Written by a senior appliance and small-engine technician with 14 years of field service experience. Based on 500+ sizing consultations. Last updated September 2026.
This page is a generator wattage calculator. It provides the calculator, the sizing steps, and the appliance wattage reference. The mistake analysis and the repair cases live on the separate generator sizing mistakes page. The URL of this page reflects an earlier structure; the current content is focused on calculation only.
Generator Wattage Calculator
Fill in the blanks. This is the calculator itself, not a description of it.
Step 1 — List Every Appliance You Will Run at the Same Time
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | ______ W | ______ W |
| Freezer | ______ W | ______ W |
| Sump pump | ______ W | ______ W |
| Well pump | ______ W | ______ W |
| Window AC | ______ W | ______ W |
| Microwave (actual draw) | ______ W | ______ W |
| Space heater | ______ W | ______ W |
| Lights | ______ W | not applicable |
| TV and electronics | ______ W | not applicable |
Step 2 — Total Running Watts
Add every running watt figure in the table. This is your continuous load.
Total running watts = ______ W
Step 3 — Highest Starting Watts
Find the single highest starting watt figure in the table. This is the largest motor surge you will face.
Highest starting watts = ______ W
Step 4 — Required Surge Capacity
Your required surge capacity is the total running watts plus the additional startup demand of the largest motor. The additional startup demand is the starting watts minus that motor’s running watts, not the full starting watts again.
Required surge = total running watts + (highest starting watts − that motor’s running watts) + 20% headroom
Example: 850W running + (1800W starting − 600W running) + 20% headroom = 850 + 1200 + 410 = 2460W required surge.
Step 5 — Minimum Generator Size
Shop for a generator whose surge rating is at or above the required surge figure from Step 4. Check the generator sizing calculator result against the appliance wattage table below before buying.
How to Use the Generator Wattage Calculator
Step 1 — List Every Appliance
Write down every device you intend to power at the same time during an outage. Include the ones you might forget: phone chargers, a modem, a sump pump that cycles automatically, a refrigerator that kicks on by itself.
Step 2 — Find Running Watts
Running watts are what the device consumes continuously. The appliance label, the owner’s manual, or a plug-in watt meter gives the true number. Manufacturer marketing pages often list only cooking power or output power, not input draw.
Step 3 — Find Starting or Surge Watts
Starting watts are what a motor-driven device needs for the first 1–3 seconds. Motors need 2–3 times their running watts to start. Appliances without motors, such as lights and electronics, have no starting surge.
Step 4 — Identify Appliances That Start Automatically
Refrigerators, freezers, and sump pumps start on their own, without warning. Two automatic-start motors can cycle on at the same moment and stack their surges. This is where most overload trips happen.
Step 5 — Calculate Your Continuous Load
Add all running watts together. This is the load the generator must carry hour after hour. It must be below the generator’s continuous rating, not the surge rating.
Step 6 — Check Your Required Surge Capacity
Use the formula in Step 4 of the calculator. The surge rating on the generator must cover the running load plus the additional startup demand of the largest motor plus the 20% headroom.
Step 7 — Add a Practical Safety Margin
Add 20% headroom. Running a generator at 100% of its continuous rating shortens engine life. Real-world sustained output can also be 5–10% below the box rating.
Generator Wattage Calculation Example
An owner wants to run a refrigerator, lights, and a TV at the same time.
| Device | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 600 W | 1800 W |
| Lights | 100 W | not applicable |
| TV | 150 W | not applicable |
| Total running | 850 W | |
| Largest starting | 1800 W |
The refrigerator starts automatically. The additional startup demand is 1800 W minus 600 W running, which is 1200 W.
Required surge = 850 W running + 1200 W additional startup + 20% headroom = 2460 W.
The owner needs a generator with a surge rating of at least 2460 W. A 2000 W surge generator is too small. A 2500 W surge generator is at the edge. A 3000 W surge generator leaves working margin.
This is why “add all the starting watts together” is wrong. The refrigerator’s running watts are already counted in the 850 W. Adding the full 1800 W starting figure again would double-count the refrigerator’s running load.
Generator Size Examples
Refrigerator + Lights + TV
Continuous load: 850 W. Largest motor surge: refrigerator at 1800 W. Required surge: about 2460 W. A 2500–3000 W surge generator is the target.
Refrigerator + Freezer + Microwave
Continuous load: 600 W refrigerator + 400 W freezer + 1200 W microwave actual draw = 2200 W. Largest motor surge: refrigerator at 1800 W or freezer at 1200 W. If both compressors cycle on together, the surge stacks: 1800 + 1200 additional = 3000 W additional, plus running load. Required surge: about 5800 W under worst-case stacking. This is why running the microwave while both compressors cycle is the most common overload scenario.
Refrigerator + Sump Pump + Lights
Continuous load: 600 W refrigerator + 800 W sump pump + 100 W lights = 1500 W. Largest motor surge: sump pump at 2000 W. Required surge: 1500 + (2000 − 800) + 20% = about 2100 W. A 2500 W surge generator is the target.
