Watt density is the whole design and it is not a constant
Everything downstream of the watt density is arithmetic. Area times watts per square foot gives the connected load; the load divided by the voltage gives the current; the load times the hours gives the energy. So the only number worth arguing about is the first one, and it is the one no calculator should hand you.
It depends on how cold the design condition is, on what is under the slab, on whether there is insulation beneath the heating element, and on how quickly the system is meant to clear. A slab with no insulation under it spends a large fraction of its energy warming the ground, and no amount of watt density fixes that — it just costs more to do the same visible job. Take the figure from the design or from the product data for what is actually being priced.
| Layout | Area | At 40 W/sq ft | Amps at 240 V |
|---|---|---|---|
| Two tire tracks, 40 x 3 ft each | 240 sq ft | 9.6 kW | 40.0 A |
| One-car apron, 20 x 20 ft | 400 sq ft | 16.0 kW | 66.7 A |
| Two-car drive, 40 x 20 ft | 800 sq ft | 32.0 kW | 133.3 A |
| Walk and steps, 60 x 4 ft | 240 sq ft | 9.6 kW | 40.0 A |
The third row is where most heated driveway projects meet reality. A hundred and thirty three amps is most of a two hundred amp service, and it is on top of the heat pump, the water heater, the range and the car charger. Whether the service carries it is a load calculation performed on the actual building by somebody qualified, and if the answer is no, the service upgrade may cost more than the heating did. The circuit load calculator and the EV charger circuit calculator deal with the same arithmetic on smaller loads.
The hours nobody counts
A snow melting slab has thermal mass measured in hours. Bringing it up from ground temperature takes time, so systems are started before the snow arrives and held for a period after it stops to dry the surface. On the defaults here that is three idle hours against six melting hours: a third of the runtime, billed at exactly the same rate.
Nine hours at 9.6 kilowatts is 86.4 kilowatt hours, which at sixteen cents is $13.82 for one storm. Twenty two storms is $304 for a winter of two tire tracks. Do the same on a full 800 square foot driveway and the load triples to 32 kilowatts, the event costs $46, and the winter is a little over a thousand dollars in electricity alone.
The control strategy therefore matters as much as the watt density. Systems left in an aggressive idle mode through a cold snap with no snow in it are paying full load for nothing, and the difference between a slab sensor that starts on moisture plus temperature and a thermostat that simply holds the slab warm is a large multiple of the number above.
Electric or hydronic is a fuel price question, then a capital question
The energy is the same either way: 9.6 kilowatts is 32,757 BTU an hour whichever way it is produced. What differs is the price of delivering that heat and what it costs to install.
Run the gas comparison field and the arithmetic falls out: 9 hours at 32,757 BTU an hour is 294,809 BTU, which at 85 percent efficiency needs 3.47 therms. At $1.40 a therm that is $4.86 an event against $13.82 on electricity. Gas wins on fuel by a wide margin at those prices, and then loses a large part of it back on a boiler, a heat exchanger, pumps, controls and a glycol loop that has to be maintained. Neither the capital cost nor the maintenance is on this page, and neither should be taken from any calculator: get two real quotes.
The boundary is where the ice goes
A heated area has an edge, and meltwater does not stop at it. Water that runs off a warm slab onto cold pavement freezes there, which is how a heated apron produces an ice sheet at the sidewalk. Tire tracks have two edges each and the same effect at both. Where the water goes when it leaves the heated area is a grading question, and it is the part of a snow melting design that gets left out most often.
What that meltwater amounts to in gallons is on the snowmelt runoff calculator. If a system is being considered as an alternative to clearing rather than an addition to it, the honest comparison is against what clearing actually costs in time and material: the snow removal time calculator puts hours on the mechanical version, and the ice melt calculator puts pounds and dollars on the chemical one.
Questions people ask
How much does it cost to run a heated driveway?
On the defaults here — 240 square feet of tire tracks at 40 watts a square foot, nine hours per event including pre-heat, 22 events, sixteen cents a kilowatt hour — it is $13.82 an event and $304.04 for the winter. Scale that to a full 800 square foot driveway and the same assumptions give a little over a thousand dollars. Your own watt density and electricity rate will move it substantially, which is why both are inputs.
How many watts per square foot does a snow melting system need?
That comes from the design, not from this page. It depends on the design temperature, on what is under the slab, on whether there is insulation beneath the heating element and on how fast the system is expected to clear. A slab with nothing under it wastes a large share of its energy heating the ground. Take the figure from the manufacturer data for the product being quoted and put it in the field.
Will my electrical service handle a heated driveway?
That is a load calculation performed on your actual building by someone qualified, and it is not something a calculator can answer. What this page will tell you is the size of the question: 800 square feet at 40 watts is 32 kilowatts, which is 133 amps at 240 volts, sitting alongside everything else in the panel. If the service will not carry it, the upgrade can cost more than the heating system.
Is a hydronic snow melt system cheaper to run than electric?
On fuel, usually yes, and by a lot at typical prices — the default comparison gives $4.86 an event on gas at $1.40 a therm against $13.82 on electricity. On installed cost, usually no: a boiler, heat exchanger, pumps, controls and a glycol loop are a different capital number and a different maintenance commitment. This page compares fuel only. Get quotes for the rest.
Do you still have to shovel with a heated driveway?
Beyond the heated area, yes. Water melting off a warm slab freezes when it reaches cold pavement, so the edge of the heated zone is where ice tends to form, and anything outside it — the apron at the street, the walk, the steps — is still a clearing job. Systems also do not run when they are off, and the hours before somebody notices the snow are hours of accumulation.