Sourced from manufacturer specs · EIA outage data · Updated Oct 27, 2026
Add the running watts of everything you need on at once, then add the single largest starting (surge) watts. Using Generac's chart, a fridge, furnace fan, sump pump and well pump need about 3,625 running and 5,925 peak watts. For whole-house standby, Generac says a 2,000 sq ft home typically needs 20–22 kW.[Generac][Generac]
Quick start: Use the calculator with your appliances, then match the size band to a battery, portable or standby generator.
Best for: Loads up to 2,200 running watts and 4,400 W peak, such as a fridge, TV and computer
Best for: 120V essentials up to 3,600 W; Jackery says two units connect for 240V loads such as well pumps and dryers
Best for: Running loads up to 7,200 W and 14,400 W surge, including 240V
Need more than about 7 kW running, or want automatic fuel backup? See standby installed cost →
How it works: Peak watts = total running watts of everything you tick + the single largest extra starting watts. This is Generac's sizing formula, and it assumes only one motor starts at a time.[Generac] Starting values come from Generac's portable generator wattage chart and are approximate; replace them with your appliance nameplate numbers.[Generac] Size bands are our own grouping, not a manufacturer standard. This is a planning estimate, not a load calculation; a licensed electrician or dealer should confirm any home connection.
| Item (tick what must run at once) | Running W | Extra starting W |
|---|---|---|
Battery Wh = running watts × hours ÷ usable %. It treats every ticked item as running nonstop, so it overstates cycling loads like a refrigerator; the fridge figure on the EnergyGuide label gives a closer daily number (see home battery backup). State hours are the 2024 total outage time per customer from EIA Form EIA-861, not the length of one outage.
Examples use Generac's approximate wattage chart.[Generac] Running = all items on together; peak adds the largest extra starting watts.
| Size band | Example load (Generac chart) | Running / peak | Type | Next step |
|---|---|---|---|---|
| About 2 kW | Refrigerator, 27 in TV, computer | 2,000 W / 4,200 W | Battery station or small inverter generator | Battery picks |
| 3–5 kW | Refrigerator, furnace fan, sump pump, well pump | 3,625 W / 5,925 W | Mid-size portable or larger battery | Power stations |
| 7.5–10 kW | The above plus microwave, washer, TV | 6,275 W / 8,575 W | Large portable or dual-fuel, with transfer switch | Connecting it |
| 14–18 kW | 2-ton central AC, fridge, well and sump pumps, electric water heater, washer, microwave, TV | 11,200 W / 16,150 W | Home standby | Installed cost |
| 20–28 kW | Heat pump, electric range element and water heater, fridge, well and sump pumps, washer, microwave, coffee maker, TV | 15,700 W / 20,200 W | Home standby (Generac air-cooled: 10–28 kW) | Generac cost by model |
Band edges are our grouping. Generac says starting watts are often two to three times running watts for motors, and that a load manager or soft start kit can let a smaller standby unit handle big AC start-ups.[Generac]
A 3–5 kW-class portable or a larger battery can cover this.[Generac]
One AC more than doubles the peak. Soft start or load management can help.[Generac]
EnergyGuide kWh ÷ 365. About 57 W on average, because the compressor cycles.[DOE]
Running (rated) watts are what an appliance draws while it runs. Starting (surge) watts are the short burst a motor needs to get going. Generac says that burst is often two to three times the running figure, and that sizing for it is what keeps a generator from tripping every time the AC cycles on.[Generac] Refrigerators, sump and well pumps, furnace blowers and air conditioners all have motors or compressors, so they're the items that set your peak.
Generac's portable generator chart lists running watts and additional starting watts for each item. Add up the running watts for everything on at once, then add the one largest additional starting figure. Generac notes its figures are approximate and assume devices start one at a time.[Generac]
Approximate figures from Generac's chart. Your appliances may differ, so check each nameplate.
