How Long Do Small Amp-hour (Ah) Batteries Last? (Explained!)

How long will a (1.3ah, 1.5ah, 3ah, 4ah, 5ah, 6ah) battery will last? these small type batteries are used to run small appliances like small tools, e-bikes, and small bulbs

so in this guide I’ll show you how long will your battery last and what amp-hour (Ah) means on a battery, and how it determines the battery backup time

How long will a battery last depends on two factors

  • Battery capacity in Ah or Wh
  • Output connected Load

Now let’s first describe what are the amp-hours (Ah) in the battery.

What does amp-hour (Ah) mean in a battery?

Amp is a unit that we use to measure the current. Amp-hour (ah) is a capacity measurement unit that we use to measure the capacity size of any battery

A rule of thumb is that you can draw 1amp from a 5Ah battery for 5 hours.

Every battery will have a different voltage but they can have the same capacity. So this means a 24v battery will be more powerful than a 12v battery of the same capacity (Ah)

Most of these small batteries are lithium-ion battery types which means you can draw 100% of the total battery from it.

But if you have a lead-acid battery then it will have a DOD (depth of discharge) limit of 50% which means you’ll be able to draw only 3Ah from a 6Ah lead-acid battery to keep the battery life

Video – What does Battery Amp-hours (Ah) means?

Convert Battery capacity in Wh (Ah-Wh)

Watt-hour (Wh) is the total battery capacity in watts. which will make the calculations easy for you because most of the tool’s capacity is measured in watts

so to convert the battery capacity in watts use this formula (Battery capacity Ah* Battery voltage = Watts (Wh)

How to calculate battery backup time?

To calculate the battery backup time convert the battery capacity in watts and divide the battery capacity in watts by total output in watts

Battery capacity (Wh)/Output connected load = Battery backup time

By using the formula you can calculate how many hours your small battery will last but for large-capacity batteries, the calculation will be different due to the inverter efficiency rate

Video – Does more Ah = More power?

Now let’s calculate the backup time of every size battery with examples

How long does a 6 ah battery last?

Most of the 6Ah batteries are used in a small drilling machine so first of all, let’s converter the Ah in watt-hours

and if you’re using a Dewalt 6Ah battery it comes in 20v. so we’ll use a 20v 6ah battery for this calculation

6 * 20 = 120Wh

Dewalt 20v 6Ah battery will last 1.2 hours while running a 100W tool or 30 minutes while connected to a 240W tool

How long does a 1.5ah battery last

Watts = Ah * voltage 

we’ll take Dewalt 12v 1.5ah battery as an example

1.5 * 12 = 18Wh

1.5Ah battery will last 1 hour while running an 18W output load appliance

Video – Dewalt 1.5ah vs. 2ah vs. 5ah battery

How long does a 3.0ah battery last

Dewalt 3.0Ah battery comes in 20v so we’ll use this for the calculations

3 * 20 = 60 watts 

20V 3.0Ah battery means 60 Watts or Watt-hours. which means you can run a 60W appliance with it for 1 hour

how long does a 5 ah battery last?

5ah battery also comes in 20v which will be equal to 100 watts in total capacity (5*20 = 100Wh)

Battery capacity in Wh / output total load (watts) = how long will a battery last 

so let’s suppose that you’re using a 400W tool kit with your 5Ah battery

100 / 400 = 15 minutes 

Dewalt 5ah battery will last 15 minutes while running 400W tool

how long does a 4.0 ah battery last

4.0 Ah battery with 20V will be equal to 80 watts or (Wh). So it’ll last 1 hour while running an 80W capacity appliance

Video – 5ah vs 4Ah battery test

how long does a 2.0 ah battery last

2.0Ah battery come in 20V so the calculations will be

2 * 20 = 40 watts 

you can run an 80W appliance with a 2.0Ah Dewalt 20V battery for 30 minutes

how long does a 2.5 ah battery last

2.5 Ah 20v battery will be equal to 50 watts so it’ll last 30 minutes while running 100W appliance or 1 hour on 50W output load

how long does a 1.3 ah battery last

1.3 Ah battery with 20v means 26 watts so it will last 30 minutes while running 50W output load

how long does a 2 ah battery last

2 Ah battery which comes in 20V will be equal to 40 watts. This means it will last 24 minutes while running a 100W output load


To determine how long will your battery will last depends on the battery capacity and output load.

You can calculate by converting the battery capacity in watts by using this formula (Watts = battery Ah x Battery voltage) and then dividing the battery capacity in watts by output load in watts

I hope this small 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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