How to Work Out a Run Time from a Fuel Supply

Why this matters

"How long will what's in the tank last?" comes up on every no-heat call in a cold snap, every generator install and every vacation house shut-down. A guess that runs long leaves a customer out of fuel on a Sunday, and a propane system that runs dry is not simply refilled and relit. A guess that runs short buys a delivery nobody needed.

The answer is one line of arithmetic: burner hours = usable gallons x heat per gallon / input in BTU per hour. The work is in getting each of the three inputs honest, and then turning burner hours into calendar days. This card builds that line by line on one house.

If anyone smells gas, everyone leaves the building, nobody touches a switch, a light or a phone inside, and the call to the gas supplier or 911 is made from outside. Reading a tank gauge is a look at a dial, nothing more; nobody opens a fill valve, a bleeder or a fixed liquid level gauge to check a level unless they are the supplier's trained person doing a fill.

The calculation, built line by line

HOUSE: 500 gal propane tank (water capacity)
       furnace input 80,000 BTU/h
       water heater input 40,000 BTU/h

1 Gauge reading          60 percent
2 Gallons now            0.60 x 500 gal      = 300 gal
3 Reserve kept           0.20 x 500 gal      = 100 gal
4 Usable gallons         300 gal - 100 gal   = 200 gal
5 Heat per gallon        91,500 BTU/gal (propane)
6 Furnace burner hours, tank to empty
    300 gal x 91,500 BTU/gal / 80,000 BTU/h  = 343 h
7 Furnace burner hours, to reserve
    200 gal x 91,500 BTU/gal / 80,000 BTU/h  = 229 h
8 Daily use in a cold week
    furnace  12 h/day x 80,000 BTU/h = 960,000 BTU/day
    heater    2 h/day x 40,000 BTU/h =  80,000 BTU/day
    total                            = 1,040,000 BTU/day
    in gallons 1,040,000 / 91,500    = 11.37 gal/day
9 Days to reserve        200 gal / 11.37 gal/day = 17.6 days

Each line is one of the steps below.

Step 1: Read the gauge, and know what it reads (lines 1 and 2)

The float gauge on a propane tank reads percent of the tank's total volume, its water capacity, not percent of a "full" fill. A 500 gal tank at 60 percent holds 0.60 x 500 = 300 gal of liquid.

Why it matters: a 500 gal tank is never filled to 500 gal. Suppliers fill to about 80 percent, set by the tank's fixed maximum liquid level gauge under NFPA 58 as your state has adopted it, so a full 500 gal tank reads about 80 percent and holds about 400 gal. The space left is there because liquid propane expands far more than water as it warms, and a tank filled solid on a cold morning can build enough pressure on a warm afternoon to open its relief valve.

What breaks if you skip it: reading 60 percent as 60 percent of a fill (0.60 x 400 = 240 gal) understates the supply by 60 gal. Reading 80 percent as "nearly empty space left" does the opposite. Float gauges are also approximate, often off by several percent, so treat a reading as a range rather than a gallon count.

Step 2: Keep a reserve, and say what it is for (lines 3 and 4)

Plan to a reserve, not to zero. A common default is to call for delivery at 20 to 30 percent, and many suppliers' auto-delivery schedules aim for something similar; check what the customer's supplier uses. This example keeps 20 percent: 0.20 x 500 = 100 gal, leaving 300 - 100 = 200 gal usable.

Why: two things happen near empty. In cold weather a tank low on liquid has less wetted surface to boil propane into vapor, so at high draw it may not keep up and burners start to starve before the gauge reads zero. And if the system does run out, service cannot simply resume. NFPA 58, as adopted by your state, in substance requires a leak check when gas service is restored after an interruption, and many suppliers will not refill an out-of-gas system without one. That is a supplier or licensed-tech visit, not a homeowner relight.

What breaks if you skip it: a plan that lands on the last gallon lands on the last gallon at the worst time, because draw is highest in the coldest week.

Step 3: Use the fuel's heat content, from one source (line 5)

The figures in common use, on the higher-heating-value basis that US appliance input ratings are stated on:

• Propane: about 91,500 BTU per gallon.
• No. 2 heating oil: about 138,500 BTU per gallon.

Use the figure for the fuel actually in the tank. Mixing them up moves the answer by half again: 300 gal of oil would carry 300 x 138,500 / 80,000 = 519 burner hours on the same furnace, against 343 on propane. The unit chain is in Energy and Power Units: BTU, Kilowatts, Tons and Horsepower.

Step 4: Divide by input, not output (lines 6 and 7)

Use the appliance's rated input in BTU per hour from its rating plate. Input is what the burner consumes; output is what reaches the house after efficiency. A 95 percent furnace with an 80,000 BTU/h input delivers about 76,000 BTU/h of heat, but it still burns fuel at 80,000.

