Emergency Shelter Heating Calculator

Why emergency heating planning matters

When the power goes out in winter, the first 24 to 48 hours decide whether you stay warm or face a hypothermia risk.
Most people badly underestimate how much fuel it takes to heat even a small space, and badly overestimate how long the two spare propane tanks in the garage will last.

The polar vortex outages of February 2021 in Texas killed over 200 people, many from hypothermia in heated homes that lost power for days. Similar events happen yearly across the US Northeast, Midwest, and Mountain West. Having a heating plan and adequate fuel is a survival issue, not a convenience.

The heat loss formula

The fundamental equation:

BTU per hour = Room volume (ft³) × Temperature difference (°F) × Infiltration factor

Where:

• Room volume: floor area × ceiling height
• Temperature difference: indoor target − outdoor temperature
• Infiltration factor: accounts for insulation quality and air leakage

Worked example: 200 sq ft room with 8-ft ceiling, 60°F target, 20°F outside, average insulation:

• Volume: 1,600 ft³
• ΔT: 40°F
• Infiltration: 0.25
• BTU/hr: 1,600 × 40 × 0.25 = 16,000 BTU/hr

That is a substantial output, roughly a typical kerosene heater running flat out around the clock. Not the setting you were imagining when you bought it.

Insulation factors

The infiltration factor varies dramatically with building quality:

The dropdown uses the middle of each band. If you are unsure which row you are in, pick the leakier one. Being told to store more fuel than you turn out to need costs you some money and some garage space; being told to store less is the failure that has a body count.

Fuel energy density (after efficiency)

Different fuels have different usable energy after combustion efficiency:

A propane tank (20 lb of gas, which is 4.7 gallons) provides about 343,000 BTU of usable heat. That runs a 16,000 BTU/hr heater for about 21 hours, so one tank is under a day.

Realistic fuel storage estimates

For a 7-day power outage at 20°F outside heating a 200 sq ft room to 60°F:

• Total heat needed: 16,000 BTU/hr × 24 hr × 7 days = 2,688,000 BTU

Note these are continuous heating estimates. With sleep-time temperature reduction (50°F at night with blankets), you can cut fuel use 30-40%.

Practical fuel storage considerations

Propane (20 lb tanks):

• Most accessible (exchange programs at every gas station)
• Each tank: ~4.7 gallons = ~343,000 BTU
• Indoor use requires extreme caution (CO risk)
• Outdoor storage required
• Tank inspection required every 12 years

Kerosene:

• High energy density
• Long shelf life with stabilizer (1-2 years)
• Requires kerosene heaters (Sengoku, Toyotomi)
• Less common in stores
• Indoor use requires ventilation

Firewood:

• Cheapest BTU per dollar for many regions
• Requires woodstove or fireplace
• Needs to be properly seasoned (6+ months drying)
• Indoor-friendly with adequate flue
• Long-term storage with proper protection

Wood pellets:

• Cleaner burning than logs
• Easier handling
• Requires pellet stove
• Susceptible to moisture damage
• Less common in some regions

Natural gas:

• Often continues working during electrical outages
• BUT requires electric thermostat for most furnaces
• Generator-powered furnace fan is the key
• No fuel storage required

The carbon monoxide reality

This is the single most important emergency heating safety issue:

Carbon monoxide (CO):

• Colorless, odorless gas produced by incomplete combustion
• Binds to hemoglobin 200x more strongly than oxygen
• Causes brain damage and death rapidly
• Kills 400+ Americans per year from heating equipment
• Symptoms: headache, dizziness, nausea, confusion, unconsciousness
• Detector required in any space with combustion heating

Sources of CO during emergencies:

• Propane/kerosene heaters used indoors
• Generators run in garages or near windows
• BBQ grills used indoors
• Gas stoves used as space heaters
• Charcoal indoors

Safety requirements:

