| Scenario | Avg Load (kW) | Autonomy (h) | System V | DoD | Eff (Inv/Batt) | Suggested Storage (kWh) |
|---|---|---|---|---|---|---|
| Small office backup | 1.0 | 8 | 48 | 80% | 92% / 95% | ~11.3 |
| Telecom site | 2.5 | 6 | 48 | 70% | 94% / 95% | ~23.4 |
| Home essentials | 0.6 | 12 | 24 | 80% | 90% / 92% | ~12.0 |
| Workshop tools | 1.8 | 5 | 48 | 85% | 92% / 95% | ~13.7 |
This calculator sizes nominal battery storage so the usable energy meets your autonomy target after losses and derating.
- Autonomy energy (AC):
E_ac = P_avg × torE_ac = (E_day/24) × t - DC energy at battery bus:
E_dc = E_ac / η_inv - Nominal storage required:
E_nom = E_dc × (1+g) × (1+r) / (DoD × η_batt × k_temp) - Capacity in amp-hours:
Ah = (E_nom × 1000) / V_sys - Module count: series
N_s = ceil(V_sys / V_mod), parallelN_p = ceil(Ah / Ah_mod) - Power check (C‑rate):
P_limit ≈ V_bank × Ah_bank × C / 1000
- Choose Average load or Daily energy, then enter autonomy hours.
- Enter system voltage and realistic efficiencies for your equipment.
- Set DoD and temperature derating based on expected conditions.
- Add growth and reserve margins to avoid undersizing.
- Pick a module voltage and amp-hour rating to get series/parallel recommendations.
- Click Calculate, then export results using CSV or PDF.
Round‑trip efficiency and autonomy realism
Autonomy is energy: average load multiplied by time. Conversion losses shift AC demand to DC draw. Typical inverter efficiency is 90–96% at mid‑load, and discharge efficiency is often 92–99%. With 94% inverter and 95% battery efficiency, 10.00 kWh AC can require about 11.19 kWh nominal before margins. Efficiency can drop at very low loads, so measure standby consumption.
Depth of discharge and usable energy
Usable energy is limited by depth of discharge (DoD). A 20 kWh bank at 70% DoD offers about 14 kWh usable; at 80% it offers about 16 kWh. The calculator divides required DC energy by DoD, then applies growth and reserve, so the usable window still meets your autonomy target.
Temperature derating and seasonal sizing
Capacity falls in cold or extreme heat, and power limits can tighten. A derating factor of 0.85 means you only rely on 85% of nameplate capacity under worst conditions. The calculator scales sizing by dividing by k_temp, helping avoid wintertime runtime shortfalls on exposed sites. Consider heater power and cold‑charge limits in your operating plan.
Growth and reserve margins as risk controls
Margins reduce the risk of undersizing. Growth covers expected load additions over 12–36 months; reserve covers metering error, inverter self‑consumption, and operating changes. A 10% growth and 10% reserve combine to 21% extra capacity (1.10 × 1.10 = 1.21). This calculator applies margins multiplicatively for clarity.
Module series/parallel choices and voltage strategy
Higher DC voltage reduces current for the same kW, cutting I²R losses and conductor size. Common systems use 24 V or 48 V, while larger packs run higher with integrated management. The calculator converts kWh to amp‑hours at your bus voltage, then estimates series and parallel strings to match module ratings. For systems above 5 kW, 48 V or higher often reduces current stress.
Power capability, C‑rate, and runtime expectations
Energy sets runtime; power sets whether the bank can support demand. At 0.5C, a 200 Ah string supports roughly 100 A continuously. The calculator estimates a continuous power limit from voltage, amp‑hours, and C‑rate to flag tight designs. If the warning appears, add parallel strings, select higher‑power modules, or raise system voltage. Also verify surge kW, peak current, and voltage sag during starts.
Required nominal is the calculated target after DoD, efficiency, temperature derating, growth, and reserve. Built bank is what your chosen module voltage and Ah produce using the recommended series/parallel counts.
Use average load when you know typical kW during the backup window. Use daily energy when you only have kWh/day and want an equivalent average during autonomy hours.
Choose based on chemistry and lifecycle goals. Conservative values (60–80%) extend life. Higher DoD increases usable energy but typically reduces cycle life and thermal margin.
Use manufacturer data where possible. For preliminary design, many inverter systems are 90–96% efficient at mid‑load, and battery discharge efficiency often falls in the 92–99% range.
Derating represents reduced deliverable capacity under worst conditions. Dividing by a factor like 0.85 increases nominal sizing so usable energy remains sufficient when the battery cannot deliver full nameplate capacity.
It focuses on continuous energy and a continuous C‑rate power check. For surges, validate inverter surge ratings, battery peak current limits, and voltage sag using vendor curves and application notes.