Natural Gas and Propane Heat Content and Generator Consumption

Why this matters

Every fuel decision on a standby job runs through one conversion: what the generator burns, in the unit the gas fitter, the utility or the propane supplier works in. Get it wrong and the unit starves at full load on a meter that was fine for the furnace, or the customer is told a tank lasts a week when it lasts four days. This card owns the fuel numbers for the shelf. Pipe sizing stays with 'Gas Supply Sizing and Connection Standard'; tank vaporization stays with 'How to Check a Propane Tank for Generator Runtime and Vaporization'.

The three constants and where they come from

Natural gas: about 1,000 BTU per cubic foot. Pipeline gas runs a little above or below that depending on the supply, and the utility's bill states the heating value it used. For sizing and runtime, 1,000 is the working figure and the one every card on this shelf uses. A therm is 100,000 BTU, so 100 cubic feet is about a therm.

Propane vapor: about 2,500 BTU per cubic foot. This is the figure for gas leaving the tank and moving through the vapor line. It is why a propane line can be smaller than a natural gas line for the same load: each cubic foot carries about two and a half times the heat.

Propane liquid: about 91,500 BTU per gallon. This is the figure for what sits in the tank and what the supplier sells. Dividing the two gives about 36.6 cubic feet of vapor per gallon (91,500 divided by 2,500). Propane weighs about 4.24 pounds per gallon, which matters when a portable cylinder is sold by the pound.

Keep the units straight, because the two propane figures look interchangeable on a scratch pad and differ by a factor of about 37. A vapor flow in cubic feet per hour goes to the pipe sizing table. A liquid burn in gallons per hour goes to the runtime estimate. Mixing them is the commonest error on a propane load sheet.

Reading the consumption table on a spec sheet

Makers publish consumption at two or three load points, usually half load and full load, sometimes quarter load and exercise. Natural gas is given in cubic feet per hour, propane in gallons per hour, cubic feet per hour of vapor, or both. Every figure belongs to its own fuel rating: the half-load propane line is half of the propane kW, the half-load natural gas line half of the lower natural gas kW. Why the two kW ratings differ is in 'Reading a Generator Spec Sheet and Derating'.

Two things the table does not tell you:

• Consumption is not proportional to load below half. An engine at no load still burns fuel to turn itself, its cooling fan and the alternator. Draw a straight line through the half-load and full-load points and it meets zero load well above zero. Treat that intercept as a floor, never scale the half-load figure down in proportion to a lighter load.
• The full-load figure is the sizing number, whatever the house normally draws. The gas supply has to deliver full-load flow at the pressure the maker requires, because the unit will see full load at some transfer: every motor calling at once after a long outage. Runtime estimates can use a realistic average load; supply sizing cannot.

Worked conversion: one sheet, both fuels

The sheet below is illustrative, written for this example; read the real figures off the sheet for the unit you are quoting.

SPEC SHEET: FUEL CONSUMPTION (illustrative)
RATING          LP 22.0 kW       NG 19.5 kW
                LP gal/h         NG ft3/h
  1/2 load        2.4              205
  Full load       3.6              310

CONVERTED TO BTU PER HOUR
  NG 1/2 : 205 x 1,000          = 205,000
  NG full: 310 x 1,000          = 310,000
  LP 1/2 : 2.4 x 91,500         = 219,600
  LP full: 3.6 x 91,500         = 329,400

LP AS VAPOR, CU FT PER HOUR
  LP 1/2 : 219,600 / 2,500      =    87.8
  LP full: 329,400 / 2,500      =   131.8

Check the sheet against itself. Full load on propane is 22.0 kW, and 22.0 times 3,412 BTU per kWh is 75,064 BTU per hour of electricity out, against 329,400 in: about 23 percent. Full load on natural gas is 19.5 kW, or 66,534 BTU per hour out of 310,000 in: about 21 percent. Small air-cooled standby engines land in roughly that range. A sheet that implies 60 percent, or 5 percent, has a typo or a unit swap, usually gallons printed where cubic feet were meant, and you have caught it before it reached a pipe table.

Check the vapor figure the other way. 3.6 gallons per hour times 36.6 cubic feet per gallon is 131.8 cubic feet per hour, the same as the BTU route. If the sheet prints a propane vapor flow, compare it with yours; makers round, so a few percent apart is normal and a factor of two is not.

