How Long Will A 100Ah Battery Run An Appliance That Requires 400W?

How Long Will A 100Ah Battery Run An Appliance That Requires 400W_featured

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How many hours will a 100Ah battery run 400 watts?

How long will a 100ah battery run 400W?

When I first saw this question my first reaction was to create a 100 Ah battery discharge calculator for appliances that require 100W to 1000W, but there’s a problem.

The word ‘appliance’ indicates something we would use in the home, so it would be AC and 120 volts (US) or 220 volts (EU). In this case an inverter is required, which also takes a little power.

What if it’s for an RV or boat? Then the 400W would be a direct DC load and could be 12, 24 or even 48 volt. Let’s assume for this post that the 100Ah battery is 12 volt lead-acid deep-cycle type.

An answer to the question would need to contain details of what kind of appliance load we are talking about, if it’s going to have any meaning and be useful.

For example:

A 100Ah 12 volt deep-cycle lead-acid battery can run a 400W continuous DC load for 1.5 hours with the recommended 50% Depth of Discharge. A 100Ah 12 volt deep-cycle lead-acid battery can run a 400W rated AC fridge for 15 hours at a rate of 40 watts per hour.

Video – Factors affecting 100Ah battery run-time


How do you calculate how long a battery will last?

All batteries have a rated capacity in ampere-hours (Ah). This is really convenient when you have a DC load and you know what the current draw is in amps.

In this case how long a battery will last in hours can be found by a simple formula:

Hours left in battery = battery capacity in Ah / current draw in amps

In AC circuits it’s not so simple. The battery DC voltage needs converting to AC and stepping up to either 120 volts or 220 volts, depending on location.

To make things worse, for some appliances, like fridges, freezers and air conditioning, the relationship between voltage and current isn’t constant. In these cases, another method of calculating battery drain is needed.

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How to calculate battery drain

There are two basic kinds of lead-acid battery in common use:

  • auto batteries for cranking car engines
  • deep-cycle for RV and marine use

Car batteries can deliver hundreds of amps for a short time and should never be discharged more than 20%. Normally this isn’t an issue as the car engine charges the battery back up after starting.

Deep cycle batteries, like the ‘leisure‘ batteries used in RVs, can deliver medium level currents for much longer periods and can be discharged 50% to 80%, although 50% is recommended to ensure long life.

Marine batteries can be dual-purpose. They can crank a boat engine and also power lights and other equipment when the engine isn’t running, like an RV leisure battery.

Everything I’m talking about on this post assumes a 100Ah deep-cycle lead-acid battery is being used.

Why are 100Ah lithium batteries better than lead acid?

Which is better lead acid battery or lithium battery? See Renogy Battery Prices

Are lithium batteries better than lead acid?

IMO lithium phosphate batteries are better than lead acid in all respects, except price. They are naturally deep-cycle and can be discharged up to 95%.

This is important. With a 100Ah deep-cycle lead acid you only have 50Ah available (50% discharge recommended). With a 100Ah lithium phosphate battery you have 95Ah!

Table – Compare Lithium Iron Phosphate With Lead Acid

Lithium Phosphate

Lead Acid

Number charge/discharge cycles

2000 (80% discharge)
5000 (50% discharge)

500 (80% DOD)


60% less than Pb

Deep cycle?

Inherently deep cycle

Special batteries

Cost comparision



How long will a 100ah battery run a fridge?

A conventional AC refrigerator is difficult to assess for battery drain. With a straight-forward DC load, it’s a simple calculation using watts, volts and amps.

With a fridge we can’t do that – this is very detailed post I published explaining why in detail.

The biggest problem is that refrigerators have compressors run by motors. Sometimes they run and sometimes they sit idle waiting for the next cycle.

This mean that for a certain percentage of the time the fridge draws no current, then a motor starting current and then a motor running current. The cycle might look like this:

  • Compressor idle – 60%
  • Compressor starting 5%
  • Compressor running – 35%

A motor start current, or surge current, is about 3 times the running current. An average current is needed to work out how this type of appliance would drain a 100Ah battery.

How many watts does a refrigerator use per year/month/day?

What is annual energy consumption fridge freezer - label?

How much energy dos a fridge consume?


The best way to assess the energy consumption of a fridge is to use the manufacturer’s data. Each fridge label carries an estimate of the energy it will consume on average over the year in kilowatt-hours (kWh).

