Time = (Ah x V x DoD x Eff) / Load W
Enter your battery specs (Ah and voltage), depth of discharge, and inverter efficiency. Add your devices with their wattage. The calculator determines usable energy capacity and divides it by your total load to estimate backup time. Lower DoD extends battery life but reduces backup time.
A Battery Backup Time Calculator helps you determine how long a battery or UPS system can power your devices during a power outage. It considers battery capacity, voltage, depth of discharge (DoD), inverter efficiency, and the total power consumption of your connected devices to give you an accurate estimate of backup duration.
Understanding backup time is essential for planning emergency power needs, sizing UPS systems for computers and networking equipment, or determining how long your solar battery system can run at night. It helps you decide whether your current battery setup is adequate or if you need additional capacity.
Start by entering your battery capacity in amp-hours (Ah) and its voltage (commonly 12V, 24V, or 48V). Set the depth of discharge -- this is the percentage of battery capacity you plan to use. For lead-acid batteries, 50% is recommended to preserve battery life; lithium batteries can safely go to 80-90%.
Enter the inverter efficiency (typically 80-90% for most inverters). Then add each device you want to power, specifying its wattage and quantity. Use the quick-add buttons for common devices. The calculator shows the total backup time for all devices running simultaneously, plus individual times if each device were running alone.
Depth of Discharge (DoD) is the percentage of battery capacity that has been used. A 100Ah battery discharged to 50% DoD means 50Ah has been used. Deeper discharges reduce battery cycle life -- for lead-acid batteries, regularly discharging below 50% can significantly shorten lifespan.
Lead-Acid Batteries
Keep DoD at 50% or less for optimal lifespan. A lead-acid battery at 50% DoD can last 1,200-1,500 cycles, while 80% DoD may reduce this to 400-600 cycles. Deep discharges cause sulfation which permanently reduces capacity.
Lithium Batteries
Lithium-ion and LiFePO4 batteries handle deeper discharges much better. They can be discharged to 80-90% DoD and still achieve 2,000-5,000+ cycles. This means a smaller lithium battery can effectively replace a larger lead-acid one.
Prioritize essential loads during outages -- disconnect non-critical devices to extend backup time for what matters most. Use energy-efficient LED lighting instead of incandescent bulbs. A single LED bulb uses 10W compared to 60W for an equivalent incandescent, effectively giving you 6x more lighting time from the same battery.
Keep batteries fully charged before anticipated outages. Battery capacity decreases in extreme cold, so keep batteries in a temperature-controlled area if possible. For UPS systems, replace batteries every 3-5 years as internal resistance increases with age, reducing effective capacity. Consider adding a second battery in parallel to double your backup time.
Important Note
This calculator provides theoretical estimates. Actual backup time varies with battery age, temperature, load fluctuations (e.g., refrigerator compressor cycling), cable losses, and inverter standby power consumption. Batteries lose capacity over time, so factor in age when planning backup needs.