Inputs
Enter energy, performance, and cost assumptions. The layout adapts from three columns to one.
Example Data Table
| Scenario | Daily kWh | Critical % | Autonomy (h) | Peak kW | DoD % | Derate % | Nominal Storage (kWh) | Recommended Inverter (kW) |
|---|---|---|---|---|---|---|---|---|
| Campus essential | 500 | 60 | 12 | 120 | 80 | 95 | ≈ 257 | ≈ 172 |
| Clinic backup | 180 | 75 | 18 | 55 | 85 | 90 | ≈ 196 | ≈ 85 |
| Warehouse continuity | 320 | 50 | 8 | 90 | 80 | 95 | ≈ 133 | ≈ 129 |
Examples are illustrative. Use your measured profiles for planning.
Formula Used
- Autonomy days = Autonomy hours ÷ 24
- Critical energy (kWh/day) = Daily kWh × (Critical % ÷ 100)
- Usable storage target (kWh) = Critical energy × Autonomy days
- Total efficiency = (Inverter eff ÷ 100) × (Round‑trip eff ÷ 100)
- Required nominal storage (kWh) = Usable target × (1 + Margin%) ÷ (DoD × Total efficiency × Derate)
- Battery bank capacity (Ah) ≈ (Nominal kWh × 1000) ÷ System voltage
- Recommended inverter (kW) = Peak kW × Surge factor × (1 + Margin%) ÷ (Inverter eff)
- Annualized cost = Total CAPEX × CRF, where CRF = r(1+r)^n ÷ ((1+r)^n − 1)
How to Use This Calculator
- Enter daily energy and peak load from metered data or load studies.
- Set the critical share to reflect which loads must stay online.
- Choose autonomy hours based on outage targets and dispatch plans.
- Adjust DoD, efficiencies, and derate to match chosen hardware and conditions.
- Add a design margin for growth, aging, and forecasting uncertainty.
- Optionally enter a module size to estimate unit counts.
- Provide cost assumptions to estimate CAPEX and annualized cost.
- Run the calculation, then download CSV or PDF for records.
Load profiling and criticality
Microgrid storage starts with measured energy and a realistic critical‑load fraction. If a site uses 500 kWh/day and 60% is critical, the design basis is 300 kWh/day. Pair this with peak demand, such as 120 kW, to ensure power electronics cover real operating peaks, not averages. Use 15‑minute interval data to capture ramping.
Autonomy targets and usable energy
Autonomy converts goals into energy. Twelve hours equals 0.5 days, so usable autonomy energy becomes 300 × 0.5 = 150 kWh in the example. Longer autonomy raises cost quickly; moving from 12 to 18 hours increases usable energy by 50%, which usually drives the battery budget more than small efficiency changes. Consider minimum state‑of‑charge rules and generator dispatch windows.
Efficiency, DoD, and derating impacts
Nominal capacity must exceed usable energy because only part of the nameplate is deliverable. With 80% depth of discharge, 96% inverter efficiency, 92% round‑trip efficiency, and a 95% derate, the deliverable fraction is 0.80 × (0.96×0.92) × 0.95 ≈ 0.671. A 10% margin then yields about 150×1.10/0.671 ≈ 246 kWh nominal. Aging allowances can be modeled by increasing margin or lowering derate.
Inverter sizing and surge coverage
Storage is useless without adequate conversion capacity. A conservative approach multiplies peak kW by a surge factor and a design margin, then adjusts for inverter efficiency. For 120 kW peak, 1.25 surge, 10% margin, and 96% efficiency, the recommended inverter is roughly 120×1.25×1.10/0.96 ≈ 172 kW. This supports motor starts and transient events.
Cost structure and decision metrics
Budgeting benefits from separating battery, inverter, and soft costs. At 250 per kWh and 140 per kW, 246 kWh of storage and 172 kW of inverter are about 61,500 and 24,100, before balance‑of‑system and contingency. Adding 20% BOS and 10% contingency produces total CAPEX near 113,000. Annualizing over 10 years at 10% gives an approximate 18,400 per year, useful for comparing tariffs or resilience value. Run sensitivity on autonomy and cost inputs to bound risk.
FAQs
What does “required nominal storage” mean?
Nominal storage is the nameplate battery energy needed so your usable autonomy target is met after depth‑of‑discharge limits, efficiency losses, derating, and design margin are applied.
How should I choose the critical load percentage?
Use a load list or interval data to classify essential circuits. Divide essential daily kWh by total daily kWh, then stress‑test with worst‑case days and operational constraints.
Why include both inverter efficiency and round‑trip efficiency?
Inverter efficiency captures conversion losses during discharge, while round‑trip efficiency reflects charging and internal battery losses. Multiplying them avoids underestimating nameplate capacity.
Is the Ah result accurate for all battery chemistries?
It is an approximation based on DC bus voltage and energy. Actual Ah varies with voltage window, chemistry, temperature, and BMS limits; use vendor curves for final engineering.
How do I interpret annualized cost?
Annualized cost converts upfront CAPEX into an equivalent yearly value using the discount rate and analysis period. It helps compare alternatives like tariffs, diesel fuel, or uptime benefits.
What inputs most affect the final size?
Autonomy hours, critical load share, DoD limit, derate, and design margin dominate. Costs are most sensitive to battery price per kWh and how much soft cost is applied.