Well Pump + Refrigerator + Lights
Continuous load: 1000 W well pump + 600 W refrigerator + 100 W lights = 1700 W. Largest motor surge: well pump at 3000 W. Required surge: 1700 + (3000 − 1000) + 20% = about 3200 W. A 3500 W surge generator is the target.
Whole-House Emergency Loads
Continuous load with refrigerator, freezer, well pump, sump pump, furnace blower, lights, and electronics: often 3000–5000 W running. Largest motor surge: well pump or furnace blower at 3000–4000 W. Required surge: about 7000–9000 W. A 7500–10000 W surge generator is the target.
What Size Generator Do I Need
Use the results from the calculator above to match your load to a generator class. This is the generator size calculator step.
| Load Class | Typical Running Load | Typical Surge Need | Generator Class |
|---|---|---|---|
| Phone, laptop, lights, TV | 100–500 W | 500–600 W | 1000 W class |
| Refrigerator, lights, TV | 500–1000 W | 2000–3000 W | 2000–3000 W class |
| Refrigerator, freezer, microwave | 1500–2500 W | 4000–6000 W | 3500–5000 W class |
| Well pump, refrigerator, lights | 1500–2500 W | 3000–4000 W | 3500–5000 W class |
| Whole-house emergency | 3000–5000 W | 7000–9000 W | 7500–10000 W class |
This table is a planning aid. The exact figure comes from the calculator, not the table. The table tells you which generator class to shop in; the calculator tells you whether a specific generator is large enough.
Appliance Wattage Table
These are planning estimates. Check the appliance label, the owner’s manual, or measure actual consumption with a plug-in watt meter when possible. This table is the reference data for the generator running watts calculator and the generator starting watts calculator steps above.
| Appliance | Typical Running Watts | Possible Starting Watts |
|---|---|---|
| Refrigerator (modern) | 400–800 W | 1200–2400 W |
| Refrigerator (older) | 600–1000 W | 1800–3000 W |
| Freezer | 300–500 W | 900–1500 W |
| Sump pump (1/3 HP) | 800 W | 2000 W |
| Sump pump (1/2 HP) | 1050 W | 2800 W |
| Well pump (1/2 HP) | 1000 W | 3000 W |
| Furnace blower | 500–800 W | 1500–2400 W |
| Window AC (5,000 BTU) | 500 W | 1500 W |
| Window AC (10,000 BTU) | 1200 W | 3000 W |
| Microwave (actual draw) | 1000–1500 W | 1000–1500 W |
| Space heater | 1500 W | 1500 W |
| Coffee maker | 900–1200 W | 900–1200 W |
| LED light bulb | 10 W | 10 W |
| Laptop charger | 60–90 W | 60–90 W |
| Phone charger | 5–20 W | 5–20 W |
FAQ
How many watts do I need for a generator?
Add the running watts of every device you will run at the same time. Find the single highest starting wattage among your devices. The required surge is running watts plus the additional startup demand of the largest motor (starting watts minus that motor’s running watts) plus 20% headroom. That is your minimum generator surge rating.
How do I calculate generator wattage?
List every appliance. Find the running watts from the label or a watt meter. Find the starting watts for motor-driven appliances. Add all running watts. Add the additional startup demand of the largest motor. Add 20% headroom. The result is your required surge capacity. This is the calculate generator wattage step.
How do I calculate running watts?
Running watts are listed on the appliance label or in the owner’s manual. For devices without a label, use a plug-in watt meter to measure actual consumption. Add all running watts together for the devices you will run at the same time.
How do I calculate starting watts?
Starting watts apply to motor-driven appliances only. They are typically 2–3 times the running watts. The appliance label or the owner’s manual often lists them. If they are not listed, multiply the running watts by 3 for a conservative estimate.
How many watts does a refrigerator need?
A modern refrigerator typically draws 400–800 W running and 1200–2400 W starting. An older refrigerator can draw 600–1000 W running and 1800–3000 W starting. Check the label on the refrigerator for the exact figures.
What size generator do I need for a well pump?
A 1/2 HP well pump typically draws 1000 W running and 3000 W starting. If the well pump is the largest motor on the circuit, the required surge is the total running load plus 2000 W additional startup plus 20% headroom. For a well pump plus a refrigerator plus lights, the required surge is about 3200 W. A 3500 W surge generator is the target.
What size generator do I need for a sump pump?
A 1/3 HP sump pump typically draws 800 W running and 2000 W starting. A 1/2 HP sump pump typically draws 1050 W running and 2800 W starting. If the sump pump is the largest motor on the circuit, the required surge is the total running load plus the additional startup demand plus 20% headroom.
How much generator capacity should I leave as a safety margin?
Add 20% headroom. Running a generator at 100% of its continuous rating shortens engine life. Real-world sustained output can also be 5–10% below the box rating. A generator sized at exactly the calculated load will be at the edge of overload under real conditions.