How to Calculate Generator Wattage: A Simple Guide
Picking a generator is mostly math. Get the wattage right and everything runs. Get it wrong and you trip breakers—or pay for a machine twice the size you need.
This guide explains the four numbers that matter, how to add them up the way electricians do, and how to turn the result into a generator size you can buy.
The four numbers that matter
1. Running watts (continuous power)
Running watts is what an appliance draws while it stays on. A refrigerator might pull 150W once the compressor is running. Lights, Wi‑Fi, and TVs are almost all running watts only.
You’ll see this on the yellow EnergyGuide label or the nameplate on the back of the unit—often called rated, continuous, or running.
2. Starting watts (surge power)
Motors need a short burst of extra power to start. That burst is starting watts (also surge or peak). A fridge rated at 150W running might need 600W for a few seconds when the compressor kicks on.
Starting watts matter most for:
- Refrigerators and freezers
- Air conditioners and heat pumps
- Pumps (well, sump, pool)
- Washers, tools, and anything with a compressor
If the generator can’t cover the surge, the breaker trips—even if the running watts look fine.
3. Continuous vs non-continuous load
The National Electrical Code (NEC) treats loads that run 3+ hours differently:
| Type | Examples | How it’s sized |
|---|---|---|
| Continuous | Fridge, lights, HVAC, water heater, sump pump, Wi‑Fi | Counted at 125% of running watts |
| Non-continuous | Microwave, coffee maker, hair dryer, tools | Counted at 100% |
Why 125%? Continuous loads shouldn’t run at the edge of a circuit’s capacity for hours. Sizing at 125% builds in a safety margin (NEC 210.20(A)).
4. Usable generator watts (the 80% rule)
A generator labeled 10,000W continuous should not be loaded at 10,000W all day. Industry practice (and NEC continuous-load thinking) keeps continuous draw at 80% of rated watts:
10,000W × 0.8 = 8,000W safe continuous load
That’s the number you compare against your appliance total—not the flashy peak on the box.
How to calculate: the simple method
Step 1 — List what you need during an outage
For home backup, most people start with:
- Refrigerator
- Lights (a few LED circuits)
- Wi‑Fi / modem
- Microwave or coffee maker
- Phone charging
- Heating or cooling (if you can)
- Sump pump (if you have one)
Step 2 — Write running and starting watts for each
| Appliance | Running W | Starting W | Type |
|---|---|---|---|
| Refrigerator | 150 | 600 | Continuous |
| LED lights | 50 | 50 | Continuous |
| Wi‑Fi router | 15 | 15 | Continuous |
| Microwave | 1,200 | 1,200 | Non-continuous |
| Washing machine | 500 | 1,200 | Non-continuous |
You can use our calculator for 100+ presets, or type custom values.
Step 3 — Two scenarios electricians care about
Scenario A — Everything running together
- Continuous running watts × 1.25
- Plus all non-continuous running watts
Scenario B — A big motor starts while other things run
- Total running watts
- Plus the largest starting surge (not every surge at once)
Take the higher of A and B. That’s your NEC load.
Step 4 — Convert to generator size
Divide by 0.8, then round up to a common size:
Example
- Continuous: fridge 150 + lights 50 + Wi‑Fi 15 = 215W → ×1.25 = 269W
- Non-continuous: microwave 1,200 + washer 500 = 1,700W
- Scenario A = 269 + 1,700 = 1,969W
- Scenario B = 1,969W running + washer surge extra 700W ≈ 2,669W
- Higher = 2,669W
- ÷ 0.8 = 3,336W → round up → 3,500W generator (continuous rating)
So a 3,500W inverter or portable generator covers this essential set with margin.
Common sizing shortcuts (essentials only)
| What you want to run | Typical continuous rating |
|---|---|
| Fridge, lights, Wi‑Fi, phone | 2,000–2,200W |
| Above + microwave or window AC | 3,500–5,000W |
| Above + well/sump pump or larger AC | 7,500–10,000W |
| Near whole-house (no central electric AC) | 10,000–15,000W |
| Whole house + central AC + electric dryer | 20,000W+ standby |
These are starting points. Always prefer a real calculation from your own list.
Mistakes that cause undersized generators
- Ignoring starting watts — You run the microwave and the fridge compressor starts; breaker trips.
- Using nameplate peak, not continuous — A “12,000W peak / 9,500W running” unit is a 9,500W generator for planning.
- Counting every surge at once — Motors rarely start at the same second. Use the largest surge, not the sum.
- Forgetting the 80% rule — Continuous loads need headroom.
- Skipping HVAC — A 3‑ton central AC can need 3,500W running and 9,000W+ to start. That’s often the whole generator budget.
How PickMyWatt does the math
Our wattage calculator uses the same approach:
- Classifies each appliance as continuous or non-continuous
- Continuous × 125% (NEC safety margin)
- Compares Scenario A (all running) vs Scenario B (largest motor surge)
- Takes the max, applies the 80% usable rule
- Rounds up to a practical generator size
You can also estimate fuel cost and see what an existing generator can run.
Next steps
- Open the generator size calculator and add the appliances you actually own.
- If you already have a machine, use What Can My Generator Run?.
- Need a new unit? Compare portable, inverter, and whole-house sizing guides.
The right generator isn’t the biggest one—it’s the one that covers your real load with a little margin. Run the numbers once, and outages get much simpler.