How long will a a 100Ah battery run a 100 watt load?
On the face of it this question is easy – I’d just create a calculator for the average 100 Ah battery discharge for all appliances that pull 50W to 1000W, but it isn’t so simple.
We use appliances in the home, so it stands to reason that the device would be a.c. at 120 volts if you live in the US, 220 volts in the EU.
You would also need an inverter, and these take some power as well.
What if the 100W was a DC load?
Then it might be be 12, 24 up to 48 volt. I’m going to assume for this article the 100Ah battery in question is 12v lead-acid deep-cycle type. (Although I prefer lihitum phosphate!)
An accurate question needs to include details of what kind of load we are connecting, if it’s going to practical:
A 100Ah 12 volt deep-cycle lead-acid battery can run a 100W continuous DC load for 6 hours if discharged to 50% as recommended. A 100Ah 12 volt lead-acid deep-cycle battery could run a 100W rated AC food-mixer for 7 hours with 50% Depth of Discharge.
Video – Battery capacity, amp-hours and watt-hours
Battery run time = battery capacity in amp-hours/current taken in amps
In AC circuits it’s not like that. The battery DC voltage has to be converted to AC and changed to either 120V or 220V, depending on which country you live in.
For some appliances it gets worse. When appliance have compressor motors on board, like fridges, freezers and AC units, the relationship of voltage to current doesn’t work in the same way.
For these kinds of appliances, a different method of calculating run time is required.
How do you calculate battery drain?
The most common type of battery is lead-acid, and there are two broad categories:
car and truck batteries for starting car engines
deep-cycle or leisure batteries for RV and marine applications
Auto batteries can supply many hundreds of amperes for a relatively short time.
In general, they shouldn’t be discharged over 20%. In normal operation in a vehicle this isn’t a problem – as soon as the engine fires, the alternator charges the battery back up.
Deep cycle lead-acid batteries, sometimes called ‘leisure’ batteries as used in campers, can supply low to medium level currents for many hours.
Deep-cycle batteries can be depleted down between 50% and 80% of their rated capacity, but 50% is normally recommended for maximum battery life.
Marine batteries are designed to be dual-purpose. That is, they can turn and start a marine diesel engine, but also power other on-board equipment when the engine is stopped.
For this this post I’m assuming you’ll be using a 100Ah lead-acid deep-cycle battery.
Which lasts longer, lead acid battery or lithium phosphate batteries?
Which is better lithium battery or lead acid battery?
In my opinion lithium phosphate batteries are far superior to lead acid deep-cycle, but it depends on your budget – they are more expensive.
LiFeP04 batteries are intrinsically deep-cycle in operation, with a recommended discharge rate up to 95%.
This is an important point. If using a 100Ah lead acid deep-cycle battery you can use just 50Ah of the available capacity (50% Depth of Discharge rate recommended).
When using a 100Ah lithium phosphate battery you can often use up to 95Ah.
Table – What is the difference between lead acid and lithium batteries?
Lithium Phosphate (LiFeP04)
Lead Acid (Pb deep-cycle)
Number of possible charge/discharge cycles
2000 (100% DOD)
5000 (80% DOD)
500 (80% Depth of Discharge)
Weight
60% less than Pb
Deep cycle capability?
Inherently deep cycle
Special batteries
Compare costs
$$$
$
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How long will a 100ah battery run a refrigerator?
A fridge running on household a.c. power is hard to assess for battery usage. When running a straight DC load, it’s an easy calculation involving volts, amps and watts.
With a refrigerator we can’t do it that way – read this detailed post I wrote explaining exactly why.
The issue here is that fridges and freezers have compressors with motors. Motors start and stop and sometimes sit idle – it’s all governed by the temperature.
For some of the time a refrigerator might pull zero amps, then it starts up and pulls a starting current several times that of the running current. It’s called inrush, or surge current.
After starting, the motor settles down and draws a steady running current.
The cycle looks something like this below:
The compressor motor isn’t running – 60% of the time
Compressor motor starting – 5%
Compressor running steadily – 35%
The motor surge, or inrush, current, can be 3 to 6 times more than the steady running current.
We need to know the average current over several cycles to calculate how long a 100Ah battery would run it.
So thta’s to say our 100Ah battery needs to supply 45 watts per hour.
So far, so good.
However, the energy consumed isn’t equal across day and night hours. More energy is consumed during day-time hours because the door is being opened and closed all the time.
For now I’ll work with an average estimate of 45 watts per hour. How can we relate this figure to battery amp-hours (Ah)?
