- Choose a battery preset, then adjust values to match your bank.
- Enter series and parallel counts to model the full battery pack.
- Set DoD, efficiencies, and aging based on your maintenance history.
- Use a temperature derate that matches actual site conditions.
- Enter the load as watts, or use VA and PF for mixed equipment.
- Add reserve minutes for alarms, surge, and cut-off margin.
- Press calculate, then export CSV or PDF for documentation.
| Scenario | Battery Bank | DoD / Health | Load | Estimated Runtime |
|---|---|---|---|---|
| Site office + router | 12V units · 4S×1P · 150Ah | 60% / 90% | 350W | ~2.2 h |
| CCTV + lighting circuit | 12V units · 4S×2P · 200Ah | 55% / 85% | 900W | ~2.0 h |
| Survey gear charging bay | 12V units · 8S×2P · 100Ah | 70% / 95% | 1200W | ~1.1 h |
| Control panel + comms | 2V cells · 24S×1P · 300Ah | 50% / 90% | 800W | ~3.0 h |
- High discharge rates can reduce usable Ah; use conservative derating for motor loads.
- Cold weather can lower capacity; confirm actual enclosure temperature.
- Surge and inrush may trip the UPS even if average watts look safe.
- Battery age matters; health factor keeps estimates realistic for older banks.
- Reserve margin prevents planning too close to cut-off voltage.
On construction projects, a UPS is often used to keep site offices, security systems, and control panels running during short outages or generator changeovers. A reliable runtime estimate helps you protect data, maintain communications, and avoid unsafe shutdowns. This calculator converts your battery bank configuration and derating assumptions into an expected operating time at a defined load.
Start by describing the battery bank correctly. Series batteries increase voltage while parallel strings increase amp-hour capacity. The theoretical energy is V × Ah, but practical runtime is always lower because batteries are not 100% efficient, inverter stages have losses, and older batteries hold less capacity. Temperature is another major factor; cold conditions can reduce available capacity even when the bank appears fully charged.
Next, define the load in a way that matches your equipment. For resistive loads, watts are usually accurate. For mixed loads (routers, CCTV, chargers, LED drivers, variable-speed drives), VA and power factor provide a better picture of how the UPS is stressed. Keep surge loads in mind: a starter or compressor may have a short inrush that trips the UPS, even when average watts seem safe.
Batteries also deliver less usable capacity at high discharge rates. If you are supporting heavy loads with a small bank, runtime can drop faster than simple energy math suggests. For lead-acid systems, this is commonly described by the Peukert effect. When you do not have manufacturer curves, reduce the health factor and keep reserve minutes to stay conservative.
In real sites, battery strings may not share current equally. Loose terminations, different cable lengths, or mismatched batteries can cause one string to work harder and reach cut-off earlier. Good practice is to use identical batteries, equal-length interconnects, and periodic inspection with torque checks. Record voltage and internal resistance readings during maintenance to track deterioration.
Use this calculator for planning and documentation, then confirm performance with a controlled test. Apply the calculated load, start a timer, and note the low-battery alarm and shutdown time. Update your derating values to reflect measured results. For critical construction controls, include the exported report in the site log so supervisors have a clear runtime expectation during outages.
- Battery: 12V units, 4S × 2P, 200Ah per unit
- Derating: DoD 60%, battery efficiency 92%, UPS efficiency 90%, health 85%, temperature derate 100%
- Load: 900W (or 1125VA @ PF 0.80), reserve 5 minutes, UPS rating 3000VA
- Result: usable AC energy ≈ 1.69 kWh, net runtime ≈ 1 h 48 min (after reserve)
- Confirm the UPS output waveform and load compatibility.
- Verify battery age, maintenance history, and charger condition.
- Allow margin for startup surge and future load growth.
- Keep ventilation clear and follow safety isolation procedures.
Battery age, high discharge rate, temperature, and inverter behavior all reduce usable energy. Use conservative derating and confirm with a timed load test for critical applications.
Watts work for simple resistive loads. VA with power factor is better for mixed electronic loads because it reflects apparent power and UPS stress more accurately.
For lead-acid, 40–60% is often used for dependable service life. Lithium systems can tolerate higher DoD, but follow the vendor limits and your maintenance strategy.
If batteries are new and tested, 95–100% is reasonable. For older banks or unknown history, use 75–90% to avoid optimistic results.
Reserve time protects you from cut-off voltage, alarms, and short surges. Many site plans use 3–10 minutes, depending on load stability and risk tolerance.
Yes. Inrush current, poor power factor, harmonic distortion, or weak batteries can cause undervoltage or overload trips. Check surge ratings and test the worst-case startup sequence.
Yes for estimating holdover time during transfer. For mission-critical systems, verify transfer timing, charger recovery, and battery recharge requirements to ensure continuous coverage.