Battery & UPS Runtime Calculator
Start with amp-hours or watt-hours, then expose the usable-capacity, efficiency, idle-load, and uncertainty assumptions that separate nameplate energy from a practical runtime plan.
Estimate ideal and adjusted runtime
Use measured real power in watts when possible. Select how the battery capacity is specified; only the matching capacity fields are used, while all loss and load inputs remain visible.
Primary result
After the entered usable-capacity and efficiency allowances, 864 Wh serves a 310 W adjusted load for about 2.79 hours.
Separate nameplate energy from a runtime plan
An ideal watt-hour division is useful as an upper arithmetic reference, but it is rarely the runtime a UPS or battery system delivers. Depth-of-discharge limits, conversion loss, the inverter’s own consumption, cutoff voltage, battery age, temperature, discharge rate, and a changing load can all shorten operation. This calculator keeps the largest planning allowances editable instead of embedding an unexplained fixed derating factor.
- Inputs: Ah and voltage or Wh, real load watts, usable capacity, efficiency, idle draw, and a symmetric uncertainty band.
- Outputs: nominal, usable, and delivered energy; adjusted load; ideal and adjusted runtime; and a planning range.
- Best use: screening backup time for a NAS, networking equipment, surveillance system, workstation, or other measured load before checking manufacturer runtime curves.
Formula and methodology
In amp-hour mode, nominal Wh = voltage × amp-hours. In watt-hour mode, the entered Wh rating is used directly. Usable energy equals nominal energy multiplied by the entered usable-capacity percentage. Delivered energy applies conversion efficiency to that usable amount. Adjusted load equals connected equipment watts plus UPS or inverter idle draw.
Ideal runtime = nominal Wh ÷ connected load W. Adjusted runtime equals delivered Wh divided by adjusted load W. The lower and upper planning values multiply adjusted runtime by one minus and one plus the entered uncertainty percentage. The range communicates input uncertainty; it is not a probability interval or warranty.
The formula assumes voltage × Ah is a suitable energy approximation and that load and efficiency remain constant. Real batteries have nonlinear discharge behavior. A manufacturer curve measured for the specific UPS, battery pack, power factor, and load is stronger evidence when available.
Worked examples
12 V, 100 Ah battery at a 300 W load
Nominal energy is 12 × 100 = 1,200 Wh. At 80% usable capacity, 960 Wh remains. Applying 90% efficiency gives 864 Wh delivered. Adding 10 W idle draw creates a 310 W adjusted load, so adjusted runtime is 864 ÷ 310 = 2.79 hours.
Why ideal runtime is longer
The ideal reference divides 1,200 Wh by only the 300 W equipment load, producing 4 hours. It ignores the 20% capacity reserve, 10% conversion loss, and 10 W idle consumption used in the adjusted scenario.
Load reduction
If measured equipment load falls while other assumptions remain the same, runtime rises because the delivered energy numerator is spread over fewer watts. Test the real operating state rather than relying only on device maximum labels.
Common mistakes
- Multiplying amp-hours by the wrong voltage for a series or parallel battery bank.
- Entering VA as if it were real watts without checking power factor and UPS limits.
- Using nameplate maximum load instead of measuring the state that must remain powered.
- Setting usable capacity and efficiency to 100% and treating the ideal result as guaranteed uptime.
- Ignoring battery aging, cold temperature, high discharge rate, cutoff voltage, and runtime required for orderly shutdown.
FAQ
Can I use this for lithium and lead-acid batteries?
You can create a planning scenario for either by entering suitable usable-capacity and efficiency assumptions, but the simple model does not reproduce chemistry-specific discharge curves, battery-management cutoffs, or manufacturer test conditions.
How do I find UPS idle draw?
Prefer a manufacturer specification or a real-power measurement. If it is unavailable, state the estimate and test several values; idle consumption matters most when the connected load is small.
How much runtime should I require?
That is an operational decision. Include detection and response time, orderly shutdown or generator transfer, aging margin, and the consequence of losing the protected equipment. This calculator supplies runtime arithmetic, not a resilience policy.
How this calculator helps
Use it to: Build an ideal and adjusted battery or UPS runtime range from explicit capacity, load, usable-capacity, efficiency, idle-draw, and uncertainty inputs.
Inputs and example scenarios
These examples show how the result changes with different inputs. Change the values to match the decision you are making.
starting example: Battery-bank voltage (V): 12; Battery capacity (Ah): 100; Battery capacity (Wh): 1200; Connected load (W): 300; UPS or inverter idle draw (W): 10; Usable capacity (%): 80; Conversion efficiency (%): 90; Runtime uncertainty (±%): 15; Capacity basis: Voltage × amp-hours.
materially different higher battery & ups runtime calculator scenario: Battery-bank voltage (V): 18; Battery capacity (Ah): 150; Battery capacity (Wh): 1800; Connected load (W): 300; UPS or inverter idle draw (W): 10; Usable capacity (%): 80; Conversion efficiency (%): 90; Runtime uncertainty (±%): 15; Capacity basis: Entered watt-hours.
What changes the result
Connected load, usable capacity, efficiency, and battery condition dominate runtime; voltage matters when amp-hours supply the capacity basis.
Formula and limitations
Nominal watt-hours come from volts times amp-hours or an entered watt-hour rating; usable and efficiency percentages reduce delivered energy, which is divided by connected load plus idle draw. The energy and power arithmetic is deterministic, while the allowances are visitor-entered planning assumptions rather than a battery discharge model.