| Appliance | Running W | Extra starting W | Peak if it starts last | Source |
|---|---|---|---|---|
| Refrigerator / freezer | 700 | 2,200 | 2,900 | Generac |
| Furnace fan blower (1/2 HP) | 875 | 2,300 | 3,175 | Generac |
| Sump pump (1/2 HP) | 1,050 | 2,150 | 3,200 | Generac |
| Well pump (1/2 HP) | 1,000 | 2,100 | 3,100 | Generac |
| Microwave (1,000 W) | 1,000 | 0 | — | Generac |
| Coffee maker | 1,500 | 0 | — | Generac |
| Television (27 in) | 500 | 0 | — | Generac |
| Computer with 17 in monitor | 800 | 0 | — | Generac |
| Washing machine | 1,150 | 2,300 | 3,450 | Generac |
| Garage door opener (1/2 HP) | 875 | 2,350 | 3,225 | Generac |
| Electric water heater | 2,000 | 1,000 | 3,000 | Generac |
| Electric range (one 8 in element) | 2,100 | 0 | — | Generac |
| Window AC (12,000 BTU) | 3,250 | 3,950 | 7,200 | Generac |
| Central AC (24,000 BTU, 2-ton) | 3,800 | 4,950 | 8,750 | Generac |
| Heat pump | 4,700 | 4,500 | 9,200 | Generac |
| Outdoor light string | 250 | 0 | — | Generac |
| Slow cooker | 250 | 0 | — | Generac |
| Wi-Fi router and modem | Check the power adapter label | Nameplate | ||
| LED light bulbs | Check the watts printed on each bulb | Nameplate | ||
| Medical devices (CPAP, oxygen concentrator) | Check the device label and ask the maker about generator or battery use | Nameplate | ||
“Peak if it starts last” = running + extra starting watts for that item alone. Generac's own sizing guide says a 3-ton central AC can need 3.5–5 kW running and up to 15–18 kW to start, far more than the 2-ton chart row, so use your unit's nameplate.[Generac]
Generac says a 2,000 sq ft house typically needs a 20 kW or 22 kW standby unit for whole-home coverage, and that smaller units can power the essentials.[Generac] It also says two homes of the same size can need very different generators: gas heat and city water need less than an all-electric home with a heat pump, a well pump or an EV charger. For a 1,550 sq ft home, Generac puts the range at 10–18 kW.[Generac] Square footage is a starting point. Your loads decide the size, which is why the calculator asks what you run, not how big the house is.
These lists use Generac's chart figures.[Generac] In each case, the generator's running watts must cover the running total and its starting watts must cover the peak. Check both numbers on the spec sheet.
Refrigerator, furnace fan, sump pump, well pump, microwave and a TV: 5,125 W running, 7,425 W peak.
The same list plus a washing machine and outdoor light string: 6,525 W running, 8,825 W peak.
A 2-ton (24,000 BTU) central AC with a refrigerator, well pump, sump pump, microwave and TV: 8,050 W running, 13,000 W peak, because the AC adds 4,950 W to start. A soft start kit on the AC may lower that peak; ask your HVAC contractor.[Generac]
How long your power stays out matters as much as how many watts you need. In 2024, customers in Texas averaged 1,614 outage minutes, North Carolina 1,518 and Florida 1,488, against a median state of 297, per EIA data on our outages by state page. Those are yearly totals per customer, mostly on major event days (usually storms), not the length of one outage.
Carbon monoxide: CPSC says to operate portable generators outside only, at least 20 feet away from the house, with the exhaust directed away from the home, and never inside a home, garage, basement, crawlspace, shed or on a porch. Install battery-operated CO alarms, or CO alarms with battery backup, on each floor and outside sleeping areas.[CPSC]
CPSC warns never to power house wiring by plugging a generator into a wall outlet, a practice called backfeeding. It says this presents an electrocution risk to utility workers and neighbors served by the same transformer and bypasses some household circuit protection.[CPSC] OSHA says attaching a generator to a building's electrical system without a properly installed transfer switch can energize wiring for great distances, and that a qualified electrician should install it.[OSHA] Generac adds that most jurisdictions require an electrical permit and inspection for a transfer switch.[Generac] Our home generator installation guide covers transfer switches, permits and timing.
Generac says a professional site survey is the only way to be fully accurate: an authorized dealer measures starting and running watts, inspects your panel and confirms the size before anything is ordered.[Generac] Budget with our whole house generator cost guide and Generac cost by model. Our method is on how we rank.
It depends on what runs at the same time, not just floor area. Generac's formula is total kW required = the sum of all running watts + the highest single starting burst. Generac says a 22 kW standby unit is the industry standard for whole-home coverage of 2,500 to 3,000 sq ft homes, and that load managers can help a unit prioritize critical appliances. A dealer load analysis gives the final number.
Generac says a 2,000 sq ft house typically needs a 20 kW or 22 kW standby unit for whole-home coverage, and that smaller units can power the essentials. Central air, a heat pump, a well pump or electric appliances change the answer, so add up your own loads with the calculator on this page.
Usually not everything at once. Using Generac's chart, a 24,000 BTU central AC alone needs 3,800 running watts plus 4,950 extra watts to start, or 8,750 W at start-up. A generator with 6,500 running watts can cover essentials such as a refrigerator, furnace fan, sump pump and well pump (3,625 running, 5,925 W peak) if its starting rating covers the peak.
Generac's chart lists a refrigerator or freezer at about 700 running watts and 2,200 extra starting watts. Two units come to 1,400 running watts and 3,600 W peak, assuming they don't start at the same moment. Check your nameplates, since actual figures vary by model.
For some homes and outages. Jackery lists the Explorer 5000 Plus at 5,040 Wh with 7,200 W rated output (14,400 W surge) and 120V/240V support, and the Explorer 2000 v2 at 2,042 Wh with 2,200 W output (4,400 W surge). A battery runs until its stored energy is used up and then needs recharging, while a fuel generator runs as long as it has fuel. See our home battery backup guide for runtime math.