts, charge and discharge mechanism)

Battery Type & Backup Time

The most commonly used battery types are Lead-acid, AGM, and Gel batteries. Which comes at reasonable prices but the drawback is you can’t discharge them 100%.

if you do it will affect the battery life or in some cases maybe you won’t be able to recharge your battery ever again

so the standard discharge limit for these types of batteries is 50%. This means you can only use 50% of their full capacity (depending on the manufacturer)

But the lithium battery type can be fully discharged without compromising on the battery lifespan

Like from this Ampere Time 12V 400Ah Deep Cycle LiFePO4 Battery, you’ll be able to drain 4800 watts of power from it.

Here’s a chart of battery types along with their recommended DOD limits and capacity which you can use in Ah and watts

400Ah battery TypeDOD limituseable AhUseable watts
Lead-acid 50%200Ah2400 watts
AGM50%200Ah2400 watts
Gel50%200Ah2400 watts
Lithium-ion100%400Ah4800 watts

So 400Ah battery never means you can drain 400 amp-hours of power.

Video – AGM vs Lithium-Ion Battery 

Output Load & AC vs DC watts

First, let’s talk about AC vs DC watts/load. Most of our household appliances need 220 volts because of home grid voltage which comes in 240 volts.

But our solar batteries come in 12,24, or 48 volts.

So to run the AC appliances you’ll need an inverter and again which are not 100% efficient which I’ll explain in a bit.

So if you’re running DC appliances on your battery you can directly connect it to your battery so there will no power loss.

Examples of DC appliances: Phone chargers, laptop chargers, LED lights, and Bulbs.

Examples of AC appliances: Fridge, Ceiling fan, Coffee maker, washing machine, AC, and much more… in simple words all heavy appliances require Alternating current (AC)

Why is this important? because if you’re running AC appliances with the help of an inverter there will be a power loss which will decrease the backup time of your battery

How long will a 400Ah battery will last with inverter

Most of the inverters are 85% efficient. This means a 100W inverter will consume 115 DC watts from the battery in order to run 100 watts of AC appliances

Make sure to check the efficiency rate of your battery in the product info session for more accurate numbers

So if you’re running appliances with the help of an inverter just simply add that extra 15 % to your total appliances watts

For Example: if you’re running 2000 Watts of AC appliances with an 85% efficient inverter

2000 + 15% = 2300

Your inverter will be draining 2300 DC watts from your battery to 2000 watts of AC appliances

Battery Runtime Calculator

Battery Capacity
Battery Volts
Select Battery Type
Load Connected with inverter?
Total Load (watts)

Battery life cycle

After every single cycle, the capacity of your battery will decrease a little bit but after a few 100 cycles, it can make a huge difference.

A battery cycle is charging your battery 100% and then discharging it to its recommended DOD limit (Depth of discharge).

And yes every battery type has different life cycles, So here’s a chart with battery types and their life cycles

Battery TypeNo. of Life Cycles

So after 500 cycles of lead-acid battery, your battery will lose its 20% of capacity so for example now your 400Ah battery can store only 320 amp-hours.

You may think that your battery is fully charged but in reality, it’s not if that’s an old battery.

This is also the most important point to consider but I was wondering why every other blog post has not mentioned it.

Examples – How long will a 400 amp hour battery last

Here are some examples of running appliances on a 400Ah battery and how long will your battery will last

ApplianceWattage requiredRun Time on 400Ah 50% DOD limit batteryRun Time on 400Ah 100% DOD limit battery
Fridge30010 hours20 hours
AC350035 minutes1.1 hour
Washing Machine8002.5 hours5 hours
Coffee maker10002 hours4 hours
Microwave10002 hours4 hours
Toaster8502.4 hours4.8 hours
Space Heater15001.3 hours2.7 hours
Iron12001.7 hours3.4 hours
Electric Blanket20010 hours20 hours

The calculation is based on an 85% efficient inverter.

Very Important Disclaimer! All these numbers are based on if your battery is fully charged.

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Chris Tsitouris is a renewable energy professional with 10+ years of experience as Director of Engineering at Solar Spectrum, previously working as Project Manager at SunPower and Energy Analyst at the National Renewable Energy Laboratory. As a thought leader, Chris has authored numerous articles and research papers.

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