• Battery-powered CO detector (not just smoke detector)
• Adequate ventilation (cracked window or vent)
• Never sleep with unvented combustion heater running
• Generators 25+ feet from any opening
• Never use grills indoors
• Heat one room only (not the whole house)

Shelter-in-place strategy

In severe emergencies, drift to one room and heat only that:

1. Select a small interior room (no exterior walls if possible)
2. Hang blankets over doors and windows
3. Block drafts at bottom of doors with rolled towels
4. Use small heater sized for the room
5. Sleep in shifts if using unvented heater (someone awake to monitor)

A 12 × 12 ft bedroom (1,152 ft³) at 60°F against 20°F outside needs 11,520 BTU/hr with average insulation.
Set that against a 1,600 sq ft house with the same ceilings and insulation, which comes to 128,000 BTU/hr. Eleven times the fuel, for the same temperature, in rooms you are not sitting in.

Temperature tolerance for survival

Humans survive at lower temperatures than most people realize:

• 65-70°F: comfortable
• 60-65°F: cool but fine with sweater
• 55-60°F: chilly; needs sweater and movement
• 50-55°F: cold; need multiple layers
• 45-50°F: very cold; risk of mild hypothermia at extended exposure
• 40-45°F: hypothermia risk if wet or inactive
• Below 40°F indoors: serious hypothermia risk

For survival, target 50-55°F minimum indoor temperature. Below this, sleeping risks become serious.

Multi-day power outage timeline

What to expect during prolonged outages:

Day 1: refrigerator items cold, residual heat in walls Day 2: refrigerator warming, freezer still working, residual heat fading Day 3: refrigerator failed, freezer thawing if not frozen solid, indoor temp dropping Day 4: indoor temp ≈ outdoor temp (without heating); food spoiling rapidly Day 7+: long-term emergency requirements

For most US locations, a 3-day outage is the planning threshold. Longer requires substantial fuel storage.

Generator vs heater trade-offs

For backup heat, two main strategies:

Direct heating (propane/kerosene/wood):

• Pros: simple, works without electricity, lower per-BTU cost
• Cons: CO risks, fuel storage logistics
• Best for: rural areas, frequent outages, prepared homeowners

Generator + electric heating (or generator + gas furnace fan):

• Pros: heats whole home, allows refrigeration and lights
• Cons: high fuel use, noise, generator costs ($500-$3000+)
• Best for: shorter outages, suburban homes, those with medical needs

Hybrid approach (most common): generator for essentials (fridge, lights, well pump) + direct heat for primary living space.

Insulation as preparation

Before storing fuel, improve your heat retention:

• Weather stripping doors and windows: $50-100 investment, dramatic difference
• Thermal curtains: blocks heat loss through windows
• Door snakes/draft stoppers: blocks gap at bottom of doors
• Window plastic film: $20-40 per window, reduces heat loss 25%+
• Pipe insulation: prevents frozen pipes
• Outlet/switch gaskets: small leaks adding up

Reducing your home’s heat loss before an emergency = stretching your fuel supply 30-50%.

Common emergency heating mistakes

1. Underestimating fuel needs: 1 day of fuel becomes 24 hours of fear
2. No CO detector: silent killer
3. Whole-house heating: wastes fuel during emergencies
4. Wet firewood: less heat, more smoke
5. Indoor charcoal grill: deadly CO production
6. Sealed room with combustion heater: CO accumulates
7. No backup plan: one method only
8. Old kerosene: degrades over 12-18 months
9. Propane outdoors in extreme cold: tanks freeze, lose pressure
10. No fire extinguisher: needed for any combustion heating

If you remember two things

Heat one room, and buy a CO detector. Everything else on this page is arithmetic around those two decisions. Shrinking the heated space from a whole house to a single bedroom cuts the fuel requirement by an order of magnitude, which is the difference between a supply you can actually store and one you cannot. And every fuel here produces carbon monoxide, which killed most of the people who died indoors during the 2021 Texas outage. A battery-powered detector costs about $25.