There are a couple of factors that determine how long a 100Ah battery lasts:
- The energy consumption of the devices you’re trying to run.
- The type of the 100Ah battery and how much of its capacity is actually usable.
After reading this comprehensive article, you’ll know exactly why these factors matter, and how to determine their values. when you do, you can easily estimate how long your 100Ah should last using this formula:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
In this process, you’ll also end up learning the difference between different electrical units and ratings, and you’ll also learn about different concepts related to batteries such as Depth of Discharge (DOD), Usable Capacity, and rate of discharge (C-rate).
Let’s get into it
How long will a 100Ah battery last?
As mentioned above, the amount of time that a 100Ah lasts depends on how much energy the appliance consumes, and how much usable capacity the battery offers.
For example, a 12V-100Ah Lithium battery, such as the LiFePO4 batteries, can store and supply 1200Wh (Watt-hours) of energy. With this amount of energy, and assuming an 85% system efficiency, you can run a 100-watt load for up to 10 hours or a 50-watt load for up to 20 hours.
On the other hand, a 12v-100ah Lead-Acid, such as the AGM batteries, can also store 1200Wh of energy, but because of its chemistry, it can only supply 600Wh of energy before it has to be recharged (50% DOD). With 600 Watt-hours of energy and a system efficiency of 85%, you can run a 100-watt load for up to 5 hours, or a 50-watt load for up to 10 hours.
In the following sections of this article, I give a detailed explanation of why these batteries – although having the same electrical ratings – offer different amounts of Usable Capacity.
But for those of you who are in a hurry, the following table estimates the different runtimes you can get out of a 12V-100Ah battery based on the type of the battery and the power usage of the appliance:
| Power Usage (Watts) | Runtime for 12V-100Ah Lithium battery | Runtime for 12V-100Ah Lead-Acid battery |
| 10W | 100 hours | 50 hours |
| 20W | 50 hours | 25 hours |
| 30W | 34 hours | 17 hours |
| 50W | 20 hours | 10 hours |
| 60W | 17 hours | 8 hours |
| 70W | 14 hours | 7 hours |
| 100W | 10 hours | 5 hours |
| 150W | 7 hours | 3 hours |
| 200W | 5 hours | 2 hours |
| 300W | 3 hours 30 minutes | 1 hour 30 minutes |
| 400W | 2 hours 30 minutes | 1 hour |
| 500W | 2 hours | 45 minutes |
| 600W | 1 hour 45 minutes | 35 minutes |
| 700W | 1 hour 30 minutes | 25 minutes |
| 800W | 1 hour 20 minutes | 20 minutes |
| 1000W | 1 hour | 15 minutes |
| 2000W | 30 minutes | 5 minutes |
| 3000W | 20 minutes | – |
Please note that the information provided in this table is based on the assumption that these loads run continuously (non-stop). For loads that run intermittently, the runtime would be significantly higher.
A more accurate way of determining these potential runtimes is to first determine the hourly energy consumption of the device in Watt-hours per hour (Wh/hour).
Also, the runtimes provided in the Lead-Acid battery column take into consideration the losses in efficiency that are due to discharge rates.
The next sections of this article explain this in detail.
How much energy consumption are you trying to offset?
How long a 100Ah battery lasts depends directly on the amount of electricity that the battery is trying to offset. This amount of electricity is measured and represented through different electrical units:
- Voltage (potential): which is measured in Volts (V)
- Current: which is measured in Amperes or Amps for short (A)
- Power: which is measured in Watts (W)
- Energy: which is measured in Watt-hours (Wh)
Although these units represent different things, they are interconnected, and their relationship to each other can be represented through these 2 formulas:
(1) Power (Watts) = Voltage (Volts) x Current (Amps)
(2) Energy (Watt-hours) = Power (Watts) x Time (hours)
What we’re going to be focusing on in this case is the second formula, which defines the relationship between power and energy.
For example, if a device uses 100 Watts of power continuously for 5 hours, the amount of energy that it would have consumed at the end of those 5 hours is:
Energy Consumption (Watt-hours) = Power Usage (Watts) x RunTime (hours)
Energy Consumption (Watt-hours) = 100 Watts x 5 hours
Energy Consumption (Watt-hours) = 500 Watt-hours
It is very important to make this distinction between power and energy.
