How Fast Does Your House Lose Heat in a Winter Outage?

When the electrical grid fails during a winter freeze, your house immediately transitions from an actively heated living space into a slowly cooling thermal battery. How fast that stored heat escapes—whether your living room stays above 50°F for 36 hours or plunges toward 35°F before sunrise—is governed by two physical properties: your building thermal envelope’s heat-loss rate (UA) and the indoor-to-outdoor temperature gap (ΔT).

Before looking at backup heaters or batteries, start with the people and medical boundaries inside your household. An indoor temperature that merely feels chilly to a healthy adult in a parka (52°F) can trigger dangerous hypothermia in an infant or an older adult, or ruin temperature-sensitive medications.

Start with the household: who cannot tolerate a cold home

Before calculating British Thermal Units (BTU), identify the biological and medical temperature floors for everyone under your roof. A building’s pipes do not begin freezing inside exterior walls until indoor temperatures drop near 40°F to 45°F during deep sub-zero weather, but human physiological thresholds are much higher:

• Infants and Toddlers (Under 2 Years): Infants have a high body-surface-area-to-mass ratio and immature thermoregulation; they cannot shiver effectively to generate internal body heat. Pediatric sleep and health guidelines warn that prolonged exposure to indoor rooms below 60°F to 65°F increases hypothermia risk, and heavy loose blankets cannot be placed in an infant crib due to suffocation hazards (use wearable insulated sleep sacks and move to a heated location early).
• Older Adults (65+) and Reduced Mobility: According to the National Institute on Aging (NIA), older adults with reduced subcutaneous fat, slower metabolic rates, diabetes, hypothyroidism, or cardiovascular medications can develop indoor hypothermia in rooms sitting between 60°F and 65°F (15.5°C–18.3°C) over several hours.
• Temperature-Sensitive Medications: Liquid insulin, biologics, EpiPens, and certain pediatric antibiotics lose potency or denature if allowed to freeze (< 36°F / 2°C) on a cold countertop or windowsill. Store freeze-sensitive medications inside an insulated lunch cooler in your warmest interior room (without ice packs if the goal is preventing freezing) along with a digital min/max thermometer.
• Pets and Service Animals: Short-haired dogs, cats, birds, and reptiles experience cold stress rapidly below 50°F (10°C) and must be brought into your consolidated warm room.

The heat-loss formula, step by step (Q = UA × ΔT)

In building thermodynamics and National Institute of Standards and Technology (NIST) residential envelope modeling, a house loses heat through two simultaneous mechanisms:

1. Conductive heat transfer (U × A): Heat flowing directly through the solid materials of your roof/attic, exterior walls, windows, doors, and foundation slab or crawlspace.
2. Convective air infiltration (ACH): Warm indoor air escaping out of upper-story ceiling penetrations and attic hatches (the stack effect) while cold outdoor air is sucked in through sill plates, window frames, and dryer vents.

Engineers combine both conductive loss and air infiltration into a single whole-house Overall Heat Loss Coefficient (UA), measured in BTU per hour per degree Fahrenheit (BTU/hr·°F).

Step 1: The hourly heat-loss equation (BTU/hr)

Hourly Heat Loss (BTU/hr) = UA (BTU/hr·°F) × (T_indoor - T_outdoor)

Where:

• UA represents your home’s overall heat-loss coefficient (BTU/hr lost for every 1°F difference between inside and outside).
• T_indoor is the current or target indoor temperature (°F).
• T_outdoor is the outdoor ambient temperature (°F).

Step 2: Worked example for a 2,000 sq ft home at 20°F outdoors

Suppose you live in a typical 2,000 sq ft detached home built in the 1980s or 1990s with moderate wall/attic insulation (UA = 450 BTU/hr·°F). The indoor thermostat is at 68°F when a winter storm knocks out grid power, and the outdoor temperature is 20°F (ΔT = 68°F - 20°F = 48°F):

1. Calculate hourly heat loss in BTU/hr: 450 BTU/hr·°F × (68 - 20)°F = 450 × 48 = 21,600 BTU/hr
2. Convert BTU/hr into electrical kilowatts (kW): Since 1 kilowatt-hour (kWh) = 3,412 BTU of thermal energy: 21,600 BTU/hr / 3,412 BTU/kWh = 6.33 kW continuous electrical heat demand

Why battery power stations cannot heat a house

Look closely at that 6.33 kW (6,330 W) number. A popular 900 dollars portable battery power station stores 1,024 Wh (~870 Wh usable AC). Even if you plugged a single 1,500 W electric space heater (which produces 1,500 W × 3.412 = 5,118 BTU/hr, less than a quarter of the house’s 21,600 BTU/hr loss) into that 1,024 Wh battery:

870 Wh usable capacity / 1,500 W space heater = 0.58 hours (35 minutes of runtime)

In 35 minutes, your entire battery station is dead (0%), leaving you with no power for your Wi-Fi router, cell phones, CPAP machine, or LED lights. Never waste lithium battery capacity on resistive electric space heaters. Instead, use batteries to run the electric blower fan and electronic ignition of an existing natural gas, propane, or oil furnace (300 W to 600 W), or to power a 60 W heated electric mattress pad/blanket underneath a high-R-value sleeping bag.

