Battery Life Calculator
Estimate battery runtime: runtime = capacity (Ah) × voltage / load (W). Essential for product designers, IoT developers, and consumers comparing battery performance across devices, brands, and usage scenarios.
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Battery Life Calculator – Estimate Run Time & Capacity
The Battery Life Calculator takes the guesswork out of off-grid power planning. By evaluating your battery's capacity against the electrical draw of your devices, it accurately predicts exactly how long your system will run before the battery is depleted.
Whether you are building a solar setup for a camper van, designing a UPS (Uninterruptible Power Supply) for a server rack, or just figuring out if your portable power station can keep a mini-fridge cold all weekend, this tool handles the complex math. It doesn't just do basic division; it factors in critical real-world variables like inverter inefficiencies, battery chemistry types, and safe depth-of-discharge limits.
⚠️ Crucial Design Note: Never plan to drain a battery to 0%. Draining a standard Lead-Acid or AGM battery past 50% will permanently damage it. Lithium (LiFePO4) batteries are much more resilient, safely discharging down to 10% or 20%. The calculator's "Usable Capacity" metric ensures you don't destroy your expensive batteries.
When to Use This Battery Calculator
🚐 Van Life & RV Solar
Calculate if your 200Ah 'house battery' bank has enough juice to run your 12V fridge, laptop charger, Maxxair fan, and LED lights through a rainy weekend without solar charging.
⚡ Home Backup Power
Size a battery bank for a power outage. Determine exactly how many Amp-hours you need to keep your critical loads (medical devices, freezer, router) running for 24 to 48 hours.
🚤 Marine Electronics
Ensure your boat's trolling motor, fish finders, and bilge pumps won't drain your deep-cycle marine battery while you are miles away from the dock.
💻 Portable Power Stations
Verify the marketing claims of Jackery, EcoFlow, or Bluetti devices. If it claims "500Wh", calculate how many hours it can actually run your 60W CPAP machine.
🚁 Drone & RC Hobbyists
Calculate the expected flight time of a drone based on the mAh (milliamp-hour) rating of the LiPo battery and the average current draw of the motors.
🔌 UPS Sizing
Calculate how many minutes of backup time a specific UPS (Uninterruptible Power Supply) will provide to safely shut down your desktop PC and monitor during a blackout.
Battery Life Formula & Calculation
Calculating real-world battery life requires three distinct steps: finding raw time, applying safe discharge limits, and factoring in system inefficiencies.
1. The Basic Formula (Theoretical)
Example: A 100Ah battery powering a 10 Amp load theoretically lasts 10 hours. But in the real world, this is never true.
2. The Real-World Formula (Usable Capacity)
Lead Acid/AGM max DoD is 50% (0.50). Lithium (LiFePO4) max DoD is typically 80% to 90% (0.80 - 0.90).
3. Converting AC Watts to DC Amps (Inverter Loss)
If you are plugging a standard wall appliance (120V AC) into an inverter connected to your 12V battery, you must account for the inverter wasting about 15% of the power as heat.
Variable Definitions
| Symbol | Quantity | Notes |
|---|---|---|
| Ah | Amp-hours | The total electrical charge the battery can hold. |
| Wh | Watt-hours | The total energy. Calculated as Ah × Volts. |
| DoD | Depth of Discharge | The safe percentage of the battery you can drain without damage. |
| Peukert | Peukert Effect | Lead-acid batteries lose total capacity when drained very quickly. |
Step-by-Step Calculation Guide
Identify Battery Specs
Look at your battery. Note the Voltage (usually 12V, 24V, or 48V) and the Capacity in Amp-hours (Ah). If it only lists Watt-hours (Wh), divide Wh by Volts to get Ah.
Select Battery Chemistry
Identify if the battery is Lead-Acid (Flooded/AGM/Gel) or Lithium (Li-ion/LiFePO4). This determines your safe Depth of Discharge (DoD). Use 50% for Lead-Acid and 85% for Lithium.
Determine Load (Watts or Amps)
Find the power consumption of the devices you want to run. Add them all together. If you have Watts, divide by the Battery Voltage to convert the load into DC Amps.