Remember, Watt-hours represent Electrical Energy (energy production, consumption, or storage), and Watts represent Electrical Power, which is the rate at which Electrical Energy is being transferred (produced or consumed) when the device is running.
Since we’re trying to figure out how long a 100Ah battery would last running your appliance (in hours), we should first determine the amount of energy that your appliance consumes each hour.
In other words: How many Watt-hours does your appliance consume per hour?
Now, when dealing with appliances such as lights, TVs, or fans, it is relatively easy to determine their hourly energy consumption.
This is because these appliances use (more or less) the same amount of power the entire time they’re turned on.
Take a 32″ LED TV for instance. Assuming this TV uses 35 Watts of power, all we have to do to calculate its hourly energy consumption is to multiply that power usage by 1 hour:
Hourly Energy Consumption (Watt-hours) = Power Usage (Watts) x 1 hour
Hourly Energy Consumption (Watt-hours) = 35 Watts x 1 hour
Hourly Energy Consumption (Watt-hours) = 35 Watt-hours
Related: How much energy does a TV use?
The issue, however, is with appliances such as air conditioners and refrigerators.
When these appliances are turned on, they start up and keep running until the set temperature is reached. Once the set temperature is achieved, they run intermittently (ON and OFF) in a way that maintains that set temperature.
This means that during the hour, a refrigerator, for example, might only really run for 15 to 30 minutes. which makes it a little tricky to estimate the energy consumption of these appliances based solely on their rated power usage.
However, it also means that these appliances consume less energy than appliances that have the same power rating but run continuously.
If you’re dealing with an air conditioner or a refrigerator, please refer to these articles in which I explain a couple of ways to determine their energy consumption:
The best way to determine the energy consumption of these appliances is to measure it using an electricity monitoring device such as the Kill-A-Watt meter.
These devices will give you exact readings, and all you have to do is plug the meter into the outlet and plug your appliance into it.
Recap:
- If you’re dealing with appliances that have the same kind of output the entire time they’re turned on, simply multiply their power rating by 1 hour to get their hourly energy consumption:
Hourly Energy Consumption (Watt-hours) = Rated Power (Watts) x 1 hour
- If you’re dealing with appliances that turn on and off during their runtime, it is better to use an electricity monitor.
In any case, once you’ve determined the hourly energy consumption of your appliance(s) in Watt-hours/hour, the next step would be to determine the usable capacity of your 100Ah battery.
How much of your 100Ah battery capacity is usable?
Batteries are generally rated in Volts (V) and Amp-hours (Ah), and these ratings indicate the amount of electrical energy (in Wh) that the battery can store.
Related: How many kWh is a 100Ah battery?
For example, a 12V-100Ah (12 Volts and 100 Amp-hours) battery is capable of storing 1200 Watt-hours of electrical energy:
Rated Electrical Energy Capacity (Watt-hours) = Rated Electrical Charge Capacity (Amp-hours) x Rated Voltage (Volts)
Rated Electrical Energy Capacity (Watt-hours) = 100 Ah x 12 V
Rated Electrical Energy Capacity (Watt-hours) = 1200 Wh
However, this rated capacity will decrease over time, and the number one factor that determines how fast the capacity of a battery decreases is how low you usually discharge it. This is why whenever we talk about battery capacity, we also mention Depth Of Discharge, or DOD for short, and Usable Capacity.
This Depth Of Discharge (DOD), simply represents the percentage (%) of a battery’s capacity that is used, and as mentioned above, it influences the lifespan of the battery.
For example, consider a battery that has an energy capacity rating of 1200Wh, and let us assume that at 100% DOD, this battery can provide 200 charge/discharge before it loses 40% of its rated capacity.
Meaning that after 200 times of fully discharging this battery, and then recharging it, the battery will be left with only 60% of its rated capacity, which is 720 Watt-hours (1200Wh x 60%).
If instead, we only half discharge this battery every time we use it (50% DOD), this battery can provide something like 600 charge/discharge cycles before it loses 40% of its rated capacity.
