Microgrid Sizing Calculator
Sizing a microgrid starts with a question most facility planners get wrong on the first pass: what load actually needs to ride through an outage, not what the facility's full peak demand is. This calculator estimates the total energy a microgrid must deliver during an islanding event and the required battery storage capacity to supply it, from the facility load to support, the desired islanding duration, and the battery round-trip efficiency. It pairs naturally with our Islanding Duration Calculator for working backward from an existing battery to the hours of ride-through it provides, and our Critical Load Backup Sizing Calculator for determining which loads are genuinely critical and how much backup capacity they require.
The combined critical and priority load the microgrid needs to sustain during an islanding event -- not necessarily the facility's full peak load.
How long the facility needs to operate independently from the grid. Multi-day Public Safety Power Shutoff (PSPS) events in wildfire-prone areas commonly last 24-72+ hours.
The percentage of energy put into the battery that can be recovered on discharge -- modern lithium-ion systems typically deliver 85-95% round-trip efficiency, with the remainder lost as heat during charging and discharging.
facility load to support (kW) × desired islanding duration (hours)
total energy required during islanding (kWh) ÷ (battery round-trip efficiency (%) ÷ 100)
This estimates storage capacity only. A complete microgrid also requires generation (solar, generator, or both) to recharge storage during extended outages, plus controls and switchgear to manage islanding and grid reconnection -- see the Microgrid vs. Grid-Tied Cost Comparison Calculator for full system cost context.
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Sizing a microgrid starts with a question most facility planners get wrong on the first pass: what load actually needs to ride through an outage, not what the facility's full peak demand is. Supporting a 500 kW critical load for 24 hours requires over 13,300 kWh of usable storage after accounting for round-trip efficiency losses -- a very different, much more achievable number than sizing for the facility's entire peak load. In 2026, microgrid capital costs have fallen to roughly $2,500-4,000/kW installed, per DOE/NREL benchmarking, down from a $2-5 million per MW range just a few years earlier, making right-sized microgrids increasingly economical for exactly this kind of targeted critical-load application.
How microgrid sizing is calculated
This calculator estimates the total energy a microgrid must deliver during an islanding event and the required battery storage capacity to supply it, from the facility load to support, the desired islanding duration, and the battery round-trip efficiency. Two quantities tie the calculation together.
Total Energy Required During Islanding (kWh) = Facility Load to Support (kW) × Desired Islanding Duration (hours). The facility load to support is the combined critical and priority load the microgrid must sustain -- not the facility's full peak load, which is typically far larger than what genuinely needs to ride through an outage. Multiplying that load by the desired islanding duration gives the total energy the microgrid must deliver. At the defaults (500 kW and 24 hours), that is 500 × 24 = 12,000 kWh.
Required Battery Storage Capacity (kWh) = Total Energy Required During Islanding (kWh) ÷ (Battery Round-Trip Efficiency (%) ÷ 100). Battery storage loses some energy in the charge/discharge cycle, so the installed (nameplate) capacity must be larger than the usable energy actually delivered. Dividing the total energy required by the round-trip efficiency expressed as a fraction gives the nameplate storage capacity needed. At the defaults (12,000 kWh and 90%), that is 12,000 ÷ 0.90 = 13,333.3 kWh.
Two notes on the model. First, the round-trip efficiency is a single representative figure, appropriate for a planning-level estimate of required storage capacity -- but actual round-trip efficiency varies by battery chemistry (modern lithium-ion systems typically deliver 85-95%, while older lead-acid designs are lower), system design, power level, and operating temperature, so the editable field lets you substitute a system-specific figure. Second, this calculator reports storage capacity only and does not model the generation side of the microgrid (solar array size, generator capacity, or expected daily recharge from on-site generation during extended outages), load shedding strategies that dynamically reduce the supported load as the outage progresses, the additional cost of controls, switchgear, and grid reconnection equipment, 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 microgrid design evaluation would include. Data sources: critical load as a fraction of facility peak load from microgrid design guidance and DOE resilience planning resources; round-trip efficiency of 85-95% for modern lithium-ion systems from battery manufacturer specifications and NREL energy storage reporting; 2026 microgrid capital costs of roughly $2,500-4,000/kW installed from DOE and NREL microgrid benchmarking. Verification: with defaults (500 kW load, 24 hours duration, 90% round-trip efficiency), Total Energy Required During Islanding = 12,000 kWh, Required Battery Storage Capacity = 13,333.3 kWh.