Planning for a Power Outage by the Numbers: Watts, Watt-Hours, and Which Backup Fits

Most outage planning starts at the wrong end. Someone sees a portable generator or a power station on sale, buys it, and only works out what it can run when the lights go out. Done that way round, you find out at the worst moment that the generator can’t start the furnace, or that the power station drains flat at 3 a.m. with the freezer still on it.

The better order is to decide what has to keep working, put two numbers on each item, and only then pick the backup. The two numbers are different, and mixing them up causes most of the mistakes. Watts describe how hard a device pulls at any one moment. A generator or an inverter has to supply that. Watt-hours describe how much energy the device uses over a stretch of time, and a battery has to store that. A generator mostly cares about the first number, since it can keep running as long as you have fuel. A battery cares about both.

This guide walks through one household’s plan from start to finish. The numbers are an example. Your house will be different, so the method is the part to take away.

Start with a short list, not the whole house

The household here is a family of four in a cold climate. They have a gas furnace and municipal water, so there’s no well pump to worry about. A three-day winter outage is the realistic worst case. They listed only what actually matters:

1. Furnace blower, because a gas furnace won’t heat the house without its electric fan
2. Refrigerator
3. Chest freezer in the basement, full of meat
4. A handful of LED lights in the evening
5. Modem and router, so phones and work laptops stay connected
6. Phone and laptop charging
7. The TV, which they marked as optional

Everything else was deliberately left off: the electric dryer, the dishwasher, the microwave, the coffee maker and the space heater. You can always add a load later if the numbers allow it. When you start with everything and try to cut back, you usually end up buying too much.

The one item that needs extra thought is the furnace. It’s hardwired, not plugged into an outlet, so a portable source can only run it through a properly installed transfer switch or an inlet that an electrician has wired for it. Keep that in mind now, because it affects the budget as much as the generator does.

Turn each item into watt-hours per day

For energy, the useful unit is kilowatt-hours per day. The Appliance Electricity Cost Calculator shows it as the first row of its results table. Enter a wattage and the hours per day and read the kWh per day line. The cost rows don’t matter here. The tool only rejects a negative price, so you can leave the rate at zero and keep just the energy figure.

Watts × hours has a catch with anything that has a compressor. The fridge and freezer don’t draw their nameplate wattage all day. They cycle on and off, which is why the electricity-bill guide scales nameplate wattage down by a duty cycle before it multiplies. This family took the other route that guide suggests. They put both appliances on a plug-in watt-meter for 24 hours. The furnace blower is harder to measure. They used its 500 W motor rating and estimated it runs about 8 hours across a cold day.

Load	How they got the figure	Energy per day
Refrigerator	Watt-meter, 24 h	1,300 Wh
Chest freezer	Watt-meter, 24 h	900 Wh
Furnace blower	500 W × 8 h	4,000 Wh
LED lights	6 bulbs × 9 W × 6 h	324 Wh
Modem + router	20 W × 24 h	480 Wh
Phones + laptops	60 W × 3 h	180 Wh
TV (optional)	100 W × 3 h	300 Wh
Total		7,484 Wh

The furnace blower uses more than half of the total on its own. The fridge and freezer, which most people worry about first, come to less than a third. In winter, heat is the big energy cost even with a gas furnace, because moving warm air around takes a fan running for hours.

Take the furnace out and the “essentials” (fridge, freezer, lights, internet, charging) come to 3,184 Wh per day. That’s an average draw of 3,184 ÷ 24 ≈ 133 W. You’ll use both of these figures again below.

Sizing a generator: the surge decides it

A generator has fuel in reserve, so total energy per day only tells you how much fuel to keep on hand. The size of the generator depends on peak watts, and the peak comes from motors starting. The site’s generator guide explains why the starting surge, not running watts, sets the size, so this post won’t repeat it. Here’s how this household’s list works out in the Generator Sizing Calculator, using its default rows:

Checked row	Running W	Starting W
Refrigerator / Freezer	700	2,200
Chest freezer	300	1,800
Furnace fan (gas furnace blower)	800	2,350
Lights (per 100W of bulbs)	100	100
Laptop / phone chargers	100	100
TV	200	200

Running total: 2,200 W. The calculator then picks the one appliance whose starting watts sit furthest above its running watts. That’s the furnace fan, which jumps 1,550 W. It adds that single jump on top: 2,200 + 1,550 = 3,750 W starting. Add the tool’s 20% margin and you get 4,500 W, which rounds up to a 5,000 W standard size.

Two points about that result.

