A Power Outage Hits-Do You Have a Generator That Can Actually Run Your Furnace, Refrigerator, and Well Pump Simultaneously? Or Will It Just Spin Up and Quit?
Most people underestimate the generator they need. That's because motors and compressors draw 2โ3 times their running watts during startup. This calculator shows you the wattage you actually need to handle both running loads and starting surges, so you're not left in the dark.
What This Calculator Does
This calculator estimates the generator capacity (in watts) required to run your essential appliances during a power outage. You list the appliances you need to run, input their running watts and whether they have motors (which draw high starting current), and the calculator sums running watts and accounts for motor starting surges. It also adds a 20% safety margin so you're not running the generator at its absolute limit. The result is the minimum generator size you should buy, plus recommendations on features like automatic transfer switches and fuel type.
How to Use This Calculator
Start by deciding which appliances are essential during an outage. Typically: furnace or heat pump, refrigerator, well pump (if on well), sump pump, electric water heater or heating system, basic lighting, and maybe a few outlets for phone charging or a laptop. Look up the running wattage for each (check the nameplate or manual). For appliances with motors (furnace, AC, well pump, sump pump, refrigerator compressor), note that they draw 2โ3 times running watts during startup. Enter each appliance's running watts and startup multiplier. For non-motor devices (lights, outlets, TVs), startup equals running watts. The calculator will show you: total running watts, total starting watts (including surge), and recommended generator size with a 20% safety margin. This is your minimum size.
The Formula Behind the Math
The math accounts for two phases: startup and steady-state running.
Steady-state running power:
Total running watts = sum of all running watts
For example: furnace (2,500 W), refrigerator (150 W average), water heater (4,000 W idle, 0 W if off), well pump (1,500 W), sump pump (750 W), lighting and outlets (500 W):
Starting surge (one motor at a time):
The largest motor starts first, drawing 2โ3ร its running watts while others run at normal watts. Well pumps and furnace compressors typically draw 3ร running watts on startup:
Recommended generator size with 20% safety margin:
Generator = starting watts ร 1.2
Why the 20% margin? Generators don't perform at rated capacity in hot weather or altitude. A 10 kW generator might only deliver 8.5โ9 kW on a hot day. The margin prevents overload and premature failure.
Our calculator does all of this instantly-but now you understand exactly what it's computing. The biggest mistake is ignoring starting surges and undersizing the generator. A 7 kW generator sounds right until a furnace tries to start and collapses the output voltage.
Portable vs. Standby Generators: Which Do You Need?
Portable generators (2โ8 kW, gas or propane) are affordable ($500โ$2,000) and mobile, but require manual setup and fuel management. Best for short outages or temporary needs. Standby generators (8โ20+ kW, natural gas or propane) automatically turn on when power fails, run for days, and require proper installation ($3,000โ$15,000+) and permits. Best for reliability and minimal disruption. Most homeowners benefit from a 7โ10 kW portable for emergency backup. Larger homes might justify a 12โ15 kW standby.
Fuel Type: Natural Gas, Propane, or Gasoline
Natural gas generators (if you have gas service) run indefinitely-no fuel management needed. Propane generators offer multi-day runtime on a tank; a 100-pound tank runs an 7 kW generator for roughly 8โ12 hours. Gasoline generators are cheapest upfront but require fuel storage (gasoline degrades over time) and frequent refueling during outages. Most people prefer propane for standby or extended portable use. Natural gas is ideal if available. Gasoline is fine for short-term portable use.
Safe Generator Operation: Placement and Exhaust
Never run a generator indoors-carbon monoxide is deadly and scentless. Run generators outside, at least 20 feet from windows and doors. Use heavy-duty extension cords rated for outdoor use (avoid overheating). If you have a standby generator, hire a licensed electrician to install it with an automatic transfer switch (ATS)-this disconnects your home from the grid when the generator powers on, preventing backfeed (dangerous to utility workers). Never operate two power sources simultaneously without a proper ATS.
