What size generator do I need is the first question Chicago homeowners ask after a ComEd outage knocks out heat, sump pumps, and refrigerators. Generators are sold by watts and kilowatts, but your correct size comes from listing what must run at the same time and accounting for startup surges from motors and compressors. Oversizing wastes money and fuel; undersizing trips breakers the moment your furnace fan and fridge start together.
Start With Essential Loads, Not the Whole House
Most Chicago bungalows and two-flats target essential circuits first:
Typical essential circuits
Furnace blower and controls (gas heat still needs electricity). Sump pump in flood-prone basements. Refrigerator and freezer. A lighting and receptacle circuit for main living areas. Modem and phone charger paths for communication. Garage door opener if vehicles must leave during extended outages.
Adding window AC, electric range, electric dryer, or whole-house central AC jumps wattage requirements sharply. Those loads often need larger standby units or deliberate load shedding, not a mid-size portable generator.
Running Watts vs Starting Watts
Appliances list steady-state running watts and higher starting (surge) watts. Motors draw several times normal current for a second at startup. Generator sizing must cover the simultaneous running total plus the largest single surge that can occur while other loads are already on.
Example pattern (your equipment may differ; verify nameplates):
Furnace blower 400 to 800 running watts, higher surge. Refrigerator 100 to 800 running, surge up to 1200. Sump pump 800 to 1500 running, surge higher. Lights and electronics 200 to 400 combined. Add running watts, then add the biggest remaining surge not already counted.
A portable 5,000 to 7,500 watt generator often covers essentials in small homes if you manage loads manually. Whole-home comfort with central AC may require 20 kW or more standby capacity plus transfer equipment rated for the service.
Load Calculation Concepts
Professional sizing uses a structured load table:
Step 1: Inventory
Walk the panel with us or note every circuit you want during an outage. Read motor nameplates where accessible.
Step 2: Continuous vs intermittent
Mark which loads run nonstop versus occasionally. Stagger starting large motors when using portable generators.
Step 3: Diversity factor
Not every circuit hits peak at once. Calculations apply reasonable diversity for essentials-only setups. Whole-home standby quotes may use more conservative totals aligned with service size.
Step 4: Match transfer equipment
Generator capacity must align with your transfer switch installation and the circuits it feeds. A 30-amp manual switch limits total load regardless of generator nameplate.
Portable vs Standby Generators
Portable generators
Lower upfront cost, manual setup, gasoline storage, and cord or inlet connection through a transfer switch or interlock. Suitable for occasional outages and managed essentials. Must operate outdoors only, far from windows and intakes, to prevent carbon monoxide entry into Chicago homes sealed tight against winter cold.
Standby generators
Permanently mounted, automatic start with ATS, natural gas or propane fuel. Higher installed cost, quieter operation, and best fit for frequent outages or medical needs. Requires gas plumbing, pad, and permitted electrical connections documented in our emergency power systems guide.
Service Size and Panel Limits
Your main electrical service (100, 150, 200 amp or larger) bounds what a whole-home generator can feed. A 100-amp Chicago bungalow with an aging panel may need service or panel upgrades before adding a large standby unit, even if the generator itself is sized correctly.
We review breaker spaces for generator breakers, interlocks, or ATS connections during sizing visits. Crowded panels may need a subpanel for backed-up circuits.
Chicago Climate Considerations
Winter outages threaten frozen pipes when furnaces stop. Summer storms flood basements when sump pumps die. Lake-front neighborhoods see longer restoration times after major weather events. Size for the season that threatens your household most, not average weather.
Electric heat homes (rare in older Chicago gas-heated stock) need substantially larger generators than gas-forced-air homes. Know your heat source before shopping.
Fuel Storage and Runtime
Portable gasoline generators need fresh fuel and stabilizer; runtime depends on tank size and load percentage. Propane and natural gas standby units run longer without refueling trips. We do not state generic runtime hours because load and fuel type dominate outcomes.
Permits and Safe Connection
Any connection to your panel beyond extension cords requires listed transfer equipment, permits, and inspection in Chicago. Backfeeding through a dryer outlet is illegal and dangerous. Budget switching work alongside generator purchase.
Related repairs to damaged service equipment after storms fall under our electrical repair service. Emergency service addresses immediate hazards; generator planning is proactive work best done before the next outage headline.
Practical Sizing Outcomes
Essentials-only portable plan: often 5 to 9 kW range after surge math, with manual load management. Large essentials plus one window AC: may step into 7 to 12 kW portable or small standby territory. Whole-home with central AC: typically standby 17 to 22 kW class units with automatic transfer and professional load analysis.
These bands are planning guides, not substitutes for a site-specific calculation. Manufacturer online calculators help for shopping; final specification should match your panel and fuel choice.
Working With an Electrician on Sizing
Bring your appliance list, fuel preference, and outage history to the consultation. We measure starting currents where possible, map circuits to a transfer switch layout, and flag panel constraints before you commit to a generator model at a big-box store.
Generator sizing and panel upgrades often land in the same conversation for older Chicago homes. Addressing both in one plan avoids buying a unit your service cannot support safely.
Load Management During an Outage
Even correctly sized portable generators require discipline. Start the largest motor load first, then add smaller circuits. Turn off window AC before restarting the refrigerator if breakers trip. Label switched circuits at the transfer switch so family members know what is available.
Noise, Placement, and Neighbors
Portable generators belong outdoors with exhaust pointed away from openings. Chicago lot lines mean neighbor noise complaints during extended outages. Standby units in sound-rated enclosures cost more but reduce conflict. Electrical scope includes inlet and switch placement; generator physical location follows manufacturer clearance rules and municipal setback expectations.
When to Revisit Generator Size
Finished basements, new EV chargers, heat pumps, or second refrigerators change your load profile. Re-run sizing math before upgrading transfer equipment or buying a larger standby unit. An undersized generator you tolerated for years may fail after a single major appliance addition.
Records to Keep After Sizing
Save your load table, generator manual, transfer switch diagram, and inspection card together. When you add circuits later, update the table before assuming the same generator still fits. Buyers and insurers increasingly ask whether backup power was installed with permits.
Start sizing conversations before the first winter storm warning or summer derecho forecast. Lead times for transfer switches, panels, and popular generator models stretch when outages dominate the news cycle.
Get a Generator Sizing Consultation
STS Electric helps Chicago homeowners size generators, specify transfer switches, and install permitted connections. We inventory your circuits, explain trade-offs between portable and standby paths, and coordinate with your generator supplier if needed.
Call (773) 721-1111. Office: 939 W North Ave #746, Chicago, IL. Schedule before storm season when demand spikes.
