01 — Running watts vs starting watts
Every generator sizing decision depends on understanding two different wattage numbers.
Running watts (also called rated or continuous watts) are the power a device needs to keep operating. Starting watts (also called surge watts) are the burst of power a motor-driven appliance needs to start — sometimes two to five times higher than running watts, and only required for a second or two. If your generator cannot supply the starting watts of the largest motor in your load, that appliance will not start — even if total running power is within range.
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | ~700 | ~2,200 |
| Window AC (10,000 BTU) | ~1,200 | ~3,600 |
| Sump pump (1/2 hp) | ~1,050 | ~2,150 |
| Well pump (1/2 hp) | ~1,000 | ~2,000 |
| LED lights (per bulb) | 10–12 | Same |
| Phone/device charger | 10–25 | Same |
How to size: List every appliance you plan to run simultaneously. Add up all running watts for your total running load. Add the starting watts of the single highest-starting device (not all of them) to your running total — that sum is the minimum generator starting capacity you need. Add 15 to 20 percent margin above that number for the actual generator you buy.
Practical guidance
For most households wanting to run a refrigerator, a few lights, phone chargers, and a window fan during an outage, a 3,000 to 5,000 watt portable generator covers essential loads. Running central air conditioning or an electric stove requires substantially more — 7,500 watts or higher, and likely a standby unit.
02 — Portable vs standby
Portable generators are the household default. Standby generators solve problems portable units cannot.
Portable generator
- Manual start, manual setup required
- Runs on gasoline (or propane/dual-fuel)
- Typically 1,000–10,000W
- Lower upfront cost
- Must operate outdoors at least 20 feet from windows/doors (CO risk)
- Requires stored fuel supply
- No automatic transfer switch; devices connect directly or via manual transfer
Standby (whole-house) generator
- Activates automatically within seconds of an outage
- Permanently installed, connected to home electrical panel via automatic transfer switch
- Runs on natural gas or propane (no manual refueling)
- Typically 10,000–26,000+W
- Professional installation required (electrician + gas line)
- Higher upfront cost
- Can power an entire home including HVAC
For most households, a portable generator is the practical starting point. A standby generator makes financial sense in areas with frequent or extended outages, households with medical equipment requiring continuous power, or situations where manual intervention during an outage is not practical.
03 — Refueling and maintenance
Never refuel a running generator. And a generator that has been sitting for months may fail to start when you need it.
Refueling rule
Always shut off a portable generator and allow it to cool before adding fuel. Gasoline spilled onto a hot engine or exhaust can ignite instantly. Do not overfill — fuel expands in the tank with heat, and overfilling can cause spillage near hot engine components.
Gasoline degrades in as little as 30 days without a stabilizer. A generator stored with old fuel will have gummed carburetors and varnish deposits that prevent starting when an outage occurs — exactly when you need it most. This is the most common reason a "stored" generator fails to start.
- Add fuel stabilizer immediately when storing the generator or its fuel supply. Stabilizer extends usable gasoline life from weeks to one year or more.
- Run the generator under load for 30 minutes every one to three months. This keeps seals lubricated, the battery charged (on electric-start models), and confirms it will start when needed.
- Change the oil according to the manufacturer's schedule — typically every 50 to 100 hours of use, or at least once a year.
- Check and clean the air filter annually or per manufacturer guidance.
- If storing long-term: run the carb dry by shutting off the fuel valve and letting it run out, or drain and dry the carb entirely — this prevents gumming far better than stabilizer alone for multi-year storage.
04 — Generator noise
Generator noise matters during an extended outage in ways that have nothing to do with comfort.
A generator running day and night during an extended outage announces to the surrounding area that your household has fuel and power. In prolonged community-wide outages, this can attract unwanted attention. Conventional open-frame portable generators typically produce 65 to 80 decibels at 25 feet — roughly the volume of a vacuum cleaner, continuously. Inverter generators run quieter (50 to 65 dB), are more fuel-efficient, and produce cleaner power for sensitive electronics. They cost more upfront but use less fuel per hour at partial loads, which adds up during a multi-day outage.
Practical noise reduction: position the generator as far from neighbors as the cord or transfer switch will allow; orient the exhaust away from adjacent structures; use a generator shed or baffle enclosure rated for outdoor use (ensuring full ventilation remains — CO risk must not be created).
Quick reference
- Size by: total running watts of all simultaneous loads + starting watts of single largest motor + 15–20% margin
- Starting watts on motors can be 2–5x the running watts — this is what most people miss when sizing
- Portable: manual, gasoline, 20-foot CO clearance, manual transfer; Standby: automatic, gas line, whole-house, installed
- Never refuel a running or hot generator — shut off and let cool first
- Add fuel stabilizer immediately to stored gasoline; plain gasoline degrades in 30 days
- Run generator under load every 1–3 months; change oil annually
- Stored generator that has never been exercised is likely to fail when you need it most
Primary sources
- Cummins — Portable Generator Sizing Guide: Running vs starting watts explained; sizing method: add running watts of all simultaneous loads plus starting watts of single highest-starting device; propane and natural gas reduce output 10–15% vs gasoline.
- Generator Bible — FAQ: Fuel stabilizer for storage; run generator regularly to maintain condition; gasoline degrades in as little as 30 days; shut off and cool before refueling; standby generators activate automatically and run on home fuel supply.