Battery Backup vs. Generators
Battery systems (Tesla Powerwall, LG Chem, etc.) are quieter, require no fuel, and start instantly. But capacity is limited: a Powerwall holds 13.5 kWh, enough to run a refrigerator and a few lights for ~36 hours, or a house's baseload for 8โ12 hours. Capacity of 50+ kWh is needed for multi-day outages without backup generation. Cost: $10,000โ$20,000 installed for 25โ50 kWh. Paired with solar, batteries are excellent for resilience and grid independence. As standalone backup, they're expensive and capacity-limited. Most people combine: solar + battery for daytime independence, generator for extended outages or cloudy weather.
Maintenance and Fuel Management
Standby generators need annual servicing: oil change, filter replacement, and load testing. Portable generators should be exercised monthly (run under load for 30 minutes) to prevent carburetor gumming. Fuel storage is critical: gasoline degrades in 3โ6 months; use fuel stabilizer if storing long-term, or drain the tank seasonally. Propane lasts indefinitely. Natural gas needs no storage. Schedule maintenance before severe weather season to ensure readiness.
Tips and Things to Watch Out For
Don't underestimate starting surges. Most outage failures are generators that undersized and can't start large motors. Always include 2โ3ร starting watts for motors.
You can't run everything simultaneously. Even if your home uses 15 kW average, a generator sized for starting surges might be smaller by limiting simultaneous loads. Manually stagger starting (start furnace first, then add other loads).
Generator load management prevents failure. With a 7 kW generator and 8 kW starting demand, you might start just the furnace, then bring on other loads as it settles. A transfer switch with load management does this automatically.
Propane is safer than gasoline for storage. Propane is inert and non-flammable when properly stored. Gasoline is volatile and dangerous to store in large quantities. For multi-day outage prep, propane wins.
Noise matters to neighbors. Portable generators produce 70โ100 dB. Standby generators are often quieter and more distant from neighbors. If noise is a concern, invest in soundproofing or natural gas (quieter) over gasoline.
Solar + battery is more resilient than generator alone. A generator with no fuel is useless. Solar + battery runs indefinitely if the sun shines, with a generator as backup for cloudy weeks. This is the modern resilience strategy.
Frequently Asked Questions
How much does a home standby generator cost?
8โ10 kW natural gas standby: $3,000โ$6,000 installed. 12โ15 kW: $5,000โ$9,000. Labor and electrical work (transfer switch, gas line) can be 30โ50% of total cost.
Can I use a portable generator to power a sump pump?
Yes, if it's sized for the pump's starting watts (typically 2โ3 kW). A 5 kW portable generator is enough for a sump pump and basic loads. For a furnace and sump pump together, you'd need 7โ10 kW.
What happens if my generator is too small?
It will shut down under overload (automatic protection) or experience voltage sag (devices run poorly). You lose power again and can't restart the overloaded motor. Undersizing is worse than not having a generator.
How long can a generator run continuously?
Portable gas generators: 8โ12 hours per tank (depending on load). Standby natural gas/propane: indefinitely (as long as fuel supply lasts). Propane tank lasts 8โ12 hours per 100-pound tank at 7 kW load. Plan for fuel delivery during extended outages.
Do I need a transfer switch?
For standby generators, yes-it's essential and often required by code. For portable generators, you can use extension cords, but a transfer switch prevents accidental backfeed. Hire an electrician if unsure.
Can a generator power my air conditioner?
Yes, but AC units are large motors. A 3-ton AC uses 3,000โ4,000 watts running and 9,000โ12,000 watts starting. You'd need a 15+ kW generator, which is expensive. Most people don't run AC during outages to conserve fuel.
How often should I service my generator?
Portable: monthly exercise (run under light load for 30 minutes), oil change every 50โ100 hours. Standby: annual professional servicing (inspection, oil change, load test). Clean or replace fuel filters seasonally.
Is a battery backup better than a generator?
Batteries are cleaner, quieter, instant, and work with solar. But expensive upfront and limited capacity. Generators are loud, need fuel, and require startup time. Most resilient: solar + battery + generator for redundancy.
Related Calculators
Use the Electricity Cost Calculator to understand the wattage of appliances you want to run (helps with sizing). The BTU Calculator helps if you need to know furnace or AC capacity. The Electricity Bill Calculator shows your normal power consumption, which informs how much backup capacity you really need.