Example Data Table
| Load |
Quantity |
Running Watts Each |
Starting Watts Each |
Use Case |
| Refrigerator |
1 |
700 |
2200 |
Food storage |
| Furnace Blower |
1 |
800 |
2350 |
Winter heat support |
| Sump Pump |
1 |
900 |
2900 |
Flood protection |
| Lighting Circuits |
8 |
75 |
75 |
Basic safety lighting |
Formula Used
Total Running Watts = sum of quantity × running watts for selected loads.
Largest Starting Extra = largest value of selected starting watts minus running watts.
Simultaneous Running Demand = total running watts × simultaneity factor.
Peak Demand = simultaneous running demand + largest starting extra.
Required Watts = peak demand × margin factor ÷ derating factor.
Margin Factor = 1 + reserve percentage + future expansion percentage.
Derating Factor = 1 - derating percentage.
Suggested kW = required watts ÷ 1000, rounded up to a common generator class.
How To Use This Calculator
Select every appliance, circuit, or motor you want to run during an outage. Enter the quantity, running watts, and starting watts. Use nameplate ratings when possible. Adjust the simultaneity factor when all loads will not run together. Add reserve margin for safer operation. Add future expansion for planned appliances. Use derating for heat, altitude, fuel limits, or aging equipment. Press the calculate button. Review the suggested generator size, current estimate, and fuel demand. Download the result for planning or installer review.
Home Standby Generator Sizing Guide
Why sizing matters
A standby generator should cover the loads you expect to use during an outage. It should also handle short starting surges. Motors in pumps, refrigerators, and air systems often need extra watts for a few seconds. A generator that is too small may trip, stall, or shorten equipment life. A unit that is too large can cost more and run with poor efficiency.
Start with essential circuits
List the items that must stay on. Common choices include lights, refrigerator, freezer, internet, furnace blower, medical equipment, and sump pump. Add well pumps, kitchen outlets, or air conditioning only when they are truly needed. Whole home coverage needs a larger unit. Essential coverage can often use a smaller unit with smarter load control.
Understand running and starting watts
Running watts are the steady load after equipment starts. Starting watts are the short peak needed by a motor. This calculator sums running watts first. Then it adds the largest starting surge. That approach assumes only one major motor starts at once. It gives a practical estimate for many homes. Homes with many motors may need professional load management.
Add margin and derating
Reserve margin keeps the generator from running at its limit. Future expansion allows new appliances later. Derating handles heat, altitude, age, and fuel limits. These factors raise the required generator size. The final result rounds up to a common generator class. Rounding up helps avoid nuisance overloads.
Use the result carefully
The suggested size is an estimate. It helps compare appliances and priorities. It does not replace an electrical load study. Local code, transfer switch type, gas pressure, breaker size, and installation method still matter. A licensed installer should confirm the final unit, fuel line, and transfer equipment.
Good planning reduces waste
A generator is easier to choose when loads are grouped by need. Start with safety loads. Add comfort loads next. Leave luxury loads for last. This method keeps cost lower and makes outages easier to manage.
Review results after each change. Remove loads that are not urgent. Try a second plan for night use. Try another for daytime comfort. This reveals the best balance between safety, cost, and convenience during outages.
FAQs
1. What size standby generator do I need?
You need enough capacity for selected running loads, starting surges, reserve margin, and derating. The calculator estimates that value and rounds up to a common generator size.
2. What are running watts?
Running watts are the steady watts used after an appliance or motor has started. These watts are added together for selected loads.
3. What are starting watts?
Starting watts are the short surge watts needed by motors. Pumps, compressors, refrigerators, and air systems often need much higher startup power.
4. Why does the calculator use the largest starting surge?
Many homes do not start every motor at the same second. Using the largest surge gives a practical estimate. Complex homes may need load management.
5. Should I include air conditioning?
Include air conditioning only if you expect to run it during outages. Central air can greatly increase generator size because compressor startup demand is high.
6. What is derating?
Derating reduces usable generator output for heat, altitude, fuel limits, age, or installation conditions. A derating allowance raises the recommended size.
7. Is the fuel demand exact?
No. Fuel demand is an estimate for planning. Actual use depends on generator model, load level, fuel pressure, maintenance, and weather.
8. Do I still need an installer?
Yes. A licensed installer should confirm load calculations, transfer equipment, fuel supply, permits, and code requirements before installation.