How to Size a Generator: Running Watts vs Starting Watts

The single most common mistake we see when customers buy a generator is choosing one that's too small. A generator that can't handle the surge when a motor starts will trip, stall, or — worse — slowly damage the equipment plugged into it. This guide walks you through sizing a generator properly so you buy once and buy right.

Running watts vs starting watts

Every electrical device has two numbers that matter:

• Running watts — the continuous power a device needs while it operates.
• Starting (surge) watts — the extra burst, often 2–3× the running figure, that motors draw for a fraction of a second at start-up.

Resistive loads such as lights, heaters and chargers have no real surge. Inductive loads — fridges, pumps, compressors, power tools — spike hard at start-up. Your generator must cover the total running watts plus the single largest starting surge on the circuit at one time.

A simple four-step calculation

1. List every device you'll run at the same time.
2. Write down the running watts for each (check the nameplate or manual).
3. Add the running watts together.
4. Add the largest single starting surge to that total.

Example: 800W fridge (running) + 600W lights + a water pump rated 900W running / 2,700W starting. Total running = 2,300W. Add the pump's 2,700W surge → you need roughly 5,000W of headroom.

Matching the load to a BASOO generator

Use case	Typical load	Suggested model
Sensitive electronics / camping	up to 2 kW	BASOO Super Silent BS-S2000ISG (inverter)
Home backup essentials	3–5 kW	BASOO BS-6500
Workshop / small site	5–6.5 kW	BASOO BS-8500
Continuous 3-phase site power	10 kW+	BASOO BS-15000DXS-3 diesel

Inverter or conventional?

If you'll run laptops, routers or any device with a circuit board, choose an inverter generator like the Super Silent range. Inverter technology delivers clean, stable sine-wave power and throttles the engine to match the load — quieter operation and far better fuel economy at part load.

Don't forget altitude and duty cycle

Engines lose roughly 3–4% of power per 300 m above sea level, so size up if you work at altitude. And never plan to run a generator continuously at 100% — aim for 70–80% of rated output for long service life.

Still unsure? Send us your load list through the contact page and our team will recommend the right model — or, if you buy for resale, ask about wholesale pricing.