Account for Inverter Loss
If you are using an inverter to power 120V/240V AC household devices, multiply your Amp load by 1.15 to account for the ~15% efficiency loss of the inverter.
Calculate Usable Run Time
Multiply Battery Ah by DoD to get Usable Ah. Divide Usable Ah by your Total Amp Load. The result is your safe run time in hours.
Worked Examples
Given:
- • Battery: 12V 100Ah Lithium (85% DoD)
- • Fridge Draw: 12V DC @ 4 Amps
- • Duty Cycle: Runs 50% of the time (equivalent to 2 Amps continuous)
Solution:
Usable Ah = 100Ah × 0.85 = 85Ah
Run Time = 85Ah ÷ 2A continuous
Run Time = 42.5 hours
With a modern Lithium battery, you can easily run an efficient 12V compressor fridge for almost two days without any solar input or driving.
Given:
- • Battery: 12V 100Ah AGM (Lead-Acid) (50% max DoD)
- • Load: 500W AC Desktop PC
- • Inverter Efficiency: 85%
Solution:
Raw DC Load = 500W ÷ 12V = 41.6 Amps
Actual Load (Inverter Loss) = 41.6A ÷ 0.85 = 49 Amps
Usable Ah = 100Ah × 0.50 = 50Ah
Run Time = 50Ah ÷ 49A = 1.02 hours
Notice how a massive 100Ah Lead-Acid battery dies in just ONE HOUR running a PC. Between the 50% DoD rule and inverter losses, the usable capacity is tiny. (Furthermore, due to Peukert's law on Lead-Acid pulling 49A rapidly, it will likely die in 45 minutes).
Given:
- • Battery: 24V 50Ah Lithium (90% DoD)
- • Trolling Motor: 24V, pulls 15 Amps at medium speed
Solution:
Usable Ah = 50Ah × 0.90 = 45Ah
Run Time = 45Ah ÷ 15A = 3 hours
Running at medium speed continuously, this battery will give you 3 hours of trolling. If you bump it to max speed (pulling 40 Amps), run time drops to roughly 1 hour.
Given:
- • Power Station: 300Wh (Lithium)
- • CPAP (No Humidifier): 12V DC @ 1.5 Amps
Solution:
Convert Station to Ah = 300Wh ÷ 12V = 25Ah
Usable Ah = 25Ah × 0.90 = 22.5 Ah
Run Time = 22.5Ah ÷ 1.5A = 15 hours
By plugging the CPAP directly into the 12V DC port (avoiding the AC inverter), a small 300Wh power station can easily provide two full 7-hour nights of sleep.
Given:
- • Critical Loads: Fridge (100W) + Router (15W) + Lights (25W) = 140W Continuous AC
- • Target Run Time: 24 Hours
- • System: 12V Lithium, 85% Inverter
Solution:
Actual DC Amps = (140W ÷ 12V) ÷ 0.85 = 13.7 Amps
Total Ah Needed = 13.7A × 24h = 328.8 Usable Ah
Total Capacity Needed = 328.8Ah ÷ 0.85 DoD = 386 Ah
To survive a 24-hour blackout running just a fridge, internet, and lights, you need a massive 12V 400Ah Lithium battery bank (equivalent to four standard 100Ah batteries).
Common Mistakes to Avoid
Treating Lead-Acid and Lithium equally
A 100Ah Lithium battery has nearly TWICE the usable capacity of a 100Ah Lead-Acid battery. Lead-Acid can only be drained to 50% without damage, while Lithium can safely go to 10-20%.
Forgetting the Inverter 'Tax'
Inverters magically turn 12V DC into 120V AC, but they waste 10-20% of the energy as heat in the process. If you calculate battery life based strictly on the AC wattage, your battery will die 15% sooner than expected.
Ignoring Peukert's Law (Lead-Acid only)
If you draw massive current (like running a microwave) from a Lead-Acid battery, the battery's total capacity physically shrinks. A 100Ah battery might only act like a 60Ah battery under heavy load.
