Calculations

Microgrid and Distributed Energy Equipment Calculations, Step by Step

The five calculations that turn battery and inverter datasheets into orderable steel and scheduled crew hours, worked with real units and numbers.

Five calculations underpin most microgrid equipment work: battery-to-inverter sizing, thermal derating, cabinet capacity, switchgear build hours, and field labor. Each takes nameplate data from a datasheet and turns it into a number you can order steel and schedule crews against. Get the ratio wrong by 10 percent and you either clip energy or strand capital. This guide runs the arithmetic with the same inputs an integrator pulls from a one-line diagram and a battery spec sheet. The Battery-Inverter Matching and Thermal Derating calculators check each step. Every formula below uses kW, kWh, amps, and degrees C, with no unit conversion hidden in the middle.

Battery-inverter matching starts with usable energy: nameplate kWh times depth of discharge. A 3,440 kWh nameplate pack at 0.90 DoD gives 3,096 kWh usable. For a 2-hour system, required power is 3,096 divided by 2, or 1,548 kW. Pick an inverter block of three 500 kW units for 1,500 kW AC, so the DC to AC ratio is 1,548 over 1,500, or 1.03. Apply round-trip loss of 0.92 and deliverable AC energy is 3,096 times 0.92, or 2,848 kWh. The Battery-Inverter Matching calculator flags ratios above 1.35, where continuous clipping wastes installed cells.

Thermal derating cuts inverter output above the knee temperature, usually 45 to 50 degrees C. The linear form is P derated equals P rated times one minus k times ambient minus knee, where k runs 0.015 to 0.02 per degree C. For a 1,500 kW unit at knee 45, ambient 55, and k of 0.017: one minus 0.017 times 10 equals 0.83, so 1,245 kW. Add an elevation derate near 1 percent per 300 m above 1,000 m. At 2,200 m that is about 4 percent more, landing near 1,195 kW. The Thermal Derating calculator combines temperature and altitude in one pass.

Inverter cabinet capacity comes from continuous bus current, not just footprint. Current equals power divided by the product of root three, line-to-line voltage, and power factor. For 1,500 kW at 480 V and unity power factor: 1,500,000 divided by 1.732 times 480 equals 1,805 A. Bus bars are rated at 80 percent continuous, so you need 1,805 over 0.80, or 2,256 A, which rounds up to a 2,500 A bus. The Inverter Cabinet Capacity calculator sizes cabinet count from module footprint and bus ampacity; a 2,500 A cabinet holds three 600 A inverter feeders plus a 400 A spare.

Switchgear assembly hours build from a per-section base plus terminations. Take an 8-section lineup at 34 base hours each, or 272 hours. Add 640 control terminations at 6 minutes each, another 64 hours. Add CT and PT wiring at 12 hours. The total is about 348 hours. Split across a crew of three, that is 116 hours per person, roughly 15 working days. The Switchgear Assembly Hours calculator takes section count, breaker count, and termination count directly. A typical 15 kV metal-clad section runs 30 to 40 build hours; padding an estimate to 45 usually signals expected rework rather than real scope.

Field install labor sums equipment set, cable, and termination time. Set 8 sections at 6 rigging hours each for 48 hours. Pull 2,400 ft of 500 kcmil at 0.08 crew-hours per foot for 192 hours. Terminate 96 lugs at 0.75 hours each for 72 hours. Add 40 hours of startup. The total is 352 hours; with a 4-person crew at 8 hours per day that is 11 days. The Field Install Labor calculator rolls these into a crew-day estimate, and the Controls Commissioning Load calculator handles I/O point checkout separately so wiring and software time never get double counted.

Every input traces to a document. Depth of discharge and round-trip efficiency come off the battery datasheet and warranty table. The derate coefficient k and knee temperature sit in the inverter thermal curve, usually page two of the spec. Bus ampacity and section counts come from the one-line and the switchgear submittal. Termination counts come from the cable schedule. Field productivity rates come from your own timecards, not a catalog. When you feed real clocked hours back into Switchgear Assembly Hours and Field Install Labor each quarter, the estimates converge to within 5 to 8 percent of actuals instead of drifting 20 percent job over job.

Published 2026-07-02.