Battery Backup Time Calculator – How Long Will Your Battery Last?
Find out how long your battery will power your devices with our Battery Backup Time Calculator. Enter battery capacity in Ah or Wh and your device's power draw in watts to get an accurate runtime estimate — ideal for solar systems, UPS, and portable power banks.
Battery Specifications
Affects depth of discharge limits
Device Specifications
Inverter/convertor losses (typically 80-95%)
Backup Time Results
Enter values and click Calculate to see results
Battery Backup Calculation Formula
| Battery Type | Recommended DoD |
|---|---|
| Lead-Acid (Flooded) | 50% |
| AGM | 60% |
| Gel | 60% |
| Lithium (LiFePO4) | 90% |
Enter your battery specifications
Input the battery capacity in amp-hours (Ah) and select the voltage. Choose your battery type to apply the correct depth of discharge limit.
Enter your device power requirements
Input the wattage of the device you want to power. You can find this on the device label or power adapter. Select the device voltage.
Click Calculate to see runtime
Get your estimated backup time in hours or minutes, plus usable energy and device current draw calculations.
| Device | Power (Watts) | Runtime on 100Ah 12V |
|---|---|---|
| LED Light Bulb (10W) | 10W | ~54 hours |
| Laptop Computer | 45-65W | ~8-12 hours |
| LED TV (42 inch) | 80-100W | ~5-6 hours |
| Mini Fridge | 50-100W (avg) | ~5-10 hours |
| CPAP Machine | 30-60W | ~8-16 hours |
| WiFi Router | 5-15W | ~30-90 hours |
| Phone Charger | 5-10W | ~50-100 hours |
Note: Runtimes assume lead-acid battery with 50% DoD and 85% inverter efficiency. Actual results vary.
Amp-Hours vs Watt-Hours
Battery capacity is often listed in amp-hours (Ah), but devices consume watts. To compare them, multiply Ah by voltage to get watt-hours (Wh). A 100Ah 12V battery stores 1200Wh of energy. This conversion is essential for accurate runtime estimates.
Depth of Discharge (DoD)
DoD is how much of the battery capacity you can safely use. Lead-acid batteries should not discharge below 50% or their lifespan drops dramatically. Lithium batteries can safely use 80-90% of their capacity. This is why lithium costs more but delivers more usable energy.
Inverter Efficiency
Inverters convert DC battery power to AC for household devices. This conversion is not perfect — some energy becomes heat. Typical efficiency is 80-95%. A 100W device might actually draw 115W from the battery after accounting for inverter losses.
Why Runtime Is an Estimate
Battery capacity ratings are measured under ideal conditions. Cold temperatures reduce capacity. Old batteries hold less charge. High current draws reduce effective capacity (Peukert effect). Motors and compressors have startup surges. Plan for 20-30% less runtime than calculated.
Use LED lighting
LED bulbs use 80-90% less power than incandescent. A 10W LED replaces a 60W bulb. This single change can extend your backup time by hours.
Prioritize essential loads
Make a list of what you really need during an outage. Lights, phone charging, and maybe a fridge. Skip the TV and microwave. Every watt counts.
Keep batteries at room temperature
Cold reduces battery capacity. Heat shortens battery life. Store batteries in a climate-controlled space when possible. A garage that freezes in winter is not ideal.
Maintain your batteries
Flooded lead-acid batteries need water checks. All batteries need clean terminals and secure connections. Check voltage periodically. Replace batteries that no longer hold a charge.
How long will a 100Ah battery last?
It depends on your load. A 100Ah 12V lead-acid battery has about 600Wh usable (50% DoD). Running a 50W device gives roughly 12 hours. A 100W device gives about 6 hours. Double the wattage, halve the runtime.
Should I choose lithium or lead-acid for backup?
Lithium costs more upfront but lasts longer and provides more usable capacity. Lead-acid is cheaper but heavier and needs more maintenance. For occasional outages, lead-acid works. For daily use or critical backup, lithium is worth the investment.
What size inverter do I need?
Add up the wattage of all devices you might run simultaneously. Choose an inverter rated 20-25% higher than that total. Motors and compressors need surge capacity — check the inverter surge rating for starting loads.
Can I connect multiple batteries together?
Yes. Connecting in parallel (positive to positive, negative to negative) increases capacity (Ah) while keeping voltage the same. Connecting in series increases voltage. Match battery type, age, and capacity when connecting batteries.
Why is my actual runtime shorter than calculated?
Several factors reduce real-world runtime. Battery capacity decreases with age. Cold temperatures reduce capacity. High current draws are less efficient. Inverter efficiency varies with load. Device wattage labels show maximum, not average consumption.
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