Whole House Generator Sizing Calculator
Portable Generator
Standby Generator
Generator Size Recommendation
Understanding Whole House Generator Sizing
Choosing the right size for a whole house generator is crucial to ensure reliable backup power during outages. An undersized generator may struggle to power your essential appliances, especially during startup (surge), leading to tripped breakers and potential damage. An oversized generator, on the other hand, can be unnecessarily expensive to purchase and operate.
This calculator helps you determine the appropriate generator size based on your total continuous power needs and the surge wattage required by your appliances. It also considers the generator's power factor.
How it Works: The Math Behind the Sizing
Generator output is typically rated in both Watts (W) and Volt-Amperes (VA). Watts represent the real power consumed by resistive loads (like lights and heaters), while Volt-Amperes represent the apparent power, which includes both real power and reactive power (needed by motors in appliances like refrigerators, pumps, and air conditioners).
The relationship between Watts and VA is defined by the Power Factor:
Watts = VA * Power Factor
This means: VA = Watts / Power Factor
1. Continuous Wattage: This is the sum of the wattage required by all the appliances you intend to run simultaneously under normal operation. This is your baseline power requirement.
2. Surge Wattage: Many appliances, especially those with electric motors, require a significantly higher amount of power for a brief moment when they start up. This is known as surge wattage. You need a generator that can handle both the continuous load and these momentary surges.
3. Power Factor: Motors have a power factor less than 1, meaning they require more apparent power (VA) than their actual power consumption (Watts) would suggest. A common power factor for motors is 0.8. For example, a 1000W motor with a 0.8 power factor actually draws 1250 VA (1000W / 0.8).
Calculator Logic
- The calculator first determines the required VA for the continuous load:
Continuous VA = Total Continuous Watts / Power Factor - Next, it determines the required VA for the surge load. We assume the surge wattage is the additional power needed above the continuous load for startup. Therefore, the total VA needed for surge is calculated by considering the surge wattage as if it were the total real power draw that needs to be converted to VA:
Surge VA = Surge Wattage / Power Factor. This provides a more conservative estimate to ensure sufficient power during startup peaks. - The total required generator capacity is the higher of the two calculated VA values (Continuous VA vs. Surge VA) to ensure the generator can handle both sustained operation and startup demands. In practice, one often adds a buffer, but for simplicity, we take the maximum of the continuous and surge VA requirements for the output.
- The calculator then presents the recommended generator size in Watts, ensuring it's at least the total continuous wattage needed, and often recommends a size that also comfortably covers the calculated VA requirement, typically rounded up to standard generator sizes. For this calculator, we will output the higher VA requirement converted back to Watts (as a minimum running capacity) and the peak VA requirement.
When to Use a Whole House Generator
- Frequent Power Outages: If your area experiences regular blackouts due to weather or grid issues.
- Critical Medical Equipment: To ensure life-support or essential medical devices remain operational.
- Home Office Needs: To keep computers, servers, and communication equipment running for business continuity.
- Comfort and Convenience: To maintain heating, cooling, refrigeration, lighting, and basic household functions.
- Home Security Systems: To power alarms, cameras, and smart home security devices.
- Well Pumps and Sump Pumps: Essential for homes with wells or basements prone to flooding.
Important Considerations:
- Appliance Startup Loads: Always check the specific surge requirements for major appliances like refrigerators, air conditioners, well pumps, and electric ovens. These can be significantly higher than their running wattage.
- Future Needs: Consider if you might add more appliances or increase your power needs in the future.
- Fuel Type: Generators run on various fuels (gasoline, propane, natural gas, diesel). Choose based on availability and cost.
- Professional Installation: For standby generators, professional installation by a licensed electrician is typically required and recommended for safety and proper integration with your home's electrical system.
- Transfer Switch: A transfer switch (manual or automatic) is necessary to safely connect the generator to your home's electrical panel, preventing backfeeding to the grid.
This calculator provides an estimate. Consulting with a qualified electrician or generator specialist is always recommended for a precise assessment tailored to your specific home and appliances.