Four steps, no spreadsheet in between.
Most solar design tools start from irradiance data and a roof — they model how much a system will produce, then assume a flat retail rate to estimate savings. That assumption is where payback numbers usually go wrong: real tariffs have time-of-use bands, demand charges, and daily supply charges that a single average rate cannot represent.
Amperage starts from the bill instead. Because the tariff structure, usage and demand charges come straight from the customer's own account, the payback figure reflects what they are actually paying today — not a state-average cents-per-kWh number. The roof and production model still matter, but the savings side of the equation is derived from the customer's real electricity costs.
That matters most for C&I sites, where demand charges and unbundled network components can be a third or more of the bill and an irradiance-first tool has no way to see them at all.