Heating Oil Gallon BTU: What a Gallon Is Worth in Heat

The energy in a gallon is only the starting point — two corrections sit between the delivery ticket and the heat that reaches a New York living room.

Heating Oil, Gallons and BTU: What a Gallon Is Worth

A gallon of number 2 heating oil is a container of chemical energy, and the
BTU figure is how that container is measured. It is the number that lets you
compare a delivery to a therm of gas, a pound of propane or a kilowatt-hour of
electricity — which is the only honest way to compare what two heating systems
cost to run.

The comparison is worth doing carefully, because two separate corrections
sit between the number on the delivery ticket and the heat that reaches a New
York living room, and skipping either one produces an answer that is wrong in a
predictable direction.

What a BTU is, and what the gallon figure describes

A BTU is a small unit of heat — roughly the energy needed to raise a pound
of water by one degree Fahrenheit. It is a quantity, like a kilowatt-hour, and
the two convert directly into one another because they measure the same thing
in different units.

The heating value published for number 2 fuel oil is the energy released by
burning one gallon completely. Two things about it matter more than the digits
themselves:

• It is a range, not a constant. Fuel oil is a refined
  product with a specification, not a pure compound, and its energy content
  varies within that specification. Blends containing biodiesel — increasingly
  common in New York, and mandated in some parts of the state — differ again.
  Take the figure from a current published source rather than from memory.
• Volume changes with temperature. Oil expands as it warms,
  so a gallon delivered on a cold morning contains marginally more mass, and
  therefore more energy, than one delivered on a warm afternoon. It is a small
  effect and it is not worth arguing about, but it is why gallons are a slightly
  soft unit in the first place.

The two corrections between the ticket and the room

This is where most household comparisons go wrong.

Correction one: combustion efficiency

Burning a gallon does not put all its heat into the house. Some leaves up
the flue, and the fraction that stays is what the appliance’s efficiency rating
describes. So the heat delivered is the gallon’s energy content multiplied by
that efficiency — and the rating is a laboratory figure for a well-maintained
unit, which an old boiler that has not been serviced will not match.

Correction two: what electricity does differently

Electric resistance heating converts essentially all of its energy into heat
in the room, so a kilowatt-hour of resistance heat is a kilowatt-hour of heat.
A heat pump does something different again: it moves heat rather than
creating it, so it delivers substantially more heat energy than the electrical
energy it consumes. Its coefficient of performance expresses that multiple, and
it varies continuously with outdoor temperature.

That is the reason a naive comparison of fuel prices is meaningless. The
correct comparison is cost per unit of heat delivered, which means dividing by
efficiency for the oil side and by the seasonal performance figure for the heat
pump side.

How to do the comparison for your own house

That last row is the one to be strict about. Heating oil is bought on a
market that moves week to week, and electricity is priced by tariffs that
change on their own schedule, so any figure printed in an article — including
this one, which prints none — is stale by the time it is read. Use your own
most recent invoice and your own bill, and redo the sum when either moves. The
method is durable; the numbers are not.

A shortcut that is good enough for a decision

You do not need a spreadsheet to get the shape of the answer. Take a full
year of oil deliveries in gallons, convert to delivered heat with the two
corrections, and compare against what the same delivered heat would cost on
electricity given a realistic seasonal performance figure. A year of deliveries
is more honest than one winter month, for the same reason
a single electricity bill misleads — and if the
electric side is going to grow, you also want to know
what the house already consumes before adding to
it.

Comparing oil with the other fuels

Once everything is expressed as delivered heat, the fuels line up on one
axis and the differences become visible. Propane carries meaningfully less
energy per gallon than fuel oil, which is why a propane household consumes more
gallons for the same heat and why gallon-for-gallon price comparisons between
the two are misleading — the same arithmetic applies there, and
the propane conversion works exactly the same way.
Natural gas is sold by the therm or the hundred cubic feet, which are already
energy units, so it needs only the efficiency correction.

