Calculations

How to Calculate the Core Metrics for EV Charging Cabinet Manufacturing

Work through the five formulas that govern an EV charger line, with real units and numbers: module yield, assembly standard time, test takt, thermal derating, and firmware flashing throughput.

An EV charging cabinet line runs on five numbers you can compute by hand before you touch a spreadsheet: rolled throughput yield on power modules, standard assembly time per cabinet, final test takt, thermal derating at the rated ambient, and firmware flashing capacity. Each pulls inputs from a different place, the process traveler, the time study, the demand plan, the datasheet, and the station log. Get these right and the rest of the plan follows. This guide walks each formula with worked numbers for a 180 kW DC fast charger built from six 30 kW SiC modules. Pricing and target ranges live in the sibling cost and benchmarks guides, so here we only do the math.

Start with yield, because it sizes every upstream order. First pass yield at a step is good units out divided by units in. Rolled throughput yield is the product across all steps: RTY = Y1 x Y2 x ... x Yn. A SiC power module with eight process steps each at 98.5% gives RTY = 0.985^8 = 0.886, or 88.6%. To ship 500 good modules you must start units = target / RTY = 500 / 0.886 = 565 starts. The Power Module Yield calculator does this across your real step list. Note the trap: averaging step yields understates loss because yield multiplies, it does not average.

Standard assembly time sets your labor headcount. Standard time = sum of measured task times x (1 + allowance), where the allowance covers personal, fatigue, and delay, typically 12 to 15% on manual electromechanical work. If your time study of a cabinet totals 210 observed minutes and you apply a 12% allowance, standard time = 210 x 1.12 = 235 minutes per cabinet. The Charger Cabinet Assembly Time calculator separates module insertion, busbar torque, and wiring, while the Cable Harness Labor calculator handles harness build as its own standard so you can balance the line and avoid double counting the same wiring minutes.

Final test takt tells you the beat the line must hold. Takt = net available time / customer demand. Two shifts of 480 minutes with 85% net availability give 480 x 2 x 0.85 = 816 net minutes. Against a demand of 40 cabinets per day, takt = 816 / 40 = 20.4 minutes per cabinet. If your measured final test cycle is 26 minutes, one station cannot hold takt and you need stations = cycle / takt = 26 / 20.4 = 1.27, so two parallel test bays. The Final Test Takt calculator runs this against your real uptime and mix.

Thermal derating protects the rating you print on the nameplate. A common model is derated power = rated x (1 minus d x (Tamb minus Tref)) for ambient above the reference. With a 350 kW cabinet derating at 2% per degree C above 40 C, at 50 C ambient the deratable term is 0.02 x (50 minus 40) = 0.20, so output = 350 x (1 minus 0.20) = 280 kW. If your spec demands 300 kW at 50 C, you must either add cooling or specify modules with a lower derating slope. The Thermal Derating calculator sweeps ambient so you see the curve, not one point.

Firmware flashing capacity gates throughput more often than people expect. Units per hour = (3600 / cycle seconds) x stations x uptime. A flash and verify cycle of 240 seconds on three stations at 90% uptime yields (3600 / 240) x 3 x 0.90 = 40.5 units per hour, or 324 over an 8 hour shift. Compare that against your cabinet demand of 40 per day and flashing is not the bottleneck, but drop to one station and capacity falls to 13.5 per hour. The Firmware Flashing Capacity calculator models station count and retry rate so you size the bench before it starves final test.

Two reserves round out the math. Burn-in load is total kWh the test racks must sink: load kWh = units x rated kW x burn hours x load factor. Twelve 180 kW cabinets at a 0.6 load factor for a 4 hour burn draw 12 x 180 x 4 x 0.6 = 5,184 kWh, which the Burn-In Test Load calculator converts into rack and regen sizing. For field spares, contactor reserve = fleet x annual failure rate x replacement lead months / 12. A 2,000 unit fleet at 1.5% annual contactor failure with a 3 month lead needs 2,000 x 0.015 x 3 / 12 = 7.5, round to 8, per the Contactor Failure Reserve calculator.

Published 2026-07-02.