What size generator do I need?

Running watts vs starting watts: what is the difference?

Running watts are the power an appliance uses after it is operating normally. Starting watts are the brief higher demand some motors and compressors need when they switch on. Refrigerators, pumps, furnace blowers, and air conditioners are common reasons this matters.

You do not add every appliance’s full starting wattage together. The current SizeDial method adds all selected running watts, then adds the largest extra starting surge among the loads. That is a practical planning approach used by generator manufacturers for estimating a set of loads that can run together.

How do I calculate how many generator watts I need?

1. Choose the appliances and devices you actually care about during the outage or trip.
2. Add their running watts.
3. Find the largest extra startup requirement among the motor-driven loads.
4. Add that one largest extra surge to the total running load.
5. Choose a generator class that can cover the resulting requirement, then verify voltage and connection needs.

The calculator does those steps for its built-in loads. Manual wattage entry is there for equipment that is not on the list or when you have a better manufacturer-specific number.

Generator wattage by appliance

These are the planning values currently built into SizeDial. They are useful for estimating, not for overruling the equipment’s own nameplate or manual.

Real products vary. Check the appliance label, manual, or manufacturer specification when you need a more exact plan.

What size generator do I need for a refrigerator?

A refrigerator is a classic example of why starting watts matter. The compressor can need a brief burst of extra power when it starts. If the refrigerator is the only important load, a smaller generator class may be enough, but most people also want lights, chargers, a furnace blower, a freezer, or a sump pump. Build the list around what you actually need during the outage.

Refrigerator, lights, and chargers

Using the current SizeDial planning values, a refrigerator/freezer load plus a few household lights and laptop/phone charging stays within a relatively modest portable-generator requirement. The refrigerator’s startup surge still drives the peak more than the lights or chargers do.

Refrigerator and furnace blower

This combination can push the requirement up quickly because the blower motor may have a larger startup demand than the refrigerator. The current presets produce a planning peak of about 2,542 watts, which maps to a 3,000-watt SizeDial class. Your actual furnace voltage and connection method still need to be verified.

Refrigerator and sump pump

With the current SizeDial planning values, this pair produces a peak around 2,000 watts. That does not mean every refrigerator and sump pump pair will fit the same generator. Pump size and actual startup current vary, so manufacturer values matter.

Refrigerator, furnace, sump pump, and lights

This is the kind of outage combination many homeowners actually care about. Using SizeDial’s current planning estimates, the set reaches a peak around 3,642 watts and maps to a 4,000-watt class. It is a useful illustration of why choosing by one appliance at a time can underestimate the generator you need.

What size generator do I need for my house?

“Run my house” can mean two very different things. One homeowner means refrigerator, furnace blower, sump pump, a few lights, and internet equipment. Another means electric water heating, central air, cooking equipment, well pumps, and nearly everything else. Those are not the same portable-generator problem.

These are planning patterns, not universal wattage promises. The actual labels or manuals for your equipment take priority over generic estimates.

SizeDial is strongest when you identify the loads you actually want during an outage. If your plan involves major 240-volt equipment, hard-wiring into household circuits, or trying to power most of the home, treat the calculator as an early planning estimate. An electrician or appropriately qualified installer should verify the transfer equipment, wiring, voltage, and generator setup.

What can a 2,000-watt generator run?

A 2,000-watt class is best thought of as an essentials-scale generator, not a whole-house unit. Small electronic loads are easy, but a refrigerator or pump can consume much of the available starting capacity for a moment. The exact combination depends on the real appliance labels.

What can a 3,500-watt generator run?

Around 3,500 watts gives more room for several household essentials, especially when only one motor load has a large startup surge at a time. It can be a practical class for an outage plan built around refrigeration, some lighting/electronics, and selected mechanical loads. It does not guarantee that every furnace, pump, or air conditioner will fit.

What can a 5,000-watt generator run?

A 5,000-watt class can cover broader essential-load combinations, but the same rules still apply. Running watts, the largest startup surge, voltage, and how the generator is connected all matter. More watts do not automatically make a setup appropriate for powering household wiring.

What can a 7,500-watt generator run?

A 7,500-watt class gives substantially more room for heavier combinations. At this point, 240-volt loads and connection details are more likely to matter. If the goal is whole-home backup rather than selected portable-generator loads, move from a simple shopping estimate to a proper electrical assessment.

Generator size classes at a glance

Should I buy a bigger generator than the calculation says?

A little breathing room can be useful if your real appliance numbers are uncertain or you expect to add another modest load. Buying dramatically larger without a reason adds cost, size, weight, fuel use, and noise. The better approach is to make the load list realistic, use manufacturer wattages when available, and choose a class that covers the requirement with sensible margin.

How to find an appliance’s wattage

Look for a label or nameplate on the appliance, then check the owner’s manual or manufacturer specification if the label is unclear. If it lists both running and starting watts, use both. If it lists only watts for a simple resistive or electronic load, the starting value may be the same or close. Motors and compressors are the equipment most likely to need a higher starting value.

Do not confuse watts with watt-hours. Watts describe power at a moment in time. Watt-hours describe energy over time and are more central when sizing batteries or portable power stations.

Common generator-sizing mistakes

• Adding only running watts and ignoring motor startup.
• Assuming every refrigerator, furnace, or pump uses the same wattage.
• Planning to run every large appliance at once because the generator’s headline wattage sounds high.
• Confusing watts with watt-hours.
• Ignoring 120-volt vs 240-volt requirements.
• Assuming a generator’s wattage rating means it can be connected directly to household wiring.

When the generator’s real-world output may differ

The wattage on a generator is a model-specific rating, and actual available output can depend on operating conditions. Altitude, temperature, and fuel type can matter on some models. SizeDial does not apply a universal derating percentage because manufacturers handle those conditions differently. If you expect to run the generator at higher elevation, in unusual temperatures, or on a different fuel, check that model’s manual and rating information before buying.

How SizeDial calculates generator size

The generator engine totals the running watts of the selected loads. For each motor or compressor, it looks at the difference between starting and running watts. It adds the largest of those extra startup demands to the total running load, then maps the result to the next configured generator class.

The result also flags loads that may involve 240 volts and very large plans that should not be treated as a routine portable-generator shopping decision. Preset wattages are planning estimates, not universal appliance specifications.