Critical Loads First: How to Size Backup Power Before You Buy a Generator

1. Why Critical Loads Come Before Any Generator Purchase

The right generator is the one sized to your list, not the one on sale.

Picture the scene. A regional grid failure has hit, and you're standing in a big-box store aisle, comparing boxes that shout about starting watts and run hours. You grab the one with the biggest number and a friendly price. At home, you learn it can't start your well pump. Or you learn it's far more machine than you need to keep a fridge, a CPAP, and a router alive.

Most backup power mistakes start at the store. People buy by brand or headline wattage instead of by what the house actually needs.

A critical load is any device your household can't safely or reasonably go without during an outage. Common examples are refrigeration for food and medicine, powered medical devices, a well pump, a furnace blower, communications, and lighting. Ready.gov tells households to make alternate plans for refrigerating medicines and powering medical devices that depend on electricity. [3] Those items anchor your list.

Solar-plus-battery guidance from DOE's Pacific Northwest National Laboratory groups selected essential circuits on a dedicated critical-loads panel rather than backing up everything. [1] Backing up a short list of essentials is cheaper and simpler than powering the whole house, and it stretches limited fuel or battery much further. [1] The size of the savings depends on your house, so treat that as a principle, not a promise.

A grid failure can knock out lighting, refrigeration, heating or cooling, and device charging all at once. That's why you have to prioritize. The next two sections walk you through a worksheet and a three-number calculator. You'll finish with a target capacity before you compare a single product.

2. Step-by-Step: Build Your Critical-Load Worksheet

Grab a pen or open a spreadsheet. You'll build one table, with one row per device, and fill it in over five steps. Here's the layout to copy:

Step 1: List every device you'd want running in a multi-day outage. Walk the house room by room. Include the refrigerator, freezer, medical devices such as a CPAP or oxygen concentrator, well pump, furnace blower, sump pump, internet router, phone and radio chargers, and LED lighting. Don't judge yet. You'll trim later.

Step 2: Record running watts. Check the nameplate or label, the owner's manual, or a plug-in watt meter. If the label lists only amps, use watts = volts × amps. Treat that result as an estimate. For motors, volts × amps gives apparent power, which usually overstates real watts, so your number will tend to run high. That's the safe direction to err.

Step 3: Record starting (surge) watts. Motor-driven loads like refrigerators, well pumps, sump pumps, and furnace blowers briefly draw much more than their running watts as they start. Check the manual or ask the manufacturer for the figure, because surge ratings vary widely between models. Leave the cell blank for anything without a motor, since those loads don't surge the same way.

Step 4: Estimate hours of use per day, then calculate daily watt-hours. The formula is running watts × hours per day. Refrigerators cycle on and off, so a watt meter left on for 24 hours beats guessing. The meter reports real energy use, and you can enter that figure directly.

Step 5: Assign a runtime priority.

• Tier 1 is life safety: medical devices, heat in freezing weather, and a well pump if it's your only water.
• Tier 2 is food preservation and communications.
• Tier 3 is comfort items.

Ready.gov advises planning ahead for anything that depends on electricity (medical devices belong at the top). [3]

Refrigeration timing also shapes your priorities. CDC and FoodSafety.gov guidance says refrigerated perishable food should be discarded after 4 hours without power, as long as you've kept the door closed. A full, closed freezer keeps food safe for about 48 hours, and a half-full one for about 24. The fridge can't wait long, but the freezer can ride out a short gap.

Check the voltage on every motor load. Some well pumps and furnaces are hardwired or run on 240V, so they may not plug into an extension cord at all. Mark those rows. They help determine which equipment and transfer gear you need, and your electrician or the equipment manual can confirm the details. You'll want that answer before you shop.

Every wattage in this article's examples is illustrative only, so check your nameplate. Appliance draws vary widely, and your numbers are the only ones that count.

Print the Critical-Load Worksheet and fill it out this weekend. You'll know your target capacity before you ever compare prices.

3. Run the Numbers: Peak Watts, Daily Energy, and Days of Runtime

Your finished worksheet feeds three numbers. Each answers a different question, and each sizes a different part of the system.