First, the default rows are conservative chart values. The 700 W fridge figure is far above what the watt-meter suggested this fridge averages. Part of the reason is that frost-free fridges run a defrost heater of several hundred watts for short periods, and a generator has to cover that moment as well. If your appliance labels give lower running and starting figures, type them in and the recommendation will fall. Don’t lower them on a hunch.

Second, check what the result is made of. Uncheck the furnace fan and the running total falls to 1,400 W. The largest jump is then shared by the fridge and the freezer at 1,500 W each, so starting watts are 2,900 W, the margin takes it to 3,480 W, and the recommendation drops to 3,500 W. A whole size bracket depends on one hardwired appliance, and that appliance also needs a transfer switch. Find this out before you shop, not after.

Sizing a battery: usable watt-hours, not the number on the box

Batteries are the opposite case. Peak power is the inverter’s job and is usually easy to meet. The hard part is the energy. The battery runtime guide covers the basic runtime formula and the efficiency factor. For outage planning, two losses come on top of each other.

Inverter loss. A power station converts its DC battery to 120 V AC, and some energy turns into heat along the way. Around 85% is a fair planning figure.

Depth of discharge. Lithium iron phosphate (LiFePO4) power stations are built to use most of their rated capacity. A lead-acid battery, such as an AGM bank, lasts much longer if you only take about half of its charge on a regular basis. A “2,400 Wh” lead-acid bank therefore gives you about 1,200 Wh before the inverter even takes its share.

The Battery Life Runtime Calculator has a single efficiency field, and you can put both losses into it. Set capacity to Wh and the load to W. The tool still needs a voltage above zero, but in Wh/W mode the value doesn’t change the result, so enter the battery’s nominal voltage. Then, for this household’s 133 W average essentials load:

Setup	Capacity	Efficiency entered	Runtime shown
LiFePO4 power station	2,048 Wh	85% (inverter only)	13h 07m
Two 12 V 100 Ah AGM batteries + inverter	2,400 Wh	43% (50% DoD × 85%)	7h 47m

The lead-acid bank has the bigger number on its label and still runs for about five hours less. To cover the full 3,184 Wh day, you need about 3,184 ÷ 0.85 ≈ 3,750 Wh of lithium, or about 7,500 Wh of lead-acid. In either case a battery-only plan for the furnace (another 4,000 Wh a day) is out of reach for most homes. Unless you can recharge from solar, the batteries will cover the essentials while something else handles the heat.

One more check: the runtime calculator assumes a steady load, but the fridge and freezer start and stop. Averaging them over the day is fine for energy, but the inverter’s continuous rating still has to exceed everything that can run at the same time, and its surge rating has to cover a compressor starting. Compare those two inverter figures with the running and starting totals from the generator table above.

You don’t have to run everything around the clock

The plan doesn’t have to keep everything running for all 24 hours. Food safety guidance from the USDA is that a closed refrigerator keeps food safe for about 4 hours without power, and a full freezer holds its temperature for about 48 hours (about 24 if it’s half full). If you keep the doors shut, the fridge and freezer don’t have to run continuously.

That changes the battery numbers. If the family powers the fridge and freezer in blocks, say two hours on and two off for the fridge and a few hours twice a day for the freezer, the battery covers the gaps instead of the whole day. With a generator, it means running for a few hours in the morning and evening to bring both appliances back down to temperature, run the furnace through a warm-up cycle and charge the power station. Overnight the power station runs the router and a light. That combination usually costs less and uses less fuel than either option alone. Check your own fridge’s temperature with a thermometer before you rely on any schedule. If you’re in doubt about food that has warmed, throw it out.

Two safety rules that override the numbers

Carbon monoxide. Never run a generator indoors, in a garage, a basement or a crawlspace, and never near a window, door or vent, even if the garage door is open. Generator exhaust contains carbon monoxide, which you can’t see or smell and which can kill in minutes. Put it outdoors and well away from the house, and install battery-powered CO alarms inside. A power station has no exhaust, which is one practical reason to use one indoors overnight.

Backfeeding. Never connect a generator to the house wiring through a wall outlet, including with a double-ended “suicide cord.” Electricity can flow back out onto the utility lines and hit a lineworker who believes the line is dead. It’s also a fire risk. The only safe way to power hardwired loads like the furnace is a transfer switch or interlocked inlet installed by a licensed electrician.

This is general information to help you plan, not electrical or professional advice. Have an electrician confirm anything that connects to your panel, and if anyone in the house depends on powered medical equipment, plan for it with the equipment supplier.

Run your own list

Start with the Appliance Electricity Cost Calculator to turn your short list into kWh per day. Check the generator option in the Generator Sizing Calculator, with your nameplate numbers in place of the defaults. Then test a battery against your average load in the Battery Life Runtime Calculator, with the efficiency field lowered to cover both the inverter and depth of discharge.