3-day power outage plan by house size: what to keep on, and how many kilowatt-hours that is

Most outage advice is written for one imaginary house. Yours has a specific refrigerator, a pump or not, a medical device or not, and a rule about generators or not, and the plan has to be priced in the unit a battery is bought in. Four households below, each with an essentials list in watt-hours a day from this site’s sourced figures, the class that covers three days, and the recharge that makes a smaller class enough. The sizing calculator builds your own version in a minute.

The arithmetic, once

Daily watt-hours at the outlet, times three days, divided by 0.85 for the inverter’s loss, equals usable battery. That last figure reads straight across to the columns on every runtime table here and to the class labels on the roundups; the usable capacity guide explains the 0.85. A daily recharge, from solar or a vehicle, divides the battery by three and moves the problem to the panel, which the solar guide prices.

Household one: the apartment

No pump, no well, and usually a lease clause that bans generators, which makes this the household a battery was built for.

Load	Daily Wh at the outlet	Source
Refrigerator, 18 cu ft	about 1,250	ENERGY STAR, refrigerator page
Phones, router, a lamp	about 600	nameplates
CPAP with humidifier, if used	about 425	ResMed, CPAP page
Total	about 2,300 Wh

Three days is about 6,900 Wh at the outlet, roughly 8,100 Wh of usable battery, which is the 8 kWh class. Or a 2 kWh unit and a recharge each day from a car’s 12 V socket or a neighbour’s generator, which is how most apartments actually do it. The 2 kWh roundup ranks that class; UPS mode ahead of the refrigerator, per the pass-through guide, is the setup.

Household two: a house on city water, no sump

The apartment list plus a chest freezer and a gas furnace’s blower if it is on a transfer switch.

Load	Daily Wh at the outlet
Apartment list above	about 2,300
Chest freezer, 10.6 cu ft	about 600, per the freezer guide
Furnace blower, if wired in by an electrician	varies with the blower motor and run time; read its nameplate
Total, without the furnace	about 2,900 Wh

About 10 kWh of usable battery for three days unassisted, or a 2 to 4 kWh unit with daily recharge. The furnace blower is the one heating load a battery can plausibly carry, and only through a transfer switch a licensed electrician installs; the transfer switch guide explains what that is.

Household three: a house with a sump pump

Same as household two in a dry week. In a wet one the pump is the plan.

Load	Daily Wh at the outlet
Household two list	about 2,900
Sump pump, 1/3 HP at 25% duty in steady rain	about 6,700, per the sump pump page
Total in a storm	about 9,600 Wh

Three days of that is about 34 kWh of usable battery, which nothing portable holds. The honest plan is a dedicated 2 kWh unit for the pump alone, expandable, recharged daily, plus a second unit for the essentials, or an expandable system on a transfer switch from the expandable roundup. The pump is the load that cannot wait, so it gets its own battery.

Household four: a well and a sump

The largest lists, and a 240 V motor on it.

Load	Daily Wh at the outlet
Household three list, storm week	about 9,600
Well pump, 1/2 HP at 10% duty	about 2,760, per the well pump wattage guide
Total	about 12,400 Wh

Three days is about 44 kWh usable. This is installed-system territory, or a 240 V expandable system with a generator input running a few hours a day, and the home battery backup pillar and the battery versus generator page are where that decision lives. Fill the bathtub before the storm regardless; the well pump’s start-up draw, not its daily energy, is what makes it the hardest load in the house.

What every plan shares

• Charge everything full before the storm and leave the UPS-capable units plugged in ahead of the refrigerator and any medical device.
• Heat is not on any list. A 1,500 W heater is 36 kWh a day; the space heater page has the arithmetic and the alternatives.
• Food windows are USDA’s: 4 hours for a closed refrigerator, 48 for a full freezer. A thermometer in each decides.
• Generators live outside, 20 feet from any opening, with CO alarms inside, or not at all. That is the red callout above and it is not negotiable.
• Recharge on day one, not day three. A unit topped up from a vehicle each afternoon turns a 2 kWh battery into a three-day battery; a unit run flat by night two turns a three-day outage into a very long one.

The winter storm plan is this page for the season when heat is the problem; the calculator is this page for your own appliances.