Islanding Duration Calculator
When the grid goes down, the question that matters is simple: how long can your battery keep the lights on? Islanding duration is the number of hours a battery system can sustain a facility's critical load while disconnected from the utility grid, and it comes down to two things — how much usable energy the battery actually holds, and how much load you're asking it to support. This calculator estimates islanding duration from the available battery storage capacity, the facility load during islanding, and the battery usable depth of discharge. It pairs naturally with our Microgrid Sizing Calculator for sizing the overall microgrid, and our Critical Load Backup Sizing Calculator for determining which loads are truly critical and how much backup capacity they require.
The total nameplate battery storage capacity available at the facility.
The load the facility will draw while islanded -- typically just critical/priority circuits, not full facility peak load.
The percentage of total battery capacity that can safely be discharged -- most modern lithium-ion systems support 90-100% usable depth of discharge, though some older or more conservative system designs limit usable capacity further to extend battery life.
available battery storage capacity (kWh) × (battery usable depth of discharge (%) ÷ 100)
usable energy (kWh) ÷ facility load during islanding (kW)
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Islanding duration is simply usable energy divided by load -- but the two levers that actually matter are how much load you're really trying to support and how deeply the battery can safely discharge. A 2,000 kWh system supporting a 200 kW critical load provides 9 hours of islanded operation at 90% usable depth of discharge -- but shed that load to 100 kW (by curtailing non-essential circuits) and the same battery now provides 18 hours, which is exactly why load prioritization and shedding strategy matter as much as raw battery capacity when planning for extended outages.
How islanding duration is calculated
This calculator estimates the usable energy a battery system can deliver and the resulting islanding duration it can sustain, from the available battery storage capacity, the facility load during islanding, and the battery usable depth of discharge. Two quantities tie the calculation together.
Usable Energy (kWh) = Available Battery Storage Capacity (kWh) × (Battery Usable Depth of Discharge (%) ÷ 100). The available battery storage capacity is the total nameplate energy capacity of the battery system; the usable depth of discharge is the fraction of that nameplate capacity that can safely be discharged without accelerating degradation or hitting built-in safety margins. Multiplying the nameplate capacity by the depth of discharge expressed as a fraction gives the energy actually available to support the load. At the defaults (2,000 kWh and 90%), that is 2,000 × 0.90 = 1,800 kWh.
Islanding Duration (hours) = Usable Energy (kWh) ÷ Facility Load During Islanding (kW). The facility load during islanding is the power the facility draws while disconnected from the grid -- typically just critical or priority circuits, not the full facility peak load. Dividing the usable energy by that load gives the number of hours the battery can sustain it. At the defaults (1,800 kWh and 200 kW), that is 1,800 ÷ 200 = 9 hours.
Two notes on the model. First, the usable depth of discharge is a single representative figure, appropriate for a planning-level estimate of islanding duration -- but actual usable capacity varies by battery chemistry (modern lithium-ion systems commonly support 90-100% usable depth of discharge, while older lead-acid designs typically support far less), system design and warranty limits (some manufacturers conservatively limit usable capacity to extend cycle life), and the battery's state of health (degraded batteries hold less usable energy than nameplate), so the editable field lets you substitute a system-specific figure. Second, this calculator reports battery-only islanding duration and does not model on-site solar generation that can offset load and recharge batteries during daylight hours (extending effective islanding duration well beyond battery capacity alone for daytime outages), load shedding strategies that dynamically reduce the islanding load as the outage progresses, the round-trip efficiency losses of converting stored DC energy to AC load (typically 85-95%), the effect of battery degradation over the system's life (usable capacity declines unless oversized upfront or augmented over time), or the probability distribution of outage duration for a given site -- all of which a full resilience planning evaluation would include. Data sources: modern lithium-ion usable depth of discharge of 90-100% from battery manufacturer specifications and NREL energy storage reporting; typical critical load as a fraction of facility peak load from microgrid design guidance and DOE resilience planning resources; islanding duration as usable energy divided by load from standard microgrid and backup power engineering references. Verification: with defaults (2,000 kWh capacity, 200 kW load, 90% usable depth of discharge), Usable Energy = 1,800 kWh, Islanding Duration = 9 hours.