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Generator Sizing
Recommended Generator Size
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Disclaimer
This calculator provides estimates for informational purposes only. Always follow manufacturer instructions and local building or electrical codes, and consult a licensed contractor or electrician before acting on this information -- especially for structural work, electrical wiring, or fuel-burning equipment.
How Generator Sizing Is Calculated
A generator needs to cover every appliance’s continuous running watts, plus the single largest startup surge on top. List each appliance’s running watts (and starting watts, if it has a motor), and this calculator adds them up.
Key Factors to Consider
Not every appliance needs to run at once. If you’re willing to stagger usage during an
outage (running the microwave and the well pump at different times, for example), you can size a
smaller generator than the “everything at once” total this calculator returns — worth deciding
in advance which appliances are truly essential versus nice-to-have.
Running watts and starting watts are printed differently across appliances. Some
manufacturers list both directly on a nameplate or in the manual; others only list running watts
or amps, in which case starting watts (for anything with a motor — refrigerators, well pumps,
air conditioners, sump pumps) may need to be estimated from the appliance type if not otherwise
available.
A generator’s continuous rating, not its peak/surge rating, is what covers running watts.
Manufacturers often advertise a generator’s higher surge wattage prominently — check the
continuous (rated) wattage specifically against your running-watts total, since that’s the
number the generator can sustain, not just briefly handle.
Fuel consumption and runtime are a separate consideration from sizing. A correctly sized
generator still needs enough fuel on hand to actually last through an outage — see the
Generator Runtime & Fuel Consumption Calculator for that half of the planning.
Useful to Know
Portable generators are a real carbon monoxide poisoning risk if run indoors, in a garage, or
too close to windows and doors. A mid-size portable generator can produce as much carbon
monoxide as several cars running at once, and CO is colorless and odorless — always run a
portable generator outdoors, away from windows, doors, and vents, never in an enclosed or
partially enclosed space. Separately, never plug a generator directly into a wall outlet
(“backfeeding”) to power a home’s wiring — without a proper transfer switch, this can send
electricity back through the grid and seriously injure or kill utility workers repairing lines
they believe are de-energized, and it’s illegal in most places. A licensed electrician can install
a transfer switch for safe whole-house backup power.
Where the surge for one appliance is max(0,Starting Watts−Running Watts), and only the single LARGEST surge across all appliances counts — appliances essentially never all start at the exact same instant.
Worked Example
A refrigerator (700W running, 2,200W starting), lights (200W), and a TV (150W):
Un generador necesita cubrir los vatios de funcionamiento continuo de cada electrodoméstico, más el mayor pico de arranque individual. Enumera los vatios de funcionamiento de cada electrodoméstico (y los de arranque, si tiene motor), y esta calculadora los suma.
Factores Clave a Considerar
No todos los electrodomésticos necesitan funcionar a la vez. Si estás dispuesto a escalonar
el uso durante un apagón (usar el microondas y la bomba de pozo en momentos distintos, por
ejemplo), puedes dimensionar un generador más pequeño que el total de “todo a la vez” que
devuelve esta calculadora — vale la pena decidir de antemano cuáles electrodomésticos son
realmente esenciales frente a cuáles son deseables.
Los vatios de funcionamiento y de arranque se imprimen de forma distinta según el
electrodoméstico. Algunos fabricantes listan ambos directamente en una placa de datos o en el
manual; otros solo listan los vatios o amperios de funcionamiento, en cuyo caso los vatios de
arranque (para cualquier cosa con motor — refrigeradores, bombas de pozo, aires acondicionados,
bombas de sumidero) podrían necesitar estimarse según el tipo de electrodoméstico si no están
disponibles de otra forma.
La clasificación continua de un generador, no su clasificación pico/de pico, es lo que cubre
los vatios de funcionamiento. Los fabricantes a menudo anuncian de forma prominente el vataje
de pico más alto de un generador — verifica específicamente el vataje continuo (nominal) contra
tu total de vatios de funcionamiento, ya que ese es el número que el generador puede sostener, no
solo manejar brevemente.
El consumo de combustible y la autonomía son una consideración aparte del dimensionamiento.
Un generador correctamente dimensionado aún necesita suficiente combustible a mano para durar
realmente todo un apagón — consulta la Calculadora de Autonomía y Consumo de Combustible de Generador para esa otra
mitad de la planificación.
Útil Saber
Los generadores portátiles son un riesgo real de intoxicación por monóxido de carbono si se usan
en interiores, en un garaje, o demasiado cerca de ventanas y puertas. Un generador portátil de
tamaño mediano puede producir tanto monóxido de carbono como varios autos funcionando a la vez, y
el CO es incoloro e inodoro — siempre usa un generador portátil en exteriores, lejos de ventanas,
puertas y respiraderos, nunca en un espacio cerrado o parcialmente cerrado. Por separado, nunca
enchufes un generador directamente a un tomacorriente de pared (“retroalimentación”) para
alimentar el cableado de una casa — sin un interruptor de transferencia adecuado, esto puede enviar
electricidad de vuelta a través de la red y lesionar gravemente o matar a trabajadores de servicios
públicos que reparan líneas que creen desenergizadas, y es ilegal en la mayoría de los lugares. Un
electricista con licencia puede instalar un interruptor de transferencia para energía de respaldo
segura para toda la casa.
La Fórmula
Vatios Recomendados=Vatios de Funcionamiento Total+Mayor Pico
Donde el pico de un electrodoméstico es max(0,Vatios de Arranque−Vatios de Funcionamiento), y solo cuenta el pico MÁS GRANDE entre todos los electrodomésticos — en la práctica, los electrodomésticos casi nunca arrancan todos exactamente al mismo tiempo.
Ejemplo Resuelto
Un refrigerador (700W en funcionamiento, 2,200W al arrancar), luces (200W) y un televisor (150W):
Vatios de funcionamiento total: 700+200+150=1,050.
Add up the continuous running watts of every appliance you want to power, then add the single largest starting-watt surge on top (many motor-driven appliances like refrigerators or well pumps briefly draw much more power to start up than to keep running). This calculator does that math from your own appliance list.
Why does starting wattage matter for generator sizing?
A generator has to survive the brief startup surge of a motor-driven appliance, not just its steady running load -- undersizing for that surge can trip the generator’s breaker or stall it, even if the running wattage alone would have been fine.
Do I need to add up every appliance’s starting watts separately?
No -- appliances essentially never all start at the exact same instant in real use, so only the single largest starting surge needs to be added on top of everything else’s combined running watts, not the sum of every appliance’s surge.
Is it safe to run a portable generator in my garage?
No -- portable generators produce significant carbon monoxide, a colorless and odorless gas, and running one in a garage (even with the door open) or indoors is a real poisoning risk. Always run a portable generator outdoors, well away from windows, doors, and vents.
Can I just plug my generator into a wall outlet to power my house?
No -- this practice, called "backfeeding," can send electricity back through the grid and seriously injure or kill utility workers repairing lines they believe are de-energized, and it's illegal in most places. A licensed electrician can install a proper transfer switch for safe whole-house backup power.
Should I size for my whole house or just essential circuits?
Most homeowners size for a subset of essential circuits -- refrigerator, well pump, some lights, a furnace blower -- rather than every circuit in the house, since running everything at once is both expensive to size for and rarely necessary during a short outage. Deciding in advance which appliances are truly essential keeps the needed generator size, and its cost, much more reasonable.
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