Assuming 12V devices actually run at 12.0V
A '12V' battery rests at 12.8V and drops to 11.5V when dead. A '12V' solar panel outputs 18V. Voltage constantly fluctuates, which is why converting everything to Watts/Watt-hours is safer than just using Amps.
Running heating appliances off batteries
Electric space heaters, coffee makers, and water heaters draw staggering amounts of power (1500W+). Running these off a battery bank will deplete it in minutes. Always use propane or diesel for heating in off-grid setups.
Tips and Best Practices
- ✓Use DC power directly whenever possible. If you have a 12V battery, buy 12V chargers for your laptop, a 12V TV, and a 12V fridge. Bypassing the AC inverter saves 15% of your power instantly.
- ✓Compare batteries using Watt-hours (Wh), not Amp-hours (Ah). Ah is useless if voltages are different. A 48V 25Ah battery has the exact same energy (1200Wh) as a 12V 100Ah battery. Wh = Volts × Ah.
- ✓Account for temperature. Batteries hate the cold. If your batteries are sitting in a freezing garage (32°F / 0°C), expect to lose 20% to 30% of their total capacity.
- ✓Turn off the inverter when not in use. Even when nothing is plugged in, an AC inverter consumes power just by being turned on (idle draw). A large 2000W inverter can draw 1-2 Amps constantly, draining a 100Ah battery dead in a few days of doing absolutely nothing.
- ✓Buy a Battery Monitor (Shunt). Voltage is a terrible way to guess battery life. Buy a 'shunt' style battery monitor (like Victron). It counts every amp going in and out, giving you a perfectly accurate 'Gas Gauge' percentage.
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Conclusion
Designing an off-grid electrical system or emergency backup requires more than just slapping a battery and an inverter together. The Battery Life Calculator proves that the 'theoretical' capacity printed on the side of a battery is rarely the capacity you actually get to use. By factoring in Depth of Discharge and Inverter inefficiency, you can design a robust system that won't leave you in the dark.
If you take away one lesson, let it be this: Chemistry matters. Upgrading from old Lead-Acid technology to Lithium (LiFePO4) effectively doubles your usable run time for the exact same Ah rating, while lasting 10x longer.
Disclaimer: This calculator provides mathematical estimates based on ideal conditions. Battery age, ambient temperature, wire resistance, and the specific discharge curve of your battery brand will all cause slight variations in real-world performance. Always build a 20% safety margin into your final system design.
Frequently Asked Questions
How do you calculate battery life?
Divide Usable Battery Capacity (Ah) by your Load Current (Amps). Time = Capacity ÷ Current. Be sure to account for safe discharge limits and inverter inefficiencies.
What is an Amp-hour (Ah)?
It is a measure of battery capacity. A 100Ah battery can theoretically deliver 1 Amp for 100 hours, or 10 Amps for 10 hours.
Why shouldn't I drain my battery to 0%?
Draining Lead-Acid batteries past 50% permanently damages their chemistry. Lithium batteries are safer but still shouldn't be drained past 10-20% to maximize their lifespan.
How do I calculate battery life for an AC appliance?
Convert AC Watts to DC Amps (Watts ÷ Battery Volts). Then divide by 0.85 to account for the ~15% energy the inverter wastes as heat. Then divide your battery Ah by this final DC Amp number.
What is the difference between Ah and Wh?
Ah is charge capacity. Wh is total energy. Wh = Ah × Volts. You should always use Wh to compare batteries of different voltages.
How long will a 100Ah battery run a 500W load?
A 500W load pulls ~49 Amps (including inverter loss). A 100Ah Lithium battery (85% usable) gives about 1.7 hours. A 100Ah Lead-Acid (50% usable) gives about 1 hour.
Do cold temperatures affect battery life?
Yes. At freezing temperatures, both Lithium and Lead-Acid batteries suffer increased internal resistance, temporarily reducing their usable capacity by 20-30%.
What is Peukert's Law?
It's a physics law affecting Lead-Acid batteries: the faster you drain them, the less total capacity they have. Pulling 100A from a 100Ah battery will kill it in about 35 minutes, not 60 minutes.
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