Comparing home thermal envelopes: how fast does indoor temperature drop?

As your house cools during an outage, the indoor-to-outdoor temperature gap (ΔT) shrinks, which causes the temperature drop to follow an exponential decay curve (cooling fastest in the first 6 hours, then slowing as T_indoor approaches T_outdoor and the thermal mass of drywall, furniture, and concrete slabs releases stored heat).

Use the table below to estimate your home’s UA coefficient and initial heat loss at a 48°F temperature delta (68°F inside, 20°F outside). Replace the square footage or UA factor with your own home’s numbers:

The “Micro-Room” strategy: cutting heat demand by 80%

Trying to heat an entire 2,000 sq ft house during a blackout requires 20,000 to 35,000 BTU/hr. By abandoning outer rooms and consolidating everyone into a single 150 to 200 sq ft interior bedroom or den (ideally on an upper floor, facing south, with at most one exterior wall and window), you radically shrink the surface area losing heat:

• A whole 2,000 sq ft house (UA = 450, ΔT = 48°F) loses 21,600 BTU/hr.
• A consolidated 180 sq ft interior bedroom (UA ≈ 55 BTU/hr·°F with a blanket over the window and a towel under the door) cuts envelope loss by nearly 90%:
  • Four resting adults or children inside that closed room generate ~350 BTU/hr per person in metabolic body heat (4 × 350 = 1,400 BTU/hr of free internal heat gain).
  • At ΔT = 40°F (65°F inside the bedroom vs. 25°F in the surrounding unheated hallway/exterior), the room loses only 55 × 40 = 2,200 BTU/hr.
  • Subtracting 1,400 BTU/hr of human body heat leaves a net deficit of just 800 BTU/hr—or 3,000 to 4,000 BTU/hr on the Low setting of a listed indoor-safe heater cycled intermittently with dual CO detectors!

60-minute winter blackout heat-retention checklist

Execute these steps during the first 60 minutes of a winter outage—while your drywall and furniture are still at 68°F—rather than waiting until the house is already freezing:

• Select and seal your single “Warm Room”: Pick a small (120–200 sq ft) south-facing or interior room, hang thick blankets or bubble wrap/foam board over its window frames, and stuff rolled towels against the bottom crack of the door.
• Close every fireplace flue damper: An open masonry chimney damper acts as a 12-inch exhaust hole sucking warm indoor air straight out of the roof (unless you are actively burning wood in that fireplace).
• Lock all window sashes: Engaging the cam locks on double-hung or sliding windows pulls the weatherstripping gaskets together, cutting window air infiltration by up to 30%.
• Pitch an indoor tent or canopy on top of the mattress: Setting up a compact camping tent on the bed inside your warm room creates a secondary micro-envelope that stays 8°F to 12°F warmer than the room air from body heat alone.
• Stage two battery-powered digital thermometers and a CO alarm: Place one thermometer inside the warm room (keep above 60°F–65°F for seniors/infants) and a remote wireless sensor under the kitchen sink on an exterior wall (shut off and drain main water if cabinet air drops below 40°F).

For complete details on sealing window glass during an outage, read our emergency window insulation film, foam, and curtains guide. If you use a portable propane heater, review our indoor propane heater ODS sensor limits guide and our frozen pipe timing and prevention math.

Frequently asked questions

How many BTU does it take to heat a house?

The heat a house needs equals its hourly heat loss: the home’s overall heat-loss coefficient (UA, in BTU per hour per degree Fahrenheit) multiplied by the difference between indoor and outdoor temperature (ΔT). A typical 2,000 sq ft home might lose roughly 450 BTU per hour for every 1°F of temperature difference, though this varies from 220 in a tight modern home to 700+ in an older uninsulated home. At 68°F inside and 20°F outside (48°F difference), that example loses 450 × 48 = 21,600 BTU per hour.

How long will a house stay warm without power?

A modern, tightly sealed home built to recent energy codes can often stay above 50°F for 24 to 48 hours during a freezing winter outage if doors and windows remain closed. However, older homes with poor attic insulation, single-pane windows, and high air leakage (> 0.60 ACH) can drop from 68°F to below 45°F in 8 to 14 hours. Closing doors to unused rooms, covering windows with insulating film or blankets, and consolidating everyone into one small interior room slows heat loss dramatically.

How many kWh is 21,600 BTU per hour?

One kilowatt-hour (1 kWh) of electricity equals 3,412 BTU of heat energy. Dividing 21,600 BTU/hr by 3,412 BTU/kWh equals 6.33 kWh of heat energy per hour (or 6.33 kW of continuous electrical heating power). That explains why portable battery stations cannot heat a house: a 1,000 Wh battery holds less than 10 minutes of a 6.33 kW whole-house heating load, or about 35 minutes of a single 1,500 W space heater.