You’re cold in one room, the thermostat is out in the hallway, and someone online swears a $40 space heater will cut your heating bill in half. Someone else swears it’ll triple it. Both can be right — and the difference comes down to about four numbers you already have on your utility bill.
This article is a calculator, not an opinion. You’ll get the exact math for what a space heater costs per hour, what your central system costs per hour, and how to compare them fairly using cost per unit of delivered heat instead of cost per appliance. Then we’ll find the break-even: how small your heated area has to get before zone heating actually wins, and which central systems a plug-in heater can almost never beat.
Along the way you’ll get the specs that matter but rarely make the blog posts — the 12.5-amp draw of a 1,500-watt heater, the 80% circuit rule, the 3-foot clearance rule, and an honest ceiling on how much floor area 5,120 BTU per hour can hold at 68°F. By the end you should be able to write your own number on a napkin and stop guessing.
| Spec or figure | Number |
|---|---|
| Max wattage, standard 120V plug-in heater | 1,500 W (1.5 kW) |
| Heat output of 1,500 W | ~5,120 BTU/hour |
| Current draw at 120 volts | 12.5 amps |
| Safe continuous load, 15A circuit (80% rule) | 12 A — one heater, nothing else |
| Cost per hour at 16¢/kWh, full output | $0.24 |
| Cost per month, 8 hrs/day at 60% duty cycle | ~$34.56 |
| Delivered heat: electric resistance @ 16¢/kWh | ~$47 per million BTU |
| Delivered heat: 95% AFUE gas furnace @ $1.50/therm | ~$16 per million BTU |
| Delivered heat: heat pump at COP 3.0 @ 16¢/kWh | ~$16 per million BTU |
| Room a 1,500 W heater can typically hold | 100–150 sq ft (10–15 W per sq ft) |
| Required clearance from anything combustible | 3 feet (0.9 m) on all sides |
| Thermostat setback savings (US DOE) | Up to 10% a year, setting back 7–10°F for 8 hrs/day |
| Lowest safe winter thermostat setting | 55°F (13°C), to reduce frozen-pipe risk |
Step 1: Forget appliances — compare cost per unit of heat
A space heater and a furnace sell you the same product: BTUs of heat inside your home. The only fair comparison is price per BTU delivered, after efficiency losses. Once you have that, everything else is arithmetic.
Here are the conversions you need. Write them down once and you’ll never need another article.
| Unit purchased | Heat content |
|---|---|
| 1 kilowatt-hour (kWh) of electricity | 3,412 BTU |
| 1 therm of natural gas | 100,000 BTU (= 29.3 kWh) |
| 1 Ccf (100 cubic feet) of natural gas | ~103,700 BTU, about 1.037 therms (US average heat content, EIA) |
| 1 gallon of propane | ~91,500 BTU |
| 1 gallon of No. 2 heating oil | ~138,500 BTU |
| 1 million BTU (MMBTU) | 293 kWh, or 10 therms |
Now the money table. Efficiency is what separates these numbers: an electric resistance heater turns essentially 100% of the electricity into heat right there in the room, an 80% AFUE furnace sends about a fifth of its fuel energy up the flue, and a heat pump moves heat rather than making it, delivering 2 to 4 units of heat per unit of electricity.
| Heat source | Fuel price assumed | Efficiency or COP | Cost per MMBTU delivered |
|---|---|---|---|
| Plug-in electric space heater | $0.16/kWh | 100% | $46.90 |
| Plug-in electric space heater | $0.30/kWh | 100% | $87.90 |
| Electric baseboard or electric furnace | $0.16/kWh | 100% (minus duct loss) | $46.90+ |
| Air-source heat pump, mild weather | $0.16/kWh | COP 3.0 | $15.63 |
| Air-source heat pump, cold weather | $0.16/kWh | COP 2.0 | $23.45 |
| Natural gas furnace, high efficiency | $1.50/therm | 95% AFUE | $15.79 |
| Natural gas furnace, older | $1.50/therm | 80% AFUE | $18.75 |
| Propane furnace | $2.60/gallon | 95% AFUE | ~$29.90 |
| Heating oil boiler | $3.80/gallon | 85% efficiency | ~$32.30 |
Read that table twice, because it’s the whole article in one glance. Per BTU, a plug-in heater costs roughly three times as much as natural gas heat and about three times as much as a heat pump in mild weather. It’s a wash against electric baseboard, and it beats nothing at all.
