Critical Load Backup Sizing Calculator
Critical load backup sizing is about protecting only the loads that actually matter during an outage -- security systems, refrigeration, servers, life-safety equipment -- rather than oversizing a backup system to carry the entire facility. The distinction matters because critical loads are typically a fraction of total facility demand, and sizing backup for just that fraction cuts backup system cost significantly. This calculator estimates the effective critical load, the required usable energy, the required battery capacity, the required inverter/power rating, and (optionally) the backup generator size needed to ride through an outage, from the total critical load, a diversity/simultaneity factor, the required backup duration, the battery round-trip efficiency, and the allowed depth of discharge. It pairs naturally with our Microgrid Sizing Calculator for sizing the overall microgrid, our Islanding Duration Calculator for checking how long an existing battery can sustain the load, and our Public Safety Power Shutoff (PSPS) Outage Cost Calculator for quantifying the outage exposure this backup system would avoid.
The combined nameplate power rating of all loads classified as critical -- life-safety equipment, refrigeration/freezers, security and access control, network/server infrastructure, and any process equipment that would cause damage or spoilage if it lost power.
Accounts for the fact that not all critical equipment runs simultaneously at full nameplate power. A diversity factor of 0.7-0.85 is typical for mixed critical loads; 1.0 assumes everything runs at full power at once.
How long the backup system must sustain the critical load. 4-8 hours covers most common outages; facilities in wildfire-prone or storm-prone areas with Public Safety Power Shutoffs often plan for 24-72 hours instead.
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.
The percentage of total battery capacity that can safely be discharged without accelerating degradation. Most modern lithium-ion systems support 80-100% usable depth of discharge; more conservative settings extend cycle life.
total critical load (kW) × diversity / simultaneity factor
effective critical load (kW) × required backup duration (hours)
required usable energy (kWh) ÷ ((allowed depth of discharge (%) ÷ 100) × (round-trip efficiency (%) ÷ 100))
effective critical load (kW) × 1.25 (starting / surge margin)
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 single biggest cost lever in backup sizing is deciding what actually counts as a critical load. Sizing backup to carry an entire facility's peak demand almost always produces an oversized, overpriced system; sizing for just the life-safety, refrigeration, security, and server loads that genuinely cannot lose power -- derated by a diversity factor that reflects that not all of them run at full nameplate power at once -- typically yields a fraction of the capacity at a fraction of the cost. At the defaults, a 50 kW critical load with a 0.8 diversity factor needs only 40 kW of effective backup, 320 kWh of usable energy, and roughly 500 kWh of installed battery capacity after depth-of-discharge and round-trip-efficiency derating -- a very different number than sizing for the whole facility.
How this is calculated
This calculator sizes a backup system to cover only a facility's critical loads during an outage, from the total critical load, a diversity/simultaneity factor, the required backup duration, the battery round-trip efficiency, and the allowed depth of discharge. Five quantities tie the calculation together.
Effective Critical Load (kW) = Total Critical Load (kW) × Diversity / Simultaneity Factor. Nameplate ratings assume every device runs at full rated power simultaneously, which almost never happens in practice -- a diversity factor of 0.7-0.85 is typical for mixed critical loads and prevents oversizing (and overpaying for) the backup system. Multiplying the total critical load by the diversity factor gives the realistic simultaneous load the backup must support. At the defaults (50 kW and 0.8), that is 50 × 0.8 = 40 kW.
Required Usable Energy (kWh) = Effective Critical Load (kW) × Required Backup Duration (hours). The backup duration is how long the system must sustain the critical load -- 4-8 hours covers most common outages, while facilities in wildfire-prone or storm-prone areas with Public Safety Power Shutoffs often plan for 24-72 hours. Multiplying the effective load by the duration gives the energy the backup must deliver. At the defaults (40 kW and 8 hours), that is 40 × 8 = 320 kWh.
Required Battery Capacity (kWh) = Required Usable Energy (kWh) ÷ ((Allowed Depth of Discharge (%) ÷ 100) × (Battery Round-Trip Efficiency (%) ÷ 100)). Usable battery capacity is always less than nameplate for two reasons: depth-of-discharge limits mean not all nameplate capacity can safely be discharged without accelerating degradation, and round-trip efficiency losses mean some energy put into the battery is lost as heat on the charge/discharge cycle. Dividing the required usable energy by both derating factors expressed as fractions gives the installed (nameplate) capacity needed. At the defaults (320 kWh, 80% DoD, 90% RTE), that is 320 ÷ (0.80 × 0.90) = 320 ÷ 0.72 = 444.4 kWh.
Required Inverter / Power Rating (kW) = Effective Critical Load (kW) × 1.25. The inverter or power conversion system must handle not just the steady-state load but the momentary starting/surge (inrush) currents that motors, compressors, and power supplies draw when they switch on -- a 1.25x margin is the standard engineering convention for covering motor starting and inrush without tripping the inverter. At the defaults (40 kW), that is 40 × 1.25 = 50 kW.
Generator Size (kW, optional) = Effective Critical Load (kW) × 1.25. When the backup generator toggle is on, the same 1.25x starting/surge margin convention is applied to size a backup generator instead of (or alongside) a battery system. At the defaults (40 kW), that is 40 × 1.25 = 50 kW.
Two notes on the model. First, the diversity factor is a single representative figure, appropriate for a planning-level backup sizing estimate -- but actual simultaneity varies by load mix (a server room runs near nameplate continuously, while a mix of refrigeration, security, and life-safety loads sees much lower simultaneity), so the editable field lets you substitute a site-specific figure. Second, this calculator reports backup capacity and power rating only and does not model on-site solar generation that can offset load and recharge batteries during daylight hours (extending effective backup duration), load shedding strategies that dynamically reduce the supported load as an outage progresses, 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 battery degradation over the system's life (usable capacity declines unless oversized upfront or augmented over time), generator fuel supply and runtime limits, or the probability distribution of outage duration for a given site -- all of which a full resilience planning evaluation would include. Data sources: diversity factor of 0.7-0.85 for mixed critical loads from microgrid design guidance and NEC load calculation practice; depth-of-discharge limits of 80-100% for modern lithium-ion systems from battery manufacturer specifications and NREL energy storage reporting; round-trip efficiency of 85-95% for modern lithium-ion systems from battery manufacturer specifications and NREL energy storage reporting; 1.25x starting/surge margin convention from standard backup power and generator sizing engineering references. Verification: with defaults (50 kW load, 0.8 diversity, 8 hours, 90% RTE, 80% DoD), Effective Critical Load = 40 kW, Required Usable Energy = 320 kWh, Required Battery Capacity = 444.4 kWh, Required Inverter / Power Rating = 50 kW, Generator Size = 50 kW.