Why I stopped using flat $/kWh to size commercial battery storage.

I've been building energy APIs for about four years. The thing that kept bothering me wasn't the code — it was watching tools I respected give completely wrong BESS cost estimates because they multiplied system size by a flat dollars-per-kilowatt-hour figure. That's not how battery storage costs work. And it matters a lot when someone is deciding whether to spend $800,000 on a system. The problem with flat $/kWh Most BESS calculators I've seen do something like this: # What most tools do — this is wrong system_cost = capacity_kwh * cost_per_kwh # e.g. 500 kWh * $400 = $200,000 The issue is that battery storage has two fundamentally different cost components that scale differently: Energy cost — scales with kWh (how much you can store) Power cost — scales with kW (how fast you can charge/discharge) A 500 kWh system with a 250 kW inverter has a completely different cost structure than a 500 kWh system with a 500 kW inverter. The flat $/kWh model treats them identically. The NREL ATB two-