40Ah Battery Backup Time: How Many Hours Will It Last?

How long does a 40Ah battery will last or backup time depends on the type of battery, its DOD limit, and output AC or DC load

So in this guide, I’ll explain how much backup time you’ll get from your 40Ah battery while running AC or DC appliances with examples

In short, 40Ah lithium battery will last 4 hours while running 100W AC output load and 1 hour & 48 minutes on 100 DC watt appliance

The answer is not that simple, there are a few things to consider which will affect the backup time of your battery

What does 40Ah mean on a battery?

To make the calculations first let’s calculate the Ah of the battery into watt-hours (Wh)

use this formula to convert Ah of battery to Watt-hours (Watts = battery Ah* Battery Volts)

Most of the 40Ah batteries are 12v so we’ll carry on with 12 voltage battery for this guide

40*12 =  480 wh

12v 40Ah means 480 watts of capacity in a battery. So you can run 10 watts of an appliance for 48 hours? Yes & No here’s why

Lead-acid vs Lithium Battery Backup Time

as we know that every type of battery has its DOD (Depth of discharge) limit which means how much percentage of power you can draw from it to keep the battery safe

Lead-acid batteries are recommended to be discharged only 50% but you can discharge your lithium-ion battery by 80-100% without compromising on its lifespan

So, in short, this means if you have a 12v 40Ah lead-acid battery you can only draw 20Ah or 240Wh and if you have a lithium battery with the same capacity you can draw 380-480Wh

Video – AGM vs Lithium batteries

AC vs DC watts & Inverter efficiency

Batteries store DC (Direct current) but most of the household appliances required AC (Alternating current). So to convert DC watts into AC we use an inverter

But the inverters are not 100%. They range between 85-95% efficiency rate so we’ll go with an 85% efficient inverter which is the most common one

so if you’re running AC load with an inverter on your 40Ah battery then multiply the Watt-hours of battery by 0.85 (inverter’s efficiency)

Lead-acid 40Ah battery with 50% DOD limit will be equal to 204 AC watts and lithium-ion 40Ah battery with 100% discharge will give you 408 AC watts

Can we use 40AH battery for inverter? if you’re running DC appliances on your battery then you don’t need an inverter, you can connect DC appliances directly to the battery like DC Fan, LED lights, phone charger, laptop & Bulbs. But for AC appliances you’ll need an inverter which will cause a power loss

How long will a 40ah battery last? – Examples

Here’s a list of appliances that you can run with your 40Ah battery and how long will your battery will last while running the device

ApplianceWattage RequiredEstimated Backup time 40Ah lead-acid batteryEstimated Backup time 40Ah lithium battery
Ceiling Fan90 AC watts2.2 hours4.4 hours
LED Lights20 DC watts12 hours24 hours
Bulb10 DC watts24 hours48 hours
Computer300 AC watts40 minutes1 hour & 20 min
Laptop100 DC watts2.4 hours4.8 hours
Phone Charger7 DC watts34 hours68 hours
DVD Player30 AC watts6.8 hours13.6 hours
Wall Fan60 AC watts3.4 hours6.8 hours
Fridge150 AC watts1.3 hours2.6 hours
Coffee Maker800 AC watts15 minutes30 minutes
how long will a 40AHh battery last?

Note: The above calculation is made on the bases of DC (240w for 50% Depth of discharge & 480w for 100% Depth of discharge) and AC (204w for 50% DOD & 408W for 100% DOD)

Battery Runtime Calculator

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

Which inverter is suitable for 40Ah battery?

The size of the inverter depends on the output total load, you can use a 1000W inverter or 100W inverter according to your output AC load.

A rule of thumb is to add 20% in total output AC when sizing the inverter for your battery. the extra 20% is added for safety to cover the inverter efficiency

I would recommend using a Renogy pure sine wave inverter with a 40Ah battery because of their longevity and overall quality I’m confident because I’m also using Renogy’s products

Video – How to size inverter? (4 Easy steps)

Battery cycle & Backup time

To keep getting the benefits from your battery you’ll have to recharge it after discharging but every single cycle will decrease the battery life

here’s a table of battery types with their recommend DOD limit and total life cycles

Battery TypeDOD%Life cycles
Lithium 80-100%2000-1600

After mentioned cycles, your battery will perform 80% of its capacity. For Example, if you have a 40Ah lead-acid battery so after 500 cycles of 50% Depth of discharge your battery capacity will decrease about 20% so now you’ll be able to store only 384Wh in total which will leave you with 192wh of useable capacity

How long does it take to charge a 40AH battery?

as we know that 12v 40Ah battery is equal to 480Wh of capacity so if you’re using a lithium-ion battery with 100% DOD so you’ll require 480w of solar power to recharge your battery and 240 DC for a lead-acid battery

so now divide the solar DC watts required for recharge by sunlight hours in your area. I live in Florida and we receive 5 hours of solar sunlight or 5 good sunlight hours 480/5 = 96 watts & 240/5 48 watts

you’ll not receive 100% from your solar panel due to many reasons like weather conditions so now add 20% to the per hour wattage require (96 + 20% = 115-watt solar panel or 48 + 20% = 57-watt solar panel)

you can charge your 40Ah fully discharged battery in 5 hours with 120W solar and 50% discharged battery in 2.5 hours or in 5 hours with a 60W solar panel under ideal sunlight conditions

Related Post: Maximum Charging Current & Voltage For 12v Battery

Final thoughts

so the backup time of your 40Ah battery will depend on its voltage, battery type or discharge limit, & total output AC or DC load which I have explained in detail

I hope this guide was helpful to you if you have any queries contact us thank you!

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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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