What this exercise usually reveals in New York is that the ranking between
fuels is not fixed. It depends on the current price of each, on the efficiency
of the specific equipment, and — for the heat pump — on how cold the location
gets, because performance falls with outdoor temperature. Two houses in
different parts of the state can reach opposite conclusions from the same
correct method, which is why
the operating-cost comparison has to be done with
local inputs.

What the gallons figure will not tell you

Running cost is one input among several, and the oil-to-BTU sum answers only
that one.

• It says nothing about the building. A house that loses
  heat quickly is expensive to heat with anything. Envelope work changes the
  number of gallons required, which is a larger lever than the fuel choice for
  many older New York houses.
• It says nothing about capital cost or incentives. What
  equipment costs to install, and what programmes contribute toward it, is a
  separate calculation with its own current terms to look up.
• It says nothing about the tank. An ageing oil tank,
  particularly one below grade, carries its own liability and its own removal
  question — which often arrives on its own schedule rather than yours.
• It assumes one system does everything. Many New York
  households end up running a heat pump for most of the season and
  keeping the oil boiler for the coldest hours, which
  changes the arithmetic into a split between two fuels rather than a choice
  between them.

If the sum is pointing you toward electrification, the practical questions
that follow — what the house needs, what the electrical service can take, and
what the sequence should be — are
their own conversation rather than a consequence of the
BTU figure.

The durable version: a gallon of heating oil holds a published quantity of
energy, that quantity varies with blend and slightly with temperature, and it
becomes useful only after you multiply by the appliance’s efficiency and
express everything as cost per unit of heat delivered. Do that with your own
invoice and your own bill, and redo it when either changes.

Frequently Asked Questions

How many BTU are in a gallon of heating oil?

Number 2 fuel oil has a published heating value per gallon, but it is a range rather than a constant: fuel oil is a refined product with a specification, and blends containing biodiesel differ again. Volume also varies slightly with temperature, since oil expands as it warms. Take the figure from a current federal energy data source rather than from memory, and note the blend if your supplier states one.

How do I convert heating oil BTU to kilowatt-hours?

BTU and kilowatt-hours are both quantities of energy, so they convert directly and the ratio never changes. What makes the comparison useful is what you do afterwards: multiply the oil figure by your appliance’s efficiency to get heat actually delivered into the house, and divide the electric side by a heat pump’s seasonal performance figure, since a heat pump delivers more heat than the electricity it consumes.

Why can’t I just compare the price of a gallon to the price of a kWh?

Because neither price buys the same amount of delivered heat. Some of a gallon’s energy leaves up the flue, so the appliance efficiency has to be applied, and a heat pump moves heat rather than creating it, so it delivers a multiple of the electrical energy it draws. The only meaningful comparison is cost per unit of heat actually delivered into the room, which requires both corrections.

Does heating oil have more energy per gallon than propane?

Yes, meaningfully more, which is why a propane household burns more gallons for the same heat and why gallon-for-gallon price comparisons between the two mislead. The correct comparison converts both to delivered heat first, using each appliance’s efficiency, and then compares cost per unit of that heat. Natural gas is already sold in energy units, so it needs only the efficiency correction.

How many gallons should my house use in a year?

That is set by how much heat the building loses, not by any general figure, and it varies enormously between houses of similar size in the same New York town. Your own delivery history over a full year is the answer for your house. If the figure looks high, envelope work changes the number of gallons required and is often a larger lever than switching fuel, particularly in older housing.

Does the efficiency rating on my boiler still apply?

It is a laboratory figure for a well-maintained unit, and an older appliance that has not been serviced will fall short of it. A technician’s combustion test on your own equipment gives a far better input than the plate rating for the purposes of this comparison, and it is worth asking for the results in writing rather than as a verbal summary at the end of a service visit.

Do I have to choose between oil and a heat pump?

Not necessarily. A common New York outcome is a heat pump carrying most of the season with the existing boiler retained for the coldest hours, which turns the question from a choice between fuels into a split between them. That changes the arithmetic, since each fuel then covers the part of the year it does best, and it also changes what has to be kept, serviced and vented.