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Microgrid Storage Duration Estimator

This calculator estimates how long a microgrid battery can support a constant critical load. It reduces nominal storage capacity for usable depth of discharge, efficiency, and a required reserve before dividing available energy by the supported load. Use it for preliminary outage-duration and resilience comparisons. It does not simulate state of charge before an outage, renewable generation during the event, battery power limits, degradation, temperature, or load variation, all of which can change real support time.

Battery and critical-load assumptions

kWh
%
%
%
kW
Result
—
Estimated battery support duration
Energy after depth-of-discharge limit—
Deliverable energy after efficiency—
Energy held in reserve—

1. Enter nominal battery energy
Use the nameplate energy capacity in kilowatt-hours.

2. Set usable depth of discharge
Limit the portion of nameplate energy allowed for routine or emergency discharge.

3. Apply discharge efficiency
Use an efficiency appropriate to the battery and inverter boundary.

4. Hold a reserve
Enter the percentage of deliverable energy that should remain unused.

5. Enter critical load
Use the expected average load during the support period and include relevant auxiliaries.

6. Review the duration
Test degraded capacity, higher load, and lower initial state-of-charge cases for a more conservative range.

Energy after DoD = Nominal capacity × Usable depth of discharge
Deliverable energy = Energy after DoD × Discharge efficiency
Available energy = Deliverable energy × (1 − Reserve fraction)
Duration = Available energy ÷ Critical load

The model assumes the battery starts fully charged relative to the entered usable window and the load remains constant.

What the result means

The result is the number of hours the battery can supply the entered load before reaching the modeled reserve threshold.

Verify that battery inverter power is at least as large as the critical load; sufficient energy capacity alone does not guarantee load support.

Given: A 1,000 kWh battery, 90% usable depth of discharge, 92% discharge efficiency, 10% reserve, and a 150 kW critical load.

Calculation:
After DoD = 1,000 × 0.90 = 900 kWh
Deliverable = 900 × 0.92 = 828 kWh
Available after reserve = 828 × 0.90 = 745.2 kWh
Duration = 745.2 ÷ 150 = 4.97 hours

Result: Estimated support duration is about 4.97 hours.

Should round-trip efficiency be used here?

For a discharge-only runtime estimate, use discharge-path efficiency when available. Round-trip efficiency also includes charging losses and may be more conservative if no separate value is known.

How does battery degradation affect the result?

Use the expected available capacity at the future operating date rather than beginning-of-life nameplate capacity.

Does solar generation during an outage extend duration?

It can, but this calculator does not model it. A chronological simulation is needed to account for weather, load, charging limits, and curtailment.

What if the critical load changes over time?

Calculate energy use by time interval and compare cumulative consumption with available battery energy instead of using one constant load.

Why keep an energy reserve?

A reserve can protect battery life, cover forecast error, maintain black-start capability, or provide margin for longer-than-expected outages.