Every UPS and inverter nameplate promises a runtime number, and almost none of them survive contact with a real connected load.
The gap between the marketing figure and the actual backup time comes down to a handful of variables most buyers never see multiplied together.
Battery backup time is calculated by dividing the usable energy a battery can deliver, its capacity in Ah multiplied by voltage, depth of discharge, and inverter efficiency, by the connected load in watts, giving the runtime in hours before the battery reaches its safe discharge limit.
A battery's Ah rating on the label is not the number that determines how long the lights stay on. Four separate factors sit between that rating and the actual runtime, and skipping any one of them is why backup estimates run optimistic almost every time.
Depth of discharge limits how much of the rated capacity can safely be pulled out. Inverter efficiency loses a slice of energy to heat during DC to AC conversion. Battery voltage sets how much energy a given Ah figure actually represents. And the connected load, not the inverter's maximum rating, is what the battery is actually working against.
Per Genus Innovation's inverter battery backup guide, getting these four variables right is the difference between a backup estimate that matches reality and one that leaves a facility dark twenty minutes earlier than planned.
This guide walks through the runtime formula, an interactive calculator, and the real-world factors, battery chemistry, load type, and temperature, that push actual backup time away from the number on the spec sheet.
Why a Full Battery Doesn't Mean Full Runtime
A battery rated for 100 Ah never delivers 100 Ah of usable runtime. Between depth of discharge limits, conversion losses, and real-world derating, the energy that actually reaches the connected load is typically 40 to 55% of the nameplate figure for a lead-acid battery, and higher for lithium.
This is the single biggest reason backup time estimates go wrong. The formula itself is simple. The inputs people plug into it are usually optimistic.
The Battery Backup Time Formula
Backup Time (hours) = (Battery Ah × Battery Voltage × Depth of Discharge × Inverter Efficiency) ÷ Connected Load in Watts
Every term on the top of that equation shrinks the theoretical energy pool. The load on the bottom is what draws it down. A battery bank sized without accounting for all four multipliers will always underperform its paper estimate.
Try It: Battery Backup Time Calculator
Enter a battery's rated capacity and voltage, its depth of discharge and inverter efficiency, and the connected load to see estimated runtime.
A Real Worked Example
A small office runs a 600W connected load on a 24V battery bank rated at 150 Ah, using a lead-acid battery with a recommended 50% depth of discharge and an inverter rated at 90% efficiency.
Total energy = 150 Ah × 24V = 3,600 Wh. Usable energy = 3,600 × 0.50 × 0.90 = 1,620 Wh. Backup time = 1,620 ÷ 600 = 2.7 hours, or 2 hours 42 minutes.
Note that the nameplate math (3,600 Wh ÷ 600 W) would have suggested 6 full hours, more than double the real figure. That gap is entirely the depth of discharge and efficiency terms doing their work.
Depth of Discharge by Battery Chemistry
This single variable is the biggest swing factor in the entire calculation, and it depends almost entirely on battery chemistry.
Discharging a lead-acid battery beyond its recommended limit sharply shortens its cycle life, which is why manufacturers rate it conservatively at 50%. Lithium iron phosphate cells tolerate much deeper discharge without the same degradation penalty, which is a major reason lithium battery banks deliver more usable runtime per Ah than lead-acid banks of the same rated capacity.
Runtime Scales Inversely With Load
The same battery bank produces very different backup times depending purely on what's connected to it.
These figures use the same 150 Ah, 24V, 50% DoD, 90% efficiency bank from the worked example above. Doubling the load roughly halves the runtime, which is why trimming non-essential circuits from the backed-up panel is often a cheaper way to extend runtime than adding battery capacity.
Factors That Push Real Runtime Below the Estimate
Cold Temperature
Lead-acid capacity drops sharply below 0°C, sometimes by 30% or more at freezing temperatures.
Battery Age
Usable capacity fades with cycle count, often to 70-80% of rated Ah after a few years of regular use.
Inrush Loads
Motors and compressors draw a startup surge well above running wattage, briefly increasing the effective load.
Peukert Effect
Lead-acid batteries deliver less total capacity at high discharge rates than at low, slow discharge rates.
Wiring Losses
Undersized cables between battery and inverter waste energy as resistive heat before it reaches the load.
Charge State
A battery not charged to 100% before an outage starts the runtime calculation from a smaller usable pool.
Do's and Don'ts of Sizing for Backup Time
✓ Do
- Use the manufacturer's recommended depth of discharge, not the full rated Ah, in every runtime calculation
- Account for inverter efficiency losses, typically 85% to 95%, rather than assuming lossless conversion
- Size for actual connected load, not the inverter's maximum rated output
- Derate lead-acid capacity for cold storage locations or aging batteries
✗ Don't
- Divide the full nameplate Ah by the load and call it the real backup time
- Discharge lead-acid batteries below their rated DoD on a routine basis
- Ignore inrush current from motors or compressors when sizing the connected load
- Assume lithium and lead-acid batteries of the same Ah rating deliver the same usable runtime
Reference Materials on Battery Backup Time
FAQs on Battery Backup Time Calculation
External References
- How to Calculate Your UPS / Inverter Battery Backup Time, Genus Innovation
- How Do You Calculate Battery Backup Hours, Lento India
What we learn today
- Battery backup time equals (Ah × Voltage × Depth of Discharge × Inverter Efficiency) ÷ Connected Load in watts.
- A worked example shows a 150 Ah, 24V lead-acid bank delivering 2.7 hours at 600W, well under the naive 6-hour nameplate estimate.
- Depth of discharge is the biggest swing factor: roughly 50% for lead-acid versus around 90% for lithium iron phosphate.
- Runtime scales inversely with load, so trimming connected circuits often extends backup time more cheaply than adding battery capacity.
- Cold temperature, battery age, inrush current, and wiring losses all push real-world runtime below the calculated estimate.