Yes, at a 50% DOD you’ll only have access to half of the battery’s capacity (600Wh of Usable Capacity), but the battery’s lifespan will triple. In other words, discharging your battery at a 50% DOD ends up being more cost-efficient in the long term.
However, some battery types are more immune to higher depths of discharge than others, and can inherently last thousands of charge/discharge cycles even at a 100% DOD.
This can get a lot more technical, but for the sake of simplicity, the following table divides batteries into 2 categories and estimates the number of charge/discharge cycles that they can deliver at different DOD levels before their rated capacity drops to 60-70%:
| Depth Of Discharge (DOD) | Lead-Acid (Sealed, Flooded, AGM) | Lithium (Li-ion, LiFePO4) |
| 30% DOD | 1000 – 1200 cycles | 8000 – 10000 cycles |
| 50% DOD (Optimal for lead-acid) | 500 – 600 cycles | 5000 – 8000 cycles |
| 80% DOD (Optimal for Lithium) | 250 – 300 cycles | 2500 – 4000 cycles |
| 100% DOD | 150 – 200 cycles | 1500 – 2500 cycles |
The values in the table above are estimates, but they’ll help you choose a DOD that works for you.
When you figure out the DOD you’ll be going for based on the type of battery you’ll be using, you can use the following formula to determine the actual usable capacity of your battery:
Usable Capacity (Watt-hours) = Rated Capacity (Watt-hours) x DOD (%)
For example, consider a LiFePO4 battery that is rated at 12 Volts and 100 Amp-hours. An optimal Depth Of Discharge for this kind of battery would be 80% DOD, which will allow the battery to maintain its rated capacity for about 10 years.
Since it’s a 12V-100Ah battery, its rated capacity is 1200 Watt-hours (Wh). At 80% DOD, the usable capacity of this battery is:
Usable Capacity (Watt-hours) = Rated Capacity (Watt-hours) x DOD (%)
Usable Capacity (Watt-hours) = 1200 Watt-hours x 80%
Usable Capacity (Watt-hours) = 1200 Watt-hours x 0.8
Usable Capacity (Watt-hours) = 960 Watt-hours
Now that we know how to determine the hourly energy consumption of our appliance(s), and the Usable Capacity of our battery, the last step is to estimate how long the 100Ah battery will be able to run our appliance(s) before it has to be disconnected.
As mentioned at the beginning of this article, this can be done using the following formula:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
However, please note that for lead acid batteries, there’s another factor that influences the capacity of the battery significantly. Before we get into that, let’s first look at a couple of examples in the case of a 100Ah lithium battery.
How long will a 100Ah lithium battery last?
At 80% DOD, a 12V-100Ah Lithium battery can last for 8.5 to 9.5 hours running a 100W appliance, or for 17 to 19 hours running a 50W appliance. At a 100% DOD, the same battery can run a 100W appliance for 10 to 11 hours, or run 50W for 20-22 hours before it is depleted.
This is, again, assuming the appliance runs continuously. If the appliance runs intermittently (automatically turns ON and OFF), the runtime can be significantly greater.
To get a better understanding of this, let’s look at a couple of examples.
Example 1: Appliance runs continuously
For this example, let’s assume we’re trying to run a 70W light bulb on a 12V-100Ah Lithium battery.
Light bulbs use the same amount of power throughout their runtime, so our power usage will be a constant 70 Watts. This means that our hourly energy consumption is going to be 70Wh/hour (70 Watt-hours per hour):
Hourly Energy Consumption (Watt-hours) = Power Usage (Watts) x 1 hour
Hourly Energy Consumption (Watt-hours) = 70 Watts x 1 hour
Hourly Energy Consumption (Watt-hours) = 70 Watts-hours
Since our lithium battery is rated at 12V and 100Ah, its Energy Capacity is rated at 1200Wh. Assuming we’d like to repeatedly discharge the battery at an optimal DOD of 80%, the Usable Capacity of our battery becomes:
Usable Capacity (Watt-hours) = Rated Capacity (Watt-hours) x DOD (%)
Usable Capacity (Watt-hours) = 1200 Watt-hours x 80%
Usable Capacity (Watt-hours) = 1200 Watt-hours x 0.8
Usable Capacity (Watt-hours) = 960 Watt-hours
Now, the last step is to estimate the amount of runtime that we should get out of the battery using our formula:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
Runtime (hours) = (960 Wh ÷ 70 Wh/hour) x 0.85
Runtime (hours) = (13.7 hours) x 0.85
Runtime (hours) = 11.65 hours
According to our calculations, we’ve estimated that our 12V-100Ah lithium battery could run our 70W light bulb for 11.65 hours, which translates to about 11 hours and 40 minutes.
