How this instrument works
Heat output here is power integrated over time, and because Power is held constant in this instrument, that integral collapses to a single multiplication: Q = P × t. This is Joule's first law in its simplest form — every joule a resistive load draws from the circuit is a joule it sheds as heat, whether the load is a bare nichrome coil, an incandescent filament, or a baseboard heater element. Nothing is stored, nothing escapes as light or motion; electrical energy in equals thermal energy out.
The two output units serve different readers. Kilowatt-hours are what the utility meter already counts, so switching Heat output to kWh turns the total into a line item that lines up directly against an electricity bill. BTU is the HVAC trade's inherited unit, tracing back to the energy needed to warm one pound of water by one degree Fahrenheit; a room's heat-load worksheet and a portable heater's box both quote it, and one BTU equals 1,055.06 joules under the modern fixed definition.
The formula's honesty has a boundary: it assumes every watt drawn becomes heat, which holds for resistive elements but not for anything that does other work with the electricity. A heat pump can deliver two, three, even four times its electrical input as heat because it moves thermal energy rather than generating it, and a motor or compressor sheds only its inefficiency as warmth while the rest becomes mechanical work. For those devices, Q = P × t badly overstates or understates the heat actually produced.
- Enter the load's steady electrical draw in the Power field — switch to kW if the nameplate rating is given in kilowatts.
- Set Duration to how long the load runs — hours suit a heating session, minutes suit a short cycle like a hair dryer.
- Read Heat output in BTU for HVAC-style sizing work, or switch it to kWh to match a line on a utility bill.
- Zero out Duration to confirm the instrument returns zero heat output — a load that never runs produces no heat.
Worked example — a 1,500 W space heater run for 3 hours
Set Power to 1,500 W, a typical portable space heater's draw, and set Duration to 3 hours. Internally the instrument works in seconds, so it multiplies 1,500 W by 10,800 s — the number of seconds in 3 hours — to get 16,200,000 joules of heat, exactly Q = P × t with nothing rounded along the way.
Switch Heat output to kWh first, as a sanity check: 1,500 W is 1.5 kW, and 1.5 kW sustained for 3 hours is 1.5 × 3 = 4.5 kWh exactly, the same figure a utility meter would log for that heater. Switch it back to BTU and the total reads about 15,355 BTU, using the standard 1,055.06-joule-per-BTU conversion — the figure a portable heater's spec sheet or an HVAC contractor's room-sizing worksheet would print for the same appliance.
Questions
Does this formula assume the heater is 100 percent efficient at making heat?
Yes, for the electrical side. Every watt a purely resistive element draws — nichrome wire, a baseboard heater, an incandescent bulb's filament — leaves as heat, so Q = P × t accounts for all of it. Devices that do other work with the electricity, like a heat pump's compressor or a fan motor, do not fit this assumption; their heat output is not simply their electrical draw times run time.
Why does the instrument report heat output in BTU by default?
BTU is the unit HVAC work has used for over a century to size heaters, furnaces, and room heat loads, so a figure in BTU drops straight into a contractor's worksheet or a space heater's spec sheet without conversion. Switch the Heat output unit to kWh instead if you want the number an electricity bill would show for the same energy.
Is heat output in BTU the same thing as a BTU/h rating on an appliance box?
No, and mixing them up is a common mistake. A BTU figure from this instrument is a total quantity of heat delivered over the Duration entered; a BTU/h rating on a furnace or air conditioner box is a rate — heat delivered per hour, regardless of how long it runs. Multiply a BTU/h rating by the hours it runs to get a total comparable to this instrument's output.
Does switching Duration between hours and minutes change the answer?
No. The instrument converts Duration to a common internal unit before multiplying, so 3 hours and 180 minutes produce the same Heat output figure. Only the number displayed in the Duration field changes; the underlying seconds used in Q = P × t stay identical, and the result is unaffected either way.
Can I use this for a heat pump or air conditioner instead of a resistive heater?
Not directly. A heat pump moves heat rather than generating it, so its output can be two to four times its electrical input, a ratio called the coefficient of performance. This instrument's Q = P × t assumes a one-to-one conversion, which understates a heat pump's real output — it is built for resistive loads like space heaters, immersion elements, and baseboard heat.
What is the historical origin of the BTU unit?
One BTU was originally defined as the energy needed to raise one pound of water by one degree Fahrenheit near its point of maximum density. Because that definition depended slightly on the starting temperature, engineering practice settled on a fixed value — the International Table BTU, equal to exactly 1,055.05585262 joules — which is the conversion this instrument uses.