So a space heater saves money exactly one way: by letting you buy far fewer BTUs. Not cheaper heat — less heat.
Step 2: Calculate your space heater’s cost per hour
The formula is short:
Cost per hour = (watts ÷ 1,000) × your electricity price per kWh × duty cycle
Your electricity price is on your bill — but don’t use the supply rate alone. Divide your total bill by the total kWh used so delivery and distribution charges are baked in. That all-in number is usually well above the headline rate, and it’s the one that actually leaves your bank account.
| Heater setting | 10¢/kWh | 13¢/kWh | 16¢/kWh | 20¢/kWh | 25¢/kWh | 30¢/kWh |
|---|---|---|---|---|---|---|
| 1,500 W (high) | $0.15 | $0.20 | $0.24 | $0.30 | $0.38 | $0.45 |
| 1,000 W (medium) | $0.10 | $0.13 | $0.16 | $0.20 | $0.25 | $0.30 |
| 750 W (low) | $0.08 | $0.10 | $0.12 | $0.15 | $0.19 | $0.23 |
| 400 W (panel or personal) | $0.04 | $0.05 | $0.06 | $0.08 | $0.10 | $0.12 |
Multiply by hours, then by days. Here’s a 1,500 W heater at 16¢/kWh:
| Hours per day | kWh/month at full output | Cost at 100% | Cost at 60% duty cycle | Cost at 35% duty cycle |
|---|---|---|---|---|
| 2 | 90 | $14.40 | $8.64 | $5.04 |
| 4 | 180 | $28.80 | $17.28 | $10.08 |
| 8 | 360 | $57.60 | $34.56 | $20.16 |
| 12 | 540 | $86.40 | $51.84 | $30.24 |
| 16 | 720 | $115.20 | $69.12 | $40.32 |
| 24 | 1,080 | $172.80 | $103.68 | $60.48 |
Step 3: Duty cycle — the number everyone forgets
A heater with a thermostat doesn’t pull 1,500 W all night. It heats until the room hits the setpoint, shuts the element off, and coasts. The share of time the element is actually energized is the duty cycle, and it’s usually what turns a scary estimate into a realistic one.
There’s no published table of duty cycles, because the number depends entirely on your room’s heat loss, your setpoint, and the weather. The figures below are illustrative planning brackets, not measurements — use them to sketch a budget, then measure your own.
| Situation | Ballpark duty cycle | Effective average draw (1,500 W unit) |
|---|---|---|
| Small, well-insulated interior room, mild outside | Low | ~400–600 W |
| Average bedroom, one exterior wall, freezing outside | Moderate | ~750–1,000 W |
| Drafty room, large single-pane window, hard freeze | High | ~1,100–1,400 W |
| Room too large for the heater | 100% (never reaches setpoint) | 1,500 W |
| Heater with no thermostat, run on high | 100% | 1,500 W |
You can replace all that guesswork with a measurement: an inexpensive plug-in energy monitor sits between the outlet and the heater and logs actual kWh. Run it through three cold days, read the total, divide by hours. Make sure the monitor is rated for the load you’re putting through it — and never substitute a power strip for a wall outlet (see the electrical section below).
Step 4: What your central heating actually costs per hour
Central systems buy cheaper BTUs — and a lot more of them. A single 1,500 W heater makes about 5,120 BTU/hour. The sample furnace capacities in the table below run 60,000–100,000 BTU/hour of input, or 12 to 20 space heaters’ worth of output. (Real furnace sizing varies widely with climate and house size, so check your own nameplate rather than assuming.)
