Adding an EV charger, hot tub, or workshop without checking the panel can trip breakers — or worse. Electrical load is the total power (watts) all appliances draw, converted to amps to compare against circuit and panel capacity. For a home with 12,400W connected, the demand load is 7,440W (31A at 240V) on a 100A panel (24,000W) — 31% loaded, with headroom for the EV. A electrical load calculator sums appliance watts, applies NEC demand and continuous-load factors, and shows panel and circuit percentages before the work starts.
This in-depth guide explains how to calculate electrical load for a home, panel, or single circuit — watts, volts, amps, connected vs demand, the 80% continuous-load rule, and how to check whether there is headroom for the next big load.
Electrical Load Basics — Watts, Volts, Amps and the NEC 80% Rule
The relationship is Power (Watts) = Volts × Amps. Rearranged: Amps = Watts ÷ Volts and Volts = Watts ÷ Amps. For resistive loads, watts ≈ volt-amps (VA); for motors and AC with power factor ~0.8, use Watts = VA × 0.8.
Common Voltages in U.S. Homes
- 120V — General outlets, lights: 15A circuit ×120V = 1,800W max (1,440W continuous at 80%); 20A ×120V = 2,400W (1,920W continuous)
- 240V — Large appliances: Dryer 5,000W, range 8,000-12,000W, AC 3,500W, EV charger 7,200W (30A), water heater 4,500W
- Panel — Total capacity: 100A ×240V = 24,000W; 200A ×240V = 48,000W
NEC 80% rule (continuous load): Per NEC Article 210.19 and NEC 220, any load running 3+ hours (EV charger, heater, water heater, continuous lights) is calculated at 125% for sizing — or limited to 80% of the circuit/panel rating. A 20A×120V circuit is 2,400W, but the continuous max is 1,920W. A 100A panel is 24,000W, but the continuous max is 19,200W. The calculator applies the 1.25× factor automatically for continuous loads.
Connected vs Demand Load
- Connected: Sum if everything were on at once — e.g., 12,400W (worst case)
- Demand: With diversity — not all loads run simultaneously — per NEC 220 demand factors. General loads ×0.4-0.6, HVAC largest only (heat or cool, not both). For the example, 12,400W ×0.6 = 7,440W demand — the realistic sizing number. Connected is the ceiling; demand is the design load.
How to Calculate Electrical Load — 3 Steps
- List appliances and watts: Enter each load's watts and volts — AC 240V 3,500W, dryer 240V 5,000W, range 240V 8,000W, lights 120V 1,200W, outlets 120V 1,800W, EV charger 240V 7,200W. Use nameplate watts or the calculator's appliance list (pre-filled per NEC Table 220.14). Sum = connected load (12,400W in the example).
- Apply demand and continuous factors: Continuous loads (3+ hours) ×1.25 — e.g., EV 7,200W ×1.25 = 9,000W for sizing. Then apply diversity: general loads ×0.6 per NEC 220.42, HVAC largest only. Result: 7,440W demand (vs 12,400W connected). Demand is less than connected — and is the number compared to panel capacity.
- Check panel and circuit: Convert demand to amps: 7,440W ÷240V = 31A. On a 100A panel (24,000W), that's 31% loaded — 69% headroom. On a 200A panel, 15%. For a single circuit, check 80%: microwave 1,200W ÷120V=10A on a 20A (2,400W) circuit is 50%, or 12.5A (62%) as continuous — still safe. Add the EV (7,200W → 9,000W continuous) → new demand ~9,120W total? Actually demand recalculated with EV as continuous: new demand ~8,900W → 37A → 37% on 100A — still safe, but closer to the threshold.
What the calculator shows: Connected vs demand watts and amps, panel % loaded with headroom, and per-circuit % — all per NEC Article 220, with 120V/240V handling, continuous ×1.25, and demand factors by load type. No manual table lookup.
Panel and Circuit Capacity — Is There Headroom?
Panel Capacity (Whole Home)
- 100A ×240V = 24,000W capacity; continuous max 19,200W (80%)
- 200A ×240V = 48,000W; continuous max 38,400W
- Example demand 7,440W is 31% of 100A (69% headroom) or 15% of 200A. Adding an EV at 7,200W (9,000W continuous) pushes 100A to ~55% — still safe, but plan a 200A upgrade if multiple large loads are coming (hot tub + workshop).
