Start with what must run together
Imagine a small worksite that needs six LED lights at 12 W each, a 120 W laptop charger, a 700 W drill, and a 900 W water pump. The running total is 72 + 120 + 700 + 900 = 1,792 W. Your own list must use the wattage on each product label or manufacturer specification; these numbers are examples only.
Add the largest startup demand
A motor may draw more power briefly when it starts. Suppose the pump needs 2,200 W at startup, or 1,300 W above its 900 W running load. Add that extra 1,300 W once to the 1,792 W running total. The example peak becomes 3,092 W. If two motors can start together, use the startup combination that can actually occur.
Check continuous and surge ratings separately
A candidate generator needs a continuous rating above the running total and a surge rating above the calculated peak. Do not treat a 3,500 W peak label as a 3,500 W continuous supply. Compare ratings under the stated fuel, temperature, altitude, and power-factor conditions.
Leave room for real operation
Avoid planning continuous operation at the exact rated limit. Allow capacity for a changing load, warm weather, elevation, and future needs. The right margin depends on the generator manual and the duty of connected equipment; it is not one universal percentage.
Make the final check before buying
Confirm voltage, frequency, outlet type, circuit protection, extension-cord rating, runtime at your expected load, fuel, noise, and service access. Never run a combustion generator indoors, in a garage, or near openings. A qualified electrician must handle any connection to building wiring.
Product-specific documentation, an accepted quotation, an order confirmation, and applicable law take priority where they provide more specific requirements.