Please note that the 0.85 coefficient represents the efficiency of the inverter. (average inverter efficiency is 85%)
Example 2: Appliance runs intermittently
For this second example, we’ll assume that we’re trying to run a 5000BTU air conditioner, which has a power rating of 400W, on a 12V-100Ah lithium battery.
Now, when you first turn an air conditioner on, it will first run at full power (400 Watts in this case), but once the set temperature is achieved, it will start turning OFF and ON automatically in a way that maintains that set temperature.
This means that our 5000BTU AC unit, will not consume the full 400Wh/hour. But how much energy will it consume?
Well, this depends on a few factors such as ambient temperature, set temperature, room insulation, etc…
However, typically, these air conditioners only run for about 75% of the time. For the sake of simplicity, we’ll just use that estimate.
So, our air conditioner uses 400 watts of power when it’s running, and runs 75% of the time, using these values, we can estimate the hourly energy consumption of the AC as such:
Hourly Energy Consumption (Watt-hours) = Power Usage (Watts) x 1 hour x 75%
Hourly Energy Consumption (Watt-hours) = 400 Watts x 1 hour x 75%
Hourly Energy Consumption (Watt-hours) = 400 Watts x 1 hour x 0.75
Hourly Energy Consumption (Watt-hours) = 300 Watt-hours
Now that we’ve estimated that our AC unit consumes 300Wh/hour, the next step is to determine our usable battery capacity.
In this example, we’ll assume that we’re using a high-end Lithium battery of which the manufacturer promises 8 years of rated capacity even at a 100% depth of discharge.
In other words, we’ll be able to make use of the full capacity of our battery, which in this case, is 1200Wh.
Since our hourly energy consumption is 300Wh/hour, and our usable capacity is 1200Wh, we can estimate how long our battery lasts as such:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
Runtime (hours) = (1200 Wh ÷ 300 Wh/hour) x 0.85
Runtime (hours) = (4 hours) x 0.85
Runtime (hours) = 3.4 hours
According to our calculations, the 12V-100Ah lithium battery should be able to run our 5000BTU air conditioner for 3.4 hours, which translates to about 3 hours and 25 minutes.
Now that we’ve discussed how long a 100Ah lithium battery lasts, let’s talk about how long a 100Ah Lead-Acid battery lasts.
How long will a 100-ah Lead-Acid battery last?
At 50% depth of discharge and a system efficiency of about 85%, a 12V-100Ah Lead-Acid battery could run a 50W appliance for 10 to 11 hours, or a 100W appliance for 4.5 to 5 hours.
However, if the load exceeds 200 Watts of power, the lead-acid battery starts to become more and more inefficient depending on the power usage.
For example, if the appliance(s) uses 300 Watts of power, the 100Ah Lead-Acid battery can lose up to 25% of its efficiency. In other words, if the load uses 300W of power, the rated capacity of a 12V-100Ah lead-acid battery becomes 900Wh instead of 1200Wh.
This drop in efficiency is due to what we refer to as the Peukert Effect.
Let me explain.
A 100Ah rating on a battery means that – theoretically – the battery can supply 1 Amp of current for 100 hours, or 100 Amps of current for 1 hour. However, in reality, this isn’t 100% true.
The discharge rate of a battery also referred to as C-rate, has a significant impact on its efficiency. The faster you discharge a battery, the less efficient it becomes. This is especially true for Lead-Acid batteries, as they are more prone to this phenomenon than Lithium batteries.