Adding the house's other gas appliances

The generator shares the supply with everything else on the meter or tank, and during an outage those appliances are the ones most likely to be running: the furnace, because the outage is a storm, and the water heater, because the family is home. NFPA 54 and the IFGC, as adopted by your jurisdiction, size gas piping on the connected load of every appliance served, so the input ratings off the appliance plates are added at full value. Read the input rating, not the output.

Continuing the natural gas case. The house has these plates:

That 537 cubic feet per hour is what the supply has to deliver at the pressure each appliance needs. It is more than double the 227 the house was built for, which is why "they already have gas" settles nothing. Two questions follow, and neither belongs to the generator tech:

• Can the meter and service regulator pass it? The utility owns the meter set and decides. Send them the connected total and the generator's required inlet pressure; many residential meters are sized close to the original house load and need an upgrade.
• Can the house piping carry it? That is the gas fitter's calculation, done in 'Gas Supply Sizing and Connection Standard', with the generator either on its own run from the meter or added to the existing system.

Gas piping and regulators go to a licensed gas fitter or plumber wherever the locality requires one; the generator tech's job is to hand them the right total.

Worked runtime: the same house on propane

Now put the same appliances on a propane tank. The supplier sets and fills it; the runtime estimate is yours, because the customer will ask.

Usable fuel. Say a 500-gallon tank filled to the usual 80 percent holds 400 gallons. Suppose the supplier's advice and the vaporization check set the floor at 20 percent, 100 gallons, so 300 gallons are usable. The floor is a cold-weather question owned by the propane tank howto; use the supplier's figure.

Burn rate during an outage. Use realistic duty, not connected load, because this is runtime, not supply sizing:

• Generator at half load, 2.4 gallons per hour.
• Furnace, 100,000 BTU per hour divided by 91,500 is 1.09 gallons per hour while firing; at half duty on a cold day, 0.55.
• Water heater and range together, assume about 0.1 gallons per hour averaged over a day.

Total 2.4 plus 0.55 plus 0.1 is 3.05 gallons per hour. 300 gallons divided by 3.05 is about 98 hours, roughly 4.1 days. Run the generator alone at half load and it is 300 divided by 2.4, or 125 hours, about 5.2 days. At full load alone, 300 divided by 3.6 is about 83 hours.

What to tell the customer. "About four days at a normal outage load with the furnace running, less if the tank isn't full when the storm hits." Give the range, name the assumption, and put it on the quote. A runtime promise written as one number from the half-load line, ignoring the furnace, is the one that comes back after a long outage.

What changes the numbers

• Altitude and supply heating value. At elevation the engine derates, which changes kW, not BTU per cubic foot of gas. A utility supplying gas well off 1,000 BTU per cubic foot shows it on the bill; use the billed figure for a tight runtime or meter question.
• Partial fills. Propane tanks are rarely full when an outage starts. A customer on a will-call delivery may be at 40 percent the day the storm arrives: 200 gallons, only 100 above the floor, which cuts every runtime above to a third. Automatic delivery or a tank monitor is the fix, and it is the supplier's program to sell.
• A dual-fuel or converted unit. The consumption table on the sheet is for the fuel the unit was built and set for. After a field conversion, the kW rating and the consumption both follow the conversion instructions, not the original sheet.
• Natural gas runtime. It has none to calculate. The constraint is pressure and flow at full load, measured at the unit ('How to Measure Gas Pressure at the Generator, Static and Under Load'), and in rare regional events, utility curtailment.

Where the figures go wrong in the field

• Gallons treated as cubic feet. 3.6 gallons per hour entered in a cubic-feet-per-hour pipe table undersizes the line by a factor of about 37, so the table returns a pipe a fraction of what the load needs.
• Output rating read for input. A furnace plate lists both. Supply sizing uses input; using output understates the furnace by its efficiency loss.
• Half load used for supply sizing. It produces a line and meter that run the house on an ordinary day and fail at the transfer that matters.
• Daily burn quoted from the exercise. A weekly exercise burns a small amount at no load; it says nothing about outage consumption.

References

• NFPA 54, National Fuel Gas Code, and the International Fuel Gas Code, in the edition your jurisdiction has adopted, for sizing on connected appliance load.
• Generator maker's specification sheet and installation manual for the consumption table and required inlet pressure.
• See related: Gas Supply Sizing and Connection Standard; How to Size Natural Gas Pipe to a Standby Generator.
• See related: How to Check a Propane Tank for Generator Runtime and Vaporization; Reading a Generator Spec Sheet and Derating.