Household Electrical Load and Capacity Estimate

Method and formulas

This planner turns a household load list into four explicit steps. Each appliance’s running and starting watts are multiplied by quantity. The simultaneity factor reduces only the running total; starting peaks are added without diversity. Planning capacity is the larger of diversified running demand and the starting total, increased by the planning margin. It is not installation approval. Service current is I = P / V at the voltage you enter. For a 120/240 V dwelling service, enter 240 V to read the equivalent current on the 240 V basis.

P_run = Σ (running watts × quantity). P_start = Σ (starting watts × quantity). P_demand = P_run × simultaneity. P_capacity = max(P_demand, P_start) × (1 + margin). I = P_capacity / V. This is not an NEC Article 220 or local-code dwelling calculation, and it does not apply a code demand-factor table.

Worked example

One 800 W running load with an 1,800 W start, quantity 1, simultaneity 1, margin 0.20 and a 240 V service. The running total is 800 W, the starting total is 1,800 W and diversified demand is 800 W. Capacity = 1,800 × 1.20 = 2,160.00 W. Current = 2,160.00 / 240 = 9.00 A.

Assumptions and limitations

• Starting peaks are treated as coincident, which is conservative.
• Simultaneity must be greater than 0 and at most 1. Margin is from 0 to 2.
• Conductor ampacity, voltage drop, breaker coordination, motor-start voltage dip and service-upgrade feasibility are not evaluated.

Related tools

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• Maximum demand
• Appliance electricity cost