Number 1: Peak capacity. Add the running watts of everything that might run at the same time. Then add the single largest starting surge.

Peak watts = (sum of simultaneous running watts) + (largest single starting surge)

That total is the minimum output rating to look for. It's a conservative rule of thumb, not a standard from our sources. It double-counts slightly, because the device with the biggest surge is already in your running total. That extra cushion helps when you're guarding against a stalled generator. A generator-sizing guide from your manufacturer can refine it.

Number 2: Daily energy. Add the daily watt-hours for all Tier 1 and Tier 2 loads.

Daily Wh = sum of (running watts × hours per day)

This figure sizes batteries and estimates fuel use. DOE explains that battery storage lets solar keep supplying electricity when the sun isn't shining. [2] Daily watt-hours, not just peak watts, determine battery size. [1]

Number 3: Runtime days. For batteries, use this formula:

Runtime days = (usable Wh × inverter efficiency) ÷ daily Wh; use usable capacity, not gross rating. [1]

Inverter losses are real, so build in a margin. For fuel generators, multiply the manufacturer's stated fuel use per hour at your expected load by the hours you'll run each day.

Worked example (illustrative only)

Take a household with five loads:

• Refrigerator: 150 W running, 600 W starting, about 8 effective hours/day = 1,200 Wh
• CPAP: 40 W, 8 hours = 320 Wh
• Router: 12 W, 24 hours = 288 Wh
• LED lights: 30 W, 5 hours = 150 Wh
• Furnace blower: 400 W running, 800 W starting, 4 hours = 1,600 Wh

Peak: 150 + 40 + 12 + 30 + 400 = 632 W running; adding the 800 W surge gives 1,432 W. Round up for margin and shop for roughly 1,500 to 1,800 W of output.

Daily energy: 1,200 + 320 + 288 + 150 + 1,600 = 3,558 Wh.

Runtime: The 85% inverter efficiency here is an assumption, so swap in the figure from your product's spec sheet. 2,000 usable Wh at 85% efficiency delivers about 1,700 Wh; 1,700 ÷ 3,558 is about half a day. A generator burning 0.25 gallons per hour for 8 hours a day uses 2 gallons per day. That burn rate is also illustrative, so look up the real number for your model at your expected load.

Trim to fit

Staggering loads lowers the peak number and the cost. Don't start the well pump while the fridge compressor is starting. Shorten blower hours with blankets and a closed-off room. Every watt you cut here saves money later.

4. Compare Your Options: Portable, Inverter, Battery Station, Solar-Plus-Storage

Now you can shop with your three numbers in hand. Each option trades capacity, fuel logistics, noise, cost, and runtime differently. The table below is general guidance, not a ranking from a single source. Prices and specs vary by brand and by year.

Portable generators

• Pros: They generally give you the most output per dollar and can start heavy motor loads.
• Cons: They're loud, depend on stored fuel, and must run outdoors only.

If your Number 1 is high because of a well pump surge, this is often the practical answer.

Inverter generators

• Pros: They're generally quieter, can use fuel more efficiently at partial load, and typically produce cleaner power for electronics.
• Cons: They usually offer lower peak output for the price, and they still need fuel and outdoor-only use.

They suit a household whose Number 1 is modest and whose Number 2 runs long hours at low draw.

Battery power stations

• Pros: They're silent and produce no exhaust fumes at the unit. Many are designed for indoor use, so check the manual.
• Cons: Watt-hours cap everything. Recharging needs the grid, solar, or a generator, and some units can't handle large motor surges.

Lithium batteries also carry fire risks, so read the safety rules in Section 5 before you bring one inside. In the worked example, one battery covers only about half a day of the full list. It could easily cover the CPAP, router, and lights overnight.

Solar-plus-storage

• Pros: It recharges without fuel and is often paired with a critical-loads panel. Solar-plus-storage can provide backup power during electrical disruptions and help keep essential services like communications running. [2] PNNL describes systems that power selected essential circuits through that dedicated panel during outages. [1]
• Cons: Upfront cost is typically the highest of the four. Output varies with weather and season, and it usually calls for professional installation.

