Hospital/Campus Microgrid Sizing Calculator
Hospital and large-campus emergency power sizing follows a branch-based approach rather than a single blanket backup number. Life safety loads must run without interruption, critical/equipment loads support patient care, and standby loads are the first to be shed if backup capacity is limited. This calculator sizes backup generation for a hospital or large campus using that NFPA 110-aligned branch concept, from the life safety branch load, the critical/equipment branch load, the optional standby load, a toggle for whether to include standby in sizing, the required runtime, and the backup system type. It pairs naturally with our Critical Load Backup Sizing Calculator for a more general critical-load view, our Microgrid Sizing Calculator for overall microgrid storage sizing, our Islanding Duration Calculator for checking how long an existing battery can sustain the load, and our Resilience Value of Lost Load Calculator for quantifying the avoided-outage value this backup system delivers.
Must-run loads per NFPA 99/110 -- egress lighting, fire alarm systems, medical gas alarm systems.
ICU, operating rooms, emergency department, elevators designated for firefighter/patient use, medical equipment.
General hospital operations -- admin areas, non-critical HVAC. Often excluded from emergency power sizing.
96 hours (4 days) is a common resilience benchmark for hospitals; check local and NFPA 110 fuel requirements for your facility class.
Generator only sizes a diesel genset for sustained runtime. Hybrid adds a battery bridge sized for a 10-minute transfer window before the generator starts.
life safety branch load (kW) + critical/equipment branch load (kW) + (optional standby load (kW) if included)
total must-run load (kW) × 1.25 (starting / inrush margin)
generator size (kW) × runtime (hours) × 0.08 (gal/kWh, typical diesel genset specific fuel consumption)
This tool gives a planning-level estimate based on NFPA 110 branch concepts and typical diesel genset fuel consumption. It is not a substitute for a licensed engineer's emergency power system design, which must account for your facility's actual load profile, code requirements, and accreditation standards.
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →The branch-based approach is what separates a code-compliant hospital emergency power design from a single blanket backup number. Life safety loads must never lose power, critical/equipment loads support patient care directly, and standby loads are the first to be shed when generator capacity is constrained -- so excluding standby from sizing (the default here) is the conservative, code-minimum approach, while including it produces a larger, comfort-and-operations-oriented system. At the defaults (150 kW life safety, 300 kW critical, 200 kW standby excluded, 96 hours), a 562.5 kW generator needs roughly 4,320 gallons of stored diesel to ride out a four-day outage -- a striking number that explains why hospital fuel logistics are a genuine resilience planning concern, not an afterthought.
How this is calculated
This calculator sizes backup generation for a hospital or large campus using the NFPA 110 emergency power system branch concept, from the life safety branch load, the critical/equipment branch load, the optional standby load, a toggle for whether to include standby in sizing, the required runtime, and the backup system type. Four quantities tie the calculation together.
Total Must-Run Load (kW) = Life Safety Branch Load (kW) + Critical / Equipment Branch Load (kW) + (Optional Standby Load (kW) if included). NFPA 110 organizes hospital essential electrical systems into branches: the life safety branch (egress lighting, fire alarms, medical gas alarms) that must never lose power, the critical branch (ICU, operating rooms, emergency department, designated elevators, medical equipment) that supports patient care directly, and the optional standby branch (general operations, admin areas, non-critical HVAC) that is the first to be shed if backup capacity is limited. Summing the life safety and critical/equipment loads -- and adding standby only when the toggle is on -- gives the total load the backup system must support. At the defaults (150 kW life safety, 300 kW critical, 200 kW standby excluded), that is 150 + 300 = 450 kW.
Generator Sizing (kW) = Total Must-Run Load (kW) × 1.25. The generator must handle not just the steady-state load but the momentary starting/inrush (inrush) currents that motors, compressors, and medical equipment power supplies draw when they switch on -- a 1.25x margin is the standard engineering convention for covering motor starting and inrush without tripping or stalling the generator. At the defaults (450 kW), that is 450 × 1.25 = 562.5 kW.
Estimated Fuel Storage Required (gallons) = Generator Size (kW) × Runtime (hours) × 0.08 (gal/kWh). Diesel genset specific fuel consumption typically runs about 0.08 gallons per kWh delivered (roughly 0.4 lb of fuel per kWh, varying somewhat by load level and engine size). Multiplying the generator size by the required runtime and by 0.08 gives the total fuel that must be stored on-site to sustain the backup for the full duration. At the defaults (562.5 kW and 96 hours), that is 562.5 × 96 × 0.08 = 4,320 gallons. Fuel storage duration requirements are often set by local code or CMS / Joint Commission accreditation requirements rather than choice -- many hospitals plan for 96 hours (4 days) of fuel autonomy as a resilience benchmark following extended grid outages during major storms, though exact requirements vary by state, accreditation body, and facility type.
Battery Bridge Capacity (kWh, hybrid only) = Total Must-Run Load (kW) × (10/60). When a hybrid generator + battery system is selected, a battery bridge is sized to cover the transfer window -- the brief gap between a grid failure and the generator starting, coming up to speed, and accepting load. A 10-minute bridge is a common starting estimate for this transfer window; dividing 10 minutes by 60 gives the fraction of an hour, and multiplying by the total must-run load gives the energy the battery must deliver to ride through that window. At the defaults (450 kW), that is 450 × (10/60) = 75 kWh. This is clearly labeled as a starting estimate, not a full UPS design -- a real hospital UPS design accounts for transfer-switch timing, inverter sizing, redundancy, and the specific ride-through requirements of life safety and critical branches.
Two notes on the model. First, the 1.25x starting/inrush margin and the 0.08 gal/kWh specific fuel consumption are single representative figures, appropriate for a planning-level estimate -- but actual inrush varies enormously by load mix (medical imaging equipment, air handlers, and chillers have very different starting profiles), and actual fuel consumption varies by engine size, load level, and fuel type, so the editable fields let you substitute site-specific figures. Second, this calculator reports backup capacity, generator size, fuel storage, and (for hybrid) a battery bridge estimate only and does not model the full essential electrical system architecture that NFPA 99 and NFPA 110 require (automatic transfer switch placement, branch separation, grounding, redundancy, and load-shedding controls), the cost of the backup system (see the Microgrid Sizing Calculator and Microgrid vs. Grid-Tied Cost Comparison Calculator for cost context), the effect of on-site solar generation that can offset load and reduce generator run-hours, the probability distribution of outage duration rather than a single runtime figure, or the specific fuel-delivery logistics and contracts that determine whether stored fuel is actually sufficient during a regional emergency -- all of which a licensed engineer's emergency power system design would include. Data sources: NFPA 110 emergency power system branch classification (life safety, critical, optional standby) and NFPA 99 healthcare facility electrical systems standards; 1.25x starting/inrush margin convention from standard backup power and generator sizing engineering references; typical diesel genset specific fuel consumption of roughly 0.08 gal/kWh from generator manufacturer specifications and engineering references; 96-hour (4-day) hospital fuel autonomy benchmark from hospital resilience planning guidance following major-storm extended outages. Verification: with defaults (150 kW life safety, 300 kW critical, 200 kW standby excluded, 96 hours, generator only), Total Must-Run Load = 450 kW, Generator Sizing = 562.5 kW, Estimated Fuel Storage Required = 4,320 gallons; with hybrid selected, Battery Bridge Capacity = 75 kWh.