Line 6: 300 gal x 91,500 BTU/gal = 27,450,000 BTU. Divided by 80,000 BTU/h = 343.1 burner hours. The gallons cancel against per-gallon, the BTU cancels against BTU per hour, and hours remain.

Check it the other way: 80,000 / 91,500 = 0.874 gal per burner hour, and 300 / 0.874 = 343 hours. Same answer.

Line 7 does the same for the 200 usable gallons: 200 x 91,500 / 80,000 = 228.75, about 229 hours.

What changes it: a two-stage or modulating furnace burns at less than its full input much of the time, so burner hours at full input understate how long the fuel lasts. Where the thermostat or control logs stage runtime, figure each stage at its own input.

Step 5: Turn burner hours into days (lines 8 and 9)

Burner hours are not calendar hours. The furnace runs part of each hour, and how much depends on the weather and the house. Get the runtime from the thermostat's usage history where it keeps one, or from the customer's last delivery interval, or estimate it for the weather you are planning for. This example assumes a cold week with the furnace running 12 hours a day, half the time.

Then add every other appliance on the same tank. Here a 40,000 BTU/h water heater firing about 2 hours a day adds 80,000 BTU a day. A range, a dryer, a fireplace or a pool heater on the same tank each add their own line.

Total 1,040,000 BTU a day / 91,500 BTU per gallon = 11.37 gal a day. 200 usable gallons / 11.37 gal a day = 17.6 days to the reserve.

What breaks if you skip it: quoting "343 hours" as if it were two weeks of heat. At 12 burner hours a day the full 300 gal would last 343 / 12 = 28.6 days of furnace alone, and the water heater shortens that. The number the customer needs is days to reserve with everything on the tank included.

Step 6: For a generator, use the maker's fuel table

A standby generator has no single input rating to divide by. Its fuel use changes with load, and the manufacturer publishes it in the installation or specification sheet as fuel per hour at stated loads (often quarter, half and full load), in gallons per hour or cubic feet per hour for propane.

Read the figure for the load the house will realistically put on it, and divide usable gallons by it. Say the sheet lists 2.0 gal per hour at half load (an illustrative figure; read your own unit's sheet). 200 usable gallons / 2.0 gal per hour = 100 hours of continuous running, a little over four days, before the tank reaches reserve, and less if the furnace and water heater draw from the same tank during the outage.

Why not the BTU method: an engine's efficiency changes with load, so heat per gallon divided by a nameplate output gives a number that is wrong in a direction you cannot predict. The maker measured it; use theirs. Ask the supplier, too, whether the tank can vaporize fast enough for the generator's full-load draw in the coldest weather it will see, because that limit sizes tanks for generators more often than the gallon count does.

What would change the answer

• A warmer or colder week. Runtime is the input with the widest swing. At 8 furnace hours a day the same house uses 8 x 80,000 + 80,000 = 720,000 BTU a day, 7.87 gal, and 200 gal lasts 25.4 days.
• An oil tank instead of propane. Oil tanks are usually filled close to full, and there is no 80 percent fill rule for them; the planning reserve still applies, since letting an oil tank run dry can draw sludge into the line and air into the burner pump.
• A vacant house at a setback. Low runtime makes the fuel last, but a tank gauge read once and not again is not a plan; schedule a check or auto-delivery.

How to know the number is honest

Recompute line 9 from the other end: 17.6 days x 11.37 gal a day = 200 gal. Then compare against history, which is the best check on the runtime assumption because it is the one input nobody measured.

Say the delivery tickets show two fills to the same level 21 days apart, the second taking 180 gal, in weather like the week being planned. That is 180 / 21 = 8.6 gal a day actually used, against the 11.37 the calculation assumed. Work it back: 8.6 gal x 91,500 BTU/gal = 786,900 BTU a day, less the water heater's 80,000 leaves 706,900, and 706,900 / 80,000 BTU/h = 8.8 furnace hours a day rather than 12. At the measured rate the 200 usable gallons last 200 / 8.6 = 23.3 days, not 17.6. The calculation was conservative, which is the safe direction for a fuel plan, but the history says by how much. A gap the other way, history using more than the calculation, points at an appliance left off line 8.

Write the gauge reading, the date, the inputs used and the days-to-reserve figure on the ticket, so the next call starts from a number instead of a memory.

References

• NFPA 58, Liquefied Petroleum Gas Code, in the edition your state has adopted (fill limits, leak check after interruption of service)
• The appliance rating plate for input in BTU per hour
• The generator manufacturer's specification sheet for fuel consumption by load
• See related: Energy and Power Units: BTU, Kilowatts, Tons and Horsepower
• See related: Write the Calculation Down: The Field Math Habit