Circuit Capacity (Per Circuit)
- 15A ×120V = 1,800W (1,440W continuous)
- 20A ×120V = 2,400W (1,920W continuous)
- 30A ×240V = 7,200W (5,760W continuous)
- Example: microwave 1,200W on 15A is 67% (83% as continuous) — borderline continuous; move to 20A or reduce load on that circuit.
When to Upgrade vs When Not To
| Demand vs Panel | Verdict | Action |
|---|---|---|
| < 80% (with continuous ×1.25) | Safe | Add small load (EV 30A) — still under 80% |
| 80-100% | Consider upgrade | Plan 200A or load shed device for EV/hot tub |
| > 100% or tripping | Must upgrade or reduce | Consult licensed electrician — do not add load |
Panel label vs usable continuous: A 100A panel is not 100A continuous — the continuous max is 80A (19,200W) per NEC 210.19. The calculator enforces the 80% limit for continuous loads automatically.
Safety and Code Notes — NEC Article 220
Continuous Load = 3+ Hours
EV chargers, heaters, water heaters, and lights left on are continuous — size at 125%. A 20A circuit's continuous max is 16A, a 100A panel's is 80A. This single rule prevents the most common overload.
Diversity (Demand Factor)
Not all loads run at once. NEC 220.42 applies 40-60% to general loads and counts only the largest HVAC (heat or cool, not both). The calculator applies the factor per load type — demand is always less than connected, and is the realistic design load.
When to Call a Licensed Electrician
- Adding an EV charger, hot tub, or workshop
- Panel tripping, warm to the touch, or demand >80% of panel
- Aluminum wiring, Federal Pacific/Zinsco panel, or no main breaker
- Any panel work — the calculator is an estimate per NEC 220 for planning, not a substitute for a licensed electrician, load study, or local code (AHJ). Always verify before panel work.
How to Use the Result — Next Steps
- Headroom >20%: Safe to add a small load (EV at 30A) — still under 80% with continuous factor
- 80-100%: Plan load management (smart EV charger that sheds when range is on) or a 200A upgrade
- >100%: Do not add load — upgrade or reduce existing loads before adding
FAQs About Calculating Electrical Load
How do I calculate the electrical load for my house?
List each appliance's watts and volts, sum to connected load, apply NEC demand factors (general ×0.6, continuous ×1.25) for demand load, then divide watts by volts for amps and compare to panel capacity (100A×240V=24,000W). A calculator does this per Article 220.
What is the difference between connected and demand load?
Connected is the sum if everything were on at once; demand applies diversity (not all at once) per NEC — demand is the realistic sizing load and is always lower than connected.
How many watts can a 100A panel handle?
100A×240V=24,000W total, with 19,200W continuous max (80% per NEC). A typical home demand of 7,440W is well within this. A 200A panel is 48,000W (38,400W continuous).
Can I add an EV charger to a 100A panel?
Often yes — a 30A EV charger is 7,200W (9,000W continuous). On a 100A panel with 31% base demand, the new demand is ~50-60% — still under 80% if other loads are managed. Check the calculator and consult an electrician for the specific panel and local code.
What is the 80% rule for circuits?
Continuous loads (3+ hours) must not exceed 80% of the circuit or panel rating per NEC 210.19. A 20A×120V circuit (2,400W) is limited to 1,920W continuous; calculate continuous loads at 125% for sizing.
Is the calculator a substitute for an electrician?
No — it is an estimate per NEC Article 220 for planning and headroom checks. Panel upgrades, new circuits, and service changes require a licensed electrician and approval from the local authority having jurisdiction (AHJ).
Conclusion
Calculating electrical load is watts to amps to panel percent — connected vs demand, with the 80% continuous-load rule — before the next big load is added. Listing appliances, applying NEC factors, and checking the percentage turns "will the panel handle it?" from a guess into a number with headroom to spare.
Enter the appliances from the list to get connected and demand watts and amps, panel and circuit percentages, and the headroom for the next addition — then verify with a licensed electrician before any panel work.