Let’s say the average amount is 350kWh/yr. The first step if to work out how much energy it takes per hour:

  • Daily fridge energy consumption = 350kWh / 365 = 958 watt/hrs
  • Hourly fridge power consumption = 958 / 24 = 40 watts

So now we know that our 100Ah battery has to deliver 40 watts every hour on average, but the energy used isn’t the same day and night – more energy is used in day-time when the door is opened regularly.

Let’s go with the average of 40 watts per hour. How does this relate to battery amp-hours?

How to calculate watt hours of a battery

For an appliance such as a 400W refrigerator, we can equate the hourly watts used to battery capacity by converting amp-hours (Ah) into watt-hours (Wh):

Battery watt-hours = amp-hours x voltage

For a 100Ah battery we have 1200Wh, but remember we can only discharge a deep-cycle battery 50%. Really, we only have 50Ah available, or 600Wh.

How long will a deep cycle battery run a fridge?

I'll ignore the inverter losses to keep it simple:

Running time in hours for 400W fridge fed from 100Ah battery = 600Wh / 40W = 15 hours

A word of warning: Look at the chart below and you'll see that the battery volts goes down as it discharges.

Once discharge approaches 40% to 50% the compressor surge current may pull the voltage down too much and trigger the inverter low-volts alarm.

Because of the above, I would revise the comfortable running time down to 10 hours.

How to measure amp hours in a battery

How do you know how much capacity is left in your 100Ah battery? There is a rough relationship between lead-acid terminal voltage, at least, good enough for an estimate of how much power you have left.

Table - 12V Lead Acid Battery Capacity Vs Terminal Volts

Percentage State Of Charge (12 V Lead-Acid Battery)

Battery terminal volts





















How Long Will A 100Ah Battery Run An Appliance - 100 watts to 1000 watts? Battery Discharge Calculator

The calculator below finds the run time for appliances running on 12V DC supplied from a deep-cycle 100Ah battery. Enter the wattage and see the run time:

If you prefer, simply look up the load in watts on the table below:

Hours Run Time 12 volts 100Ah deep-cycle lead-acid battery (50% recommended discharge)

Load supplied in watts

Run time in hours

DC Load

AC Load (inverter losses subtracted)




























Battery discharge time calculator - 100Ah battery with AC loads + inverter

Calculating run time for AC appliances is a bit more complex, because an inverter will be used and also some AC appliances have compressors installed.

The calculator below doesn't account for refrigerators and similar appliances (freezers, air conditioning, heat pumps) as I covered that quite extensively earlier on in the post:

Table: Running times for various AC loads

If you prefer, simply look up the load in watts on the table below to see the run time:

100Ah deep-cycle lead-acid battery with AC loads with inverter - 50% recommended discharge

Load (watts)

Run time (hours)

100 watts

5.16 hours



















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How long will a 200ah battery run an appliance that requires 400W?

A 200Ah 12 volt lead-acid deep-cycle battery will run a 400W DC load for 3 hours at 50% Depth of Discharge. A 200Ah 12 volt deep-cycle lead-acid battery will power a 400W AC fridge for 30 hours, drawing power at a rate of 40 watts per hour.

Other questions asked about 100Ah batteries:

How long will a fridge run on a 100AH battery?

A 100Ah 12 volt deep-cycle lead-acid battery can run a 400W rated AC fridge for 15 hours at a rate of 40 watts per hour. The average energy consumption of a refrigerator is 350 kilowatt-hours/year.

How long will a 100Ah battery run an appliance that requires 600w?

A 100Ah battery will run a 600W DC appliance for 1 hour. An AC appliance rated at 600W will only run for 0.86 hours because of the losses in the DC/AC inverter needed for the battery.

How many solar panels does it take to charge a 100Ah battery?

A 100 watt solar panel using an MPPT solar charge controller will charge up a 50% discharged 100Ah 12 volt battery in 15 hours.

How long will a 100Ah battery run a trolling motor?

A 100Ah lead acid deep-cycle battery will run a 30lbs thrust trolling motor on Speed 4 for 4 hours.

What does 100Ah mean on a battery?

It's often said that a 100Ah (amp-hours) battery can deliver 1 amps for 100 hours or 100 amps for 1 hour. This is not really an accurate description of battery operation.

Batteries should never be discharged completely so the rated capacity isn't available for use. A deep-cycle battery should only be discharged between 50% to 80%, only that percentage of the total capacity is available for useful power.

When higher currents are drawn the battery capacity reduces significantly, so the amps vs hours relationship isn't linear at all.

How many solar panels does a grid-tie system need?

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