Example data table
| Load | Qty | Watts each | Hours/day | Surge | Daily energy (Wh) |
|---|---|---|---|---|---|
| LED site lighting string | 4 | 60 | 6 | 1.0× | 1,440 |
| Jobsite laptop | 2 | 90 | 5 | 1.0× | 900 |
| Wi‑Fi router | 1 | 15 | 24 | 1.0× | 360 |
| Water pump (small) | 1 | 400 | 1 | 3.0× | 400 |
| Ventilation fan | 1 | 120 | 8 | 2.0× | 960 |
Formula used
- DailyWh = Σ(qty × watts × hours_per_day)
- AutonomyWh = DailyWh × (autonomy_hours ÷ 24)
- BatteryWhNeeded = (ACWh ÷ inverter_eff + DCWh) ÷ battery_eff
- DesignWh = BatteryWhNeeded × (1+reserve) × (1+temp) × (1+aging)
- NominalWh = DesignWh ÷ DOD
- RequiredAh = NominalWh ÷ system_voltage
- Series = ceil(system_voltage ÷ unit_voltage)
- Parallel = ceil(RequiredAh ÷ unit_Ah)
How to use this calculator
- List every critical load you need during an outage window.
- Enter average hours per day based on your work shift plan.
- Use a surge multiplier for starting currents on motor loads.
- Select a system voltage and conservative depth of discharge.
- Adjust margins for temperature, aging, and reserve headroom.
- Press calculate, then export results for your documentation.
Battery autonomy planning on construction sites
Battery autonomy is typically set by critical loads and the outage window you can tolerate. For temporary power, a practical planning range is 4–12 hours for most projects. This calculator converts your daily energy into an autonomy-energy slice so sizing stays consistent.
Energy, power, and why both matter
Energy (kWh) determines how long the bank runs; power (kW) determines whether it can start and carry equipment. A lighting and IT package may be energy-heavy but low surge, while pumps and compressors demand high surge headroom. Enter surge multipliers per load and add an allowance to reflect uncertainty and simultaneous starts.
Accounting for losses and site realities
Construction deployments often see inverter losses, battery round‑trip losses, cold temperature reduction, and gradual aging. This tool applies inverter efficiency only to the AC portion of loads, then applies battery efficiency to the whole bank. Temperature derate and aging factor add capacity margin to keep performance predictable through seasonal changes and lifecycle fade.
Selecting voltage and battery configuration
Higher DC bus voltage reduces current for a given kW, which helps with cable size, voltage drop, and protective device selection. The recommended series count matches your target system voltage using your chosen battery unit voltage. Parallel strings scale the Ah capacity. The result is a simple S×P wiring plan that aligns with common 12V blocks or 48V modules.
Documenting results for procurement and safety
Use the exported report to support equipment selection and job documentation. Key procurement numbers include required Ah, nominal kWh, estimated usable energy at your depth of discharge, and peak/surge power targets. For safety planning, note the estimated peak DC current and confirm conductor sizing, overcurrent protection, isolation, ventilation, and enclosure ratings with a qualified professional.
FAQs
1) Why does autonomy use a fraction of daily energy?
Many sites have 8–12 active hours but need backup for only part of that day. The calculator scales daily Wh by autonomy/24 to reflect the outage window.
2) Should I size using watts or VA for inverters?
Use watts for load energy and typical running power. If loads have low power factor, check the kVA estimate and select equipment rated for both kW and kVA.
3) What depth of discharge should I choose?
A conservative value like 70–85% is common for many chemistries. Lower DOD increases bank size but can improve cycle life and reduce voltage sag.
4) How do surge multipliers affect sizing?
Surge affects power capability, not energy. High surge multipliers push inverter and cabling requirements upward, even when daily kWh stays modest.
5) Why include temperature and aging factors?
Cold reduces available capacity and aging reduces capacity over time. Adding these margins helps the bank deliver the planned autonomy later in its service life.
6) Can I use this for hybrid generator plus battery setups?
Yes. Size the battery for silent or peak-shaving periods, then use the peak/surge figures to coordinate generator, charger, and transfer strategy.