Enter loads and design limits
- Wh/day per load = Watts × Quantity × (Duty%/100) × Hours/day
- Total load energy for autonomy = (Σ Wh/day) × AutonomyDays
- Battery-supplied energy = EnergyToLoads / OverallEfficiency
- Installed energy = BatteryEnergy / (DoD × TempFactor) × (1+Aging) × (1+Reserve)
- Required capacity (Ah) at system voltage = InstalledWh / SystemVoltage
- Parallel strings = ceil(RequiredAh / UnitAh), Series = SystemV / UnitV
- List your loads with watts, quantity, duty cycle, and hours per day.
- Pick autonomy days to cover outages or off-grid operation.
- Select system voltage that matches your inverter or DC bus.
- Set usable depth of discharge based on your battery’s recommendations.
- Choose efficiency values to reflect inverter and other losses.
- Apply temperature, aging, and reserve margins for robust sizing.
- Enter a common battery unit rating to get a practical bank layout.
- Calculate, then export results to CSV or PDF for documentation.
Load energy profile and Wh/day
Start by listing every device, its rated watts, quantity, duty cycle, and daily operating hours. Duty cycle models intermittent equipment such as pumps or compressors without overestimating energy. The calculator multiplies these inputs to produce watt‑hours per day, then sums them for a site total. For example, a 12 W router running 24 hours uses 288 Wh/day, while four 10 W lights used five hours consume 200 Wh/day.
From daily energy to required capacity
Once total Wh/day is known, autonomy days scale the requirement for longer outages or off‑grid periods. The bank must deliver that energy at the selected system voltage, so the tool converts installed watt‑hours into amp‑hours using Ah = Wh ÷ V. A 24 V system halves the amp‑hours needed compared with 12 V for the same energy.
Accounting for conversion and wiring losses
Real systems lose energy in inverters, DC‑DC converters, cabling, and controllers. The calculator combines “other system efficiency” with optional inverter efficiency to form an overall efficiency factor. If overall efficiency is 0.85, the battery must supply 1 ÷ 0.85 ≈ 1.18 times the load energy, so plans stay realistic. Use measured values to tighten estimates.
Depth limits, reserve, and aging margins
Usable depth of discharge protects cycle life and prevents low‑voltage cutoffs. If DoD is 80%, only 0.80 of nominal capacity is counted as usable. Reserve margin adds headroom for forecast errors, and aging margin oversizes the bank for capacity fade. Many designs use 10–20% reserve and 15–30% aging, adjusted for criticality and service life.
Temperature derating and practical bank layout
Cold conditions reduce available capacity, especially for lead‑acid. In auto mode, the tool applies a temperature factor based on chemistry and ambient temperature, or you can enter a datasheet factor manually. Finally, the calculator suggests series and parallel counts from your unit voltage and amp‑hour rating, helping you specify a buildable battery bank. After sizing, confirm surge power, fusing, and cable ampacity.
FAQs
What does Wh/day represent in the results?
It is the daily energy your loads consume. It equals watts multiplied by operating hours, adjusted by quantity and duty cycle. The calculator sums all rows to estimate total daily energy demand.
Do I size batteries for peak watts or total energy?
Battery capacity is primarily an energy problem, so Wh/day and autonomy drive sizing. Peak watts matter for inverter rating, surge current, and cable sizing. Use both: energy for capacity, peak for power components.
How should I choose a safe depth of discharge?
Use the battery manufacturer guidance first. Typical planning values are about 50% for lead‑acid, 70–80% for Li‑ion, and 80–90% for LiFePO4. Lower DoD increases capacity but improves cycle life.
When should I include inverter efficiency?
Include it when the battery supplies AC loads through an inverter. If everything runs on DC at the system voltage, you can exclude it and model only wiring and converter losses using the system efficiency field.
Why does cold temperature increase the required bank size?
Cold reduces available capacity, so the temperature factor drops below 100%. The calculator divides by this factor, which increases installed watt‑hours and amp‑hours. Manual mode lets you match a specific datasheet curve.
What do the suggested series and parallel counts mean?
Series strings raise voltage to match the system (for example, two 12 V units in series for 24 V). Parallel strings add amp‑hours and current capability. Total batteries equals series multiplied by parallel.
| Load | Watts | Qty | Duty% | Hours/day | Wh/day |
|---|---|---|---|---|---|
| Router | 12 | 1 | 100 | 24 | 288 |
| LED lights | 10 | 4 | 100 | 5 | 200 |
| Laptop charging | 60 | 1 | 50 | 4 | 120 |
| Fan | 45 | 1 | 80 | 6 | 216 |
| Total | 824 | ||||