Solar guide

Calculation Methodology

Equations, assumptions, and limits used by the Focus Solar planning calculators.

Updated July 20, 20263 min read Focus Solar Editorial Team Reviewed for technical accuracy through the Focus Solar editorial process

Focus Solar calculators provide first-pass planning estimates. They are designed to expose the variables that drive an answer, not to produce a permit-ready design.

System size

The system-size estimate divides daily electricity use by peak sun hours and the remaining system efficiency:

array kW = daily kWh ÷ (peak sun hours × (1 − loss fraction))

Panel count rounds the resulting array wattage up to a whole number of modules. Roof area is a broad allowance of roughly 2 m² per module and must be checked against actual module dimensions, access setbacks, shading, and roof geometry.

Battery capacity

nominal battery kWh = backup energy ÷ (depth of discharge × system efficiency)

Temperature, battery power limits, ageing, surge loads, reserve state of charge, and manufacturer requirements may justify additional capacity.

Inverter, panel output, ROI, and cable estimates

Inverter sizing compares the expected peak load with array capacity divided by a target DC-to-AC ratio. Panel output uses nameplate power, peak sun hours, panel count, and a loss allowance. The ROI tool shows simple payback and an illustrative 25-year cash-flow path; it is not a tax or investment forecast. The cable tool uses a copper voltage-drop equation and does not replace ampacity, temperature, conduit, protection, or local-code checks.

Always verify local solar-resource data, tariffs, incentives, equipment data, structural constraints, electrical rules, and permitting requirements.