How long it runs depends on how well your array and battery are sized to your load.

A hybrid setup is worth considering. A battery can handle overnight quiet loads, and a generator can recharge it in short daytime runs. That pairing can cut noise, fuel burn, and exposure all at once. Whatever you pick, compare the product's output against Number 1 and its usable watt-hours against Number 2 before you trust the marketing.

5. Non-Negotiable Safety Rules: CO, Backfeed, Fuel, and Batteries

⚠️ Read before you buy, plug in, or fill anything

CARBON MONOXIDE KILLS.

• Run generators outdoors only, in well-ventilated locations away from doors, windows, and vent openings; never run one in an attached garage, even with the door open. [4]
• Never run a generator in a basement, crawlspace, or carport. [4]
• CDC warns that carbon monoxide from generators can kill. Install battery-operated CO detectors.
• CO is invisible and odorless. Never rely on smell.
• Follow the manufacturer's instructions and the CPSC, NFPA, and CDC guidance.

BACKFEED KILLS UTILITY WORKERS.

• Never plug a generator into a wall outlet; never connect to house wiring without a properly rated transfer switch or interlock installed to code by a qualified electrician. [4]
• Backfeed can send power back onto the grid and endanger line crews working to restore your power. It can also damage your home.

FUEL CAUSES FIRES.

• Store gasoline only in approved containers, away from living spaces and ignition sources, per CPSC guidance. [4]
• Keep containers in a cool, well-ventilated area.
• Never refuel a hot or running generator. Shut it off and let it cool first. [4]

LITHIUM BATTERIES CAUSE FIRES.

• Use only the battery and charging equipment designed for the device, and follow the manufacturer's instructions. [5]
• Don't charge devices on beds, couches, or under pillows. [5]
• Don't use damaged batteries. [5]
• Don't cover a charging device, and keep its vents clear so heat can escape.

These rules aren't suggestions to weigh against convenience. People get hurt when a storm, a cold night, and a tired household push someone to move a generator "just inside the garage for a few hours." Your worksheet already told you what you need, so you never need a shortcut.

Treat the transfer switch as part of the purchase price. A generator without a safe way to feed hardwired loads leaves you running extension cords through windows or, worse, improvising. An electrician may recommend a transfer switch or an interlock, depending on your panel.

Before buying anything that connects to your home, schedule a qualified electrician to quote a transfer switch or interlock.

6. Plan for the Long Outage: Fuel Limits, Sheltering in Place, and a Test Run

Imagine day four. The gas stations are dark, the roads are quiet, and your neighbor asks if you have any fuel to spare. Fuel may not be moving, so assume you can't buy more. Take your gallons-per-day figure from the worksheet and decide how many days you can safely and legally store. Then rotate that fuel according to the container and stabilizer instructions.

Sheltering in place changes the plan again. A multi-day stay at home favors quiet, fume-free power for overnight Tier 1 loads. Run the fridge in blocks instead of continuously, and keep freezer doors shut. CDC and FoodSafety.gov guidance says a full, closed freezer can stay safe for about 48 hours, or about 24 if half full, so every unopened door buys you time.

Keep the medicine plan alive too. Ready.gov tells households to make alternate plans for refrigerating medicines and powering medical devices that depend on electricity. [3] Ask your pharmacist how long each medicine can safely stay unrefrigerated.

Keep your setup quiet in every sense. Don't post about your generator, fuel, or layout publicly. Share the plan only with household members and trusted neighbors.

Finally, run a test. Hold a planned lights-off weekend and check your worksheet against real use. Confirm the CO alarms work, then update your priorities. Your watt meter will humble your estimates in a useful way.

Your action list:

1. Finish the worksheet.
2. Compute your three numbers.
3. Call an electrician about transfer equipment.
4. Buy CO alarms.
5. Then shop.

Next step: read our guide to storing fuel safely for extended outages, and lock in your fuel plan before the next storm.