Inputs
Example Data
Use this sample to sanity-check your settings.
| Scenario | Inputs (key) | Typical outcome |
|---|---|---|
| Small Office Router | Load 60 W, Surge 80 W, 120 min, 24 V bus, 12 V 18 Ah | 2S × 1P (2 batteries), ~18–25 Ah string |
| Home Workstation | Load 350 W, Surge 600 W, 30 min, 48 V bus, 12 V 100 Ah | 4S × 1P (4 batteries), ~25–40 A DC nominal |
| Critical Comms Rack | Load 1200 W, Surge 1800 W, 30 min, 48 V bus, 12 V 100 Ah | 4S × 1P (4 batteries) to 4S × 2P, by margins |
Formulas Used
How to Use
- Enter average load and the maximum surge power.
- Set the required backup time in minutes.
- Choose the UPS DC bus voltage and battery unit voltage.
- Enter the battery unit amp-hour rating from its datasheet.
- Adjust DoD, aging, temperature, and margin factors.
- Press Calculate to see sizing and configuration.
- Download CSV or PDF for records and quoting.
Load profiling and runtime targets
Start with a measured watt profile, not nameplate labels. A 1,200 W rack often averages 700–900 W. Use the peak surge for inrush and fan start. Convert minutes to hours for energy planning. For 30 minutes at 1,200 W, load energy equals 600 Wh before any losses. If power factor is 0.9, the apparent demand is about 1,333 VA, so select UPS capacity with headroom.
Battery rating and discharge behavior
Battery amp-hour ratings are usually quoted at a 20-hour rate. Higher discharge currents reduce usable capacity and increase voltage sag. A 100 Ah VRLA unit may deliver 60–80 Ah at a 1C discharge. If the calculated DC current exceeds 0.2C per string, increase parallel strings or select a larger Ah model. Lithium packs hold voltage better, but require certified battery management and matching charge profiles.
Derating, aging, and temperature
Plan for end-of-life capacity and the site environment. Many designs reserve 20–30% for aging across three to five years. Temperature effects are practical: at 0°C, lead-acid capacity can fall near 80% of rated, and at -10°C it can approach 70%. Heat also shortens life. Use a temperature factor when rooms are unconditioned. Depth of discharge matters: limiting DoD to 0.8 improves cycle life but increases required capacity.
Series-parallel configuration planning
Series batteries set the DC bus voltage; parallel strings set capacity. For a 48 V bus with 12 V units, series count is four and the actual bus is 48 V. Required amp-hours at the bus are Ah = Wh ÷ V. Example: 900 Wh ÷ 48 V is 18.75 Ah before derating. If the required capacity is 160 Ah, 100 Ah units need two parallel strings. Fuse each string, balance cable lengths, and verify connector ratings for the expected surge current.
Validation, documentation, and exports
After calculating, validate against manufacturer discharge tables and a runtime test. Record assumptions for auditability: efficiency, ripple factor, DoD, margins, and battery age. Log temperature and installation date. Keep a wiring sketch showing series count, parallel strings, and protective devices. Use the CSV for procurement comparisons and the PDF for maintenance manuals, inspections, and handover packages.
FAQs
What load value should I enter?
Enter the typical steady running watts you want to support. If you only know amperes, convert using real power, not VA. When unsure, measure with a power meter during normal operation.
Why does surge power matter?
Surge covers motor start, compressor inrush, and short transients. It affects surge current, inverter stress, and UPS headroom. Ignoring surge can cause unexpected shutdowns even when average load seems safe.
How should I choose depth of discharge?
For lead-acid, 0.5–0.8 DoD is common. Lower DoD extends life but increases battery count. For lithium, higher usable DoD is possible, but follow the manufacturer’s guidance and warranty limits.
What if my DC bus and battery voltage do not match?
The calculator sets series count using the ceiling of bus voltage divided by unit voltage. That creates a slightly higher actual bus voltage. Confirm the UPS accepts the resulting string voltage range.
Does the temperature factor replace environmental design?
No. Derating helps sizing, but ventilation and temperature control protect battery life and safety. Aim for stable room conditions and follow spacing and airflow recommendations from the UPS and battery suppliers.
When should I add more parallel strings?
Add strings when required amp-hours exceed a single battery’s rating, or when current per string is too high. More strings reduce discharge rate, improve voltage stability, and often increase real-world runtime consistency.