This is why when a manufacturer gives a rating for their batteries (100Ah), this rating comes with a condition, and this condition is the rate of discharge.
For example, in the image below, you can see that these battery cases provide an Amp-hour rating (100Ah), but also specify the discharge rate in hours:
In the case of the first battery, the manufacturer specifies 100Ah at 20 hours (0.05C rate), which means that the battery can only supply 100Ah if the appliance pulls 5 amps (@12V). In the case of the 2nd battery, the manufacturer specifies 100Ah at 10 hours (0.1C rate).
If the appliance pulls more than 5 amps at 12 Volts, the charge capacity (Ah) of the battery will be lower than 100Ah.
These losses in efficiency will differ from battery to battery, so to determine the efficiency of your lead-acid battery at different rates of discharge (C-rates), you’ll probably have to look at the datasheet provided by the manufacturer.
For example, the following table is from the datasheet of a 12V – 100Ah Lead-Acid battery from Renogy:
To understand this better, we’ll use the same examples from earlier, except this time, we’ll be using this 12V-100Ah lead-acid battery from Renogy.
Example 1:
For this example, we’ll see how long we can run a 70W light bulb that consumes 70Wh/hour on our 12V-100Ah Lead Acid battery.
Since our light bulb uses 70 Watts of power, at 12 Volts, it pulls about 5.8 amps (70W ÷ 12V). This is equivalent to a 0.058C discharge rate, which shouldn’t cause a significant drop in efficiency.
We know that our battery is rated at 1200Wh of capacity (12V x 100Ah), so at an optimal depth of discharge of 50%, our Usable capacity becomes 600Wh.
We know the hourly energy consumption of our appliance is 70Wh, and our Usable Capacity is 600Wh, therefore the runtime is:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
Runtime (hours) = (600Wh ÷ 70Wh/hour) x 0.85
Runtime (hours) = (8.57 hours) x 0.85
Runtime (hours) = 7.28 hours
According to these calculations, our 12V – 100Ah lead-acid battery should run the 70W light bulb for 7.28 hours, which translates to about 7 hours and 15 minutes.
Example 2:
For this 2nd example, we’ll see how long we can run a 400W air conditioner that consumes 300Wh/hour on our 12V-100Ah Lead-Acid battery from Renogy.
Since our air conditioner uses 400 watts of power, at 12 Volts, it pulls about 33 amps (400W ÷ 12V). This is equivalent to a 0.33C discharge rate.
As a rule of thumb, a 0.33C discharge rate should decrease the efficiency of the battery by about 35%.
Now that our battery is only 65% efficient, its rated capacity becomes:
Rated Capacity @ 0.33C-rate (Wh) = Rated Capacity @ 0.05C-rate (Wh) x 65%
Rated Capacity @ 0.33C-rate (Wh) = 1200 Wh x 65%
Rated Capacity @ 0.33C-rate (Wh) = 1200 Wh x 0.65
Rated Capacity @ 0.33C-rate (Wh) = 780 Wh
However, this is the rated capacity, and what we need is the Usable Capacity. Since the optimal Depth Of Discharge is 50%, the usable capacity of our battery (at 0.33C discharge rate) is:
Usable Capacity (Watt-hours) = Rated Capacity (Watt-hours) x DOD (%)
Usable Capacity (Watt-hours) = 780 Watt-hours x 50%
Usable Capacity (Watt-hours) = 390 Wh
Now that we know both the hourly energy consumption of our appliance and the usable capacity of our battery, we can estimate the runtime that we should be able to get out of the battery:
Runtime (hours) = (Usable Capacity of the battery (Wh) ÷ Hourly Energy Consumption of the appliance (Wh/hour)) x 0.85
Runtime (hours) = (390 Wh ÷ 300 Wh/hour) x 0.85
Runtime (hours) = (1.3 hours) x 0.85
Runtime (hours) = 1.1 hours
According to these calculations, our 12V-100Ah lead acid battery should run the air conditioner for 1.1 hours, which translates to 1 hour and about 5 minutes.
Related topics:
How long will a 100Ah battery run an appliance that requires 100W?
How long will a 100Ah battery run an appliance that requires 400W?