How to find watt hours of a battery
For a home appliance like a 400W rated fridge, we can associate the watt-hours consumed with battery capacity by translating the battery amp-hours (Ah) into watt-hours (Wh):
Watt-hours (battery) = Ah x volts
So a 100Ah battery equals 1200 watt-hours.
However, we should only discharge a lead-acid deep-cycle battery 50% of its rated capacity. Effectively, this means we can only use 50Ah, which is the same as saying 600Wh.
How long can a deep cycle battery power a fridge?
I’ll ignore inverter losses for now to make it simple. The formula is:
400W fridge running time in hours (fed from 100Ah battery) = 600 watt-hours/45W = 13.33 hours
Please note: See the chart below – note that the battery voltage reduces as the battery discharges.
This terminal voltage reduction means that when the battery is discharge to 40% to 50% of its capacity, the compressor motor inrush current may pull the voltage down so much that it triggers the inverter low-voltage alarm.
Because of this low voltage alarm issue, I would say that a more reasonable running time for a 400 watt fridge on a 100Ah battery would be 9 to 10 hours.
How do you measure amp hours in a battery?
How can you work out how much capacity is left in a 100Ah battery?
We can use the rough relationship between lead-acid terminal volts and Ah capacity to estimate remaining battery capacity.
Table – 12V Deep-cycle Lead Acid Battery Capacity Vs Terminal Volts
% State Of Charge
(12 V Lead-Acid Battery)
Battery terminal voltage
100%
12.73
90
12.62
80
12.50
70
12.37
60
12.24
50
12.10
40
11.96
30
11.81
20
11.66
10
11.51
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How Long Will A 100Ah Battery Run Home Appliances – 100 watts to 1000 watts?
Use the below to find the approximate run time for RV or marine equipment running on 12V DC powered by a deep-cycle 100Ah battery.
Simply enter the wattage of the equipment and see the run time in hours:
Battery Discharge Calculator
You could also look up the load in the table below to find running time in hours:
Table: Running times for various DC and AC loads
Hours Run Time 12 volts 100Ah deep-cycle lead-acid battery (50% recommended discharge)
Battery running time calculator – 100Ah battery running AC loads with inverter
Estimating running time for AC appliances with a battery is a bit more complicated.
An inverter needs to be used, so there are addditional losses and also some AC appliances use compressor motors, as previously described.
The calculator that appears underneath doesn’t account for fridges or any appliances with compressors or large motors (such as freezers, ac units or heat pumps):
Table: Running times for various AC loads
You could also just look up any load in watts using the table below to find the running time:
100Ah 12V deep-cycle lead-acid battery with AC loads and inverter – 50% discharge
On the face of it you would think that a 150Ah battery has 33% more power than a 100Ah battery, but let’s look a little closer …
We know that battery capacity is constrained by the recommended Depth of Discharge (DoD) – 50% for lead-acid deep-cycle, and 80% for lithium iron phosphate.
This means the actual capacity available for a 100Ah battery before recharge is:
Lead-acid (50% DoD) = 50Ah
LiFeP04 (80% DoD) = 80Ah
Now I’ll apply the same logic to 150Ah capacity battery of each type:
Lead-acid (50% DoD) = 75Ah
LiFeP04 (80% DoD) = 120Ah
Other questions about 100Ah batteries:
How long will a 100ah battery power a fridge?
A 12V 100Ah lead-acid deep cycle battery will power a 450 watt fridge for about 13 hours (45 watts per hour.) Typical energy usage for a fridge is about 400 kWh/year (found on the fridge labels).
How long will a 100Ah battery run an appliance that pulls 600w?
A 100Ah deep-cycle battery will power a 600W DC device for about 1 hour. (A 100Ah battery equals 1200 watt-hours but only 50% should be used.)
At 600W AC appliance will only run for about 0.86 hours because an inverter is needed to convert voltage from DC to AC and these devices have losses.
How many solar panels would it take to charge a 100Ah battery?
A 100 watt solar panel using an MPPT solar charge controller will recharge a 50% depleted 100Ah 12 volt deep-cycle battery in about 15 hours.
How long can a 100Ah battery run a trolling motor?
A 12 volt 100Ah lead acid deep-cycle battery will power a 30lbs thrust Minn Kota trolling motor on Speed 4 setting for about 4 hours.
What does 100Ah mean on a battery?
It is sometimes said that a 100Ah lead-acid battery can supply 2 amp for 50 hours or 50 amps for 2 hours. This isn’t the case.
Batteries should never be completely discharged so full capacity isn’t really available.
Depth of Discharge for a deep-cycle battery should be between 50% to 80%, which means only 50 to 80Ah of a 100Ah battery is available for use.
In addition, when higher currents are pulled from a battery, its capacity is significantly less – the amperes vs hours ratio isn’t at all linear.
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