Battery amp hours (Ah) measure how much electric charge a battery can store and deliver over time. A 100 Ah battery can, in theory, supply 1 amp for 100 hours, 10 amps for 10 hours, or 50 amps for 2 hours. It’s the single most common way to express battery capacity, and you’ll find it printed on everything from phone batteries (in milliamp hours) to car batteries to solar energy systems.
But the number on the label doesn’t always match what you get in real life. Temperature, discharge speed, and battery chemistry all change how many of those amp hours you can actually use.
How Amp Hours Work
An amp hour is simply current multiplied by time. If a device draws 2 amps from a battery and runs for 5 hours before the battery dies, that battery delivered 10 amp hours. The formula works in reverse too: divide a battery’s Ah rating by the current draw of your device, and you get a rough estimate of runtime.
For example, a 50 Ah battery powering a device that pulls 5 amps would last approximately 10 hours under ideal conditions. In practice, you’ll want to multiply by a factor of 0.8 to 0.9 to account for real-world inefficiency, which brings that estimate closer to 8 or 9 hours.
Phone and small electronics batteries use milliamp hours (mAh) instead, where 1,000 mAh equals 1 Ah. A phone battery rated at 5,000 mAh is the same as 5 Ah.
Amp Hours vs. Watt Hours
Amp hours tell you how much charge is in a battery, but they don’t tell you how much energy it holds. That’s because amp hours ignore voltage. Two batteries can both be rated at 100 Ah, but if one runs at 12 volts and the other at 48 volts, they carry very different amounts of energy.
To get energy, you multiply: watt hours equals amp hours times volts. That 100 Ah battery at 12 volts holds 1,200 watt hours. The same capacity at 48 volts holds 4,800 watt hours, four times the energy. Think of it this way: amp hours tell you how much water is in a bucket, while watt hours tell you how much work that water can do based on how high the bucket is. Voltage is the height.
If you’re comparing batteries at different voltages (choosing between a 12V and 24V system for an RV, for instance), watt hours give you the apples-to-apples comparison. If everything in your system runs at the same voltage, amp hours work fine on their own.
Why You Never Get the Full Rating
The Ah number printed on a battery is a nominal rating measured under controlled conditions, typically at a moderate, steady discharge rate and room temperature. Several factors chip away at that number in real use.
Discharge Speed
The faster you drain a battery, the less total capacity you get out of it. This effect is dramatic in lead-acid batteries. At very high discharge rates (around six times the battery’s rated capacity per hour), a lead-acid battery can lose up to 80% of its rated amp hours. You’d pull far less total charge than the label suggests.
Lithium-ion batteries handle this much better. Even at high discharge rates, lithium batteries lose less than 15% of their rated capacity. This is one of the practical reasons lithium batteries outperform lead-acid in applications with heavy or variable power demands, like electric vehicles and power tools.
Temperature
Cold weather is a battery’s worst enemy. A battery that delivers 100% of its rated capacity at 27°C (80°F) will typically deliver only about 50% at negative 18°C (0°F). That’s not damage; the chemical reactions inside the battery simply slow down in the cold. The capacity usually returns when the battery warms up, but if you’re sizing a battery bank for a cabin in Minnesota winters, you need to plan for significantly less usable capacity than the label states.
Depth of Discharge
Most batteries aren’t designed to be drained to zero. Manufacturers set upper and lower voltage limits that keep the battery in a safe operating range, and the slice of capacity between those limits is your usable capacity. The percentage of total capacity you actually use is called depth of discharge (DoD).
For lithium iron phosphate (LFP) batteries, common in home energy storage, the typical usable DoD is around 95%. Nickel-based lithium cells (the type in most EVs and laptops) are usually operated at 90 to 95% DoD, with a small buffer held in reserve at both the top and bottom of the charge range. So a 100 Ah lithium battery gives you roughly 90 to 95 Ah of usable capacity. Traditional lead-acid batteries are worse: draining them below 50% regularly shortens their life significantly, so your usable capacity from a 100 Ah lead-acid battery is closer to 50 Ah in practice.
Sizing a Battery for Your Needs
To figure out how many amp hours you need, start by listing the devices you want to power and how long you’ll run them. Multiply each device’s current draw (in amps) by the number of hours you’ll use it, then add everything up. That total is your minimum Ah requirement.
Say you’re setting up a small off-grid system. A 12V LED light drawing 1 amp for 8 hours uses 8 Ah. A 12V fan pulling 3 amps for 4 hours uses 12 Ah. A phone charger drawing 1.5 amps for 2 hours uses 3 Ah. Your total is 23 Ah per day. If you’re using a lithium battery with 95% usable DoD, a 25 Ah battery would just barely cover one day. You’d want more headroom than that, especially if you factor in cloudy days or cold temperatures.
If your devices list power consumption in watts instead of amps, divide watts by your battery’s voltage to get amps. A 60-watt device on a 12-volt system draws 5 amps.
Common Amp Hour Ratings by Application
- Smartphones: 3,000 to 5,000 mAh (3 to 5 Ah)
- Laptops: 40 to 100 Wh, which works out to roughly 3 to 7 Ah depending on the battery voltage
- Car starter batteries: 40 to 80 Ah at 12V
- Deep-cycle RV/marine batteries: 100 to 300 Ah at 12V
- Home solar storage: 100 to 400 Ah at 48V
- Electric vehicle packs: Large EV battery packs operate at much higher voltages (around 370V for a Tesla Model Y) and store energy in the tens of thousands of watt hours, though the pack’s low-voltage auxiliary battery is a modest 6.9 to 12 Ah
These ranges highlight why voltage matters. A 100 Ah deep-cycle RV battery at 12 volts stores 1,200 Wh. A 100 Ah battery at 48 volts in a home solar system stores 4,800 Wh. The Ah number alone doesn’t tell you which one holds more energy.
Amp Hours and Battery Lifespan
A battery’s Ah rating describes what it can deliver when new. Over hundreds of charge cycles, that capacity gradually shrinks. A lithium battery rated at 100 Ah might deliver 80 Ah after 2,000 to 3,000 cycles, depending on the chemistry and how hard it’s been used. Lead-acid batteries degrade faster, often losing significant capacity within 300 to 500 deep cycles.
How deeply you discharge a battery each cycle, how fast you charge it, and what temperatures it operates in all affect how quickly the rated amp hours fade. Keeping a battery in a moderate temperature range and avoiding full discharges when possible are the two simplest ways to preserve its capacity over time.