| System | Rated capacity | Energy price assumed | Cost per hour running | Heat delivered per hour |
|---|---|---|---|---|
| Gas furnace, 80% AFUE | 60,000 BTU input | $1.50/therm | $0.90 + blower | 48,000 BTU |
| Gas furnace, 80% AFUE | 80,000 BTU input | $1.50/therm | $1.20 + blower | 64,000 BTU |
| Gas furnace, 95% AFUE | 100,000 BTU input | $1.50/therm | $1.50 + blower | 95,000 BTU |
| Blower motor electricity | Varies widely by motor type — read the nameplate | $0.16/kWh | About $0.016/hour per 100 W of draw | — |
| Heat pump, 3 ton, COP 3.0 | 36,000 BTU output | $0.16/kWh | ~$0.56 | 36,000 BTU |
| Heat pump with electric backup strips on | 10 kW strips | $0.16/kWh | $1.60 (strips alone) | 34,120 BTU |
| Electric furnace | 15 kW | $0.16/kWh | $2.40 | 51,180 BTU |
| Electric baseboard, per linear foot | 250 W/ft | $0.16/kWh | $0.04 per foot | 853 BTU/ft |
To get a monthly figure, estimate daily run time: a few hours a day in shoulder season, many more during a cold snap. Eight hours a day from an 80,000 BTU/hr furnace at $1.50/therm works out to $9.60/day in gas, or roughly $288 over a 30-day cold month, plus blower electricity. Better still, use the therm total on last winter’s bill — that’s ground truth, not an estimate.
Step 5: The break-even — when zone heating actually wins
Zone heating pays only when the gas or electricity you stop buying is worth more than the electricity you start buying. Here are four common setups, all illustrated on the same 1,600 sq ft house over the same 30-day cold month. Treat the dollar figures as a worked example, not a prediction for your house.
| Scenario | Central cost | Space heater cost | Combined total | Verdict |
|---|---|---|---|---|
| Gas furnace at 70°F, no space heater | $250 | $0 | $250 | Baseline |
| Gas furnace set back to 62°F + one 1,500 W heater, 10 hrs/day at 60% duty | ~$200 | $43 | $243 | Saves ~$7 — basically a wash |
| Gas furnace at 55°F, family lives in 2 rooms, two heaters 12 hrs/day | ~$150 | $104 | $254 | No savings, less comfort |
| Heat pump (COP 3) at 70°F, no space heater | $240 | $0 | $240 | Baseline |
| Heat pump set back to 62°F + one heater 10 hrs/day | ~$195 | $43 | $238 | Essentially no savings |
| Electric furnace or baseboard at 70°F | $620 | $0 | $620 | Baseline |
| Electric heat off, heat only a 130 sq ft room, one heater 14 hrs/day | $0 | $60 | $60 | Big win — same fuel price, far fewer BTUs |
| Propane or oil central heat, deep setback + one heater | ~30% less fuel | $43–$60 | Often lower | Usually a modest win |
Three rules fall out of this:
- Electric resistance central heat (electric furnace, baseboards, wall heaters): space heaters can cut your bill sharply. The fuel price is identical, so every square foot you stop heating is pure savings.
- Natural gas or a working heat pump: a space heater almost never pays unless you shrink the heated area to one or two rooms and hold the rest of the house near 55–60°F.
- Propane or oil: you land in between. Do the math with your actual delivery price, which swings a lot from year to year.
One non-money factor outranks all of this: an underheated house can freeze pipes. Don’t set a central thermostat below 55°F (13°C) in freezing weather, especially with plumbing in exterior walls, crawlspaces, or an unheated garage. One burst supply line costs more than a decade of thermostat setbacks.
Step 6: Are some space heaters cheaper to run than others?
For plug-in electric models, no. Every electric resistance heater converts watts to heat at effectively 100%. A 1,500 W oil-filled radiator, a 1,500 W ceramic tower, and a 1,500 W quartz infrared unit cost exactly the same per hour at the same setting. Any “93% more efficient” claim about a resistance heater isn’t physics; it’s copywriting.
What genuinely moves your bill is how many watt-hours you need — a function of controls, placement, and whether the heat lands on you or on the ceiling.
| Type | Typical wattage | Cost/hr on high @ 16¢ | Efficiency | Best use | Notes |
|---|---|---|---|---|---|
| Ceramic fan-forced | 750 / 1,500 W | $0.24 | 100% | Fast warm-up of small rooms | Fan noise; heats air, so drafts steal it |
| Oil-filled radiator | 600 / 900 / 1,500 W | $0.24 | 100% | Steady, quiet bedroom heat | Slow to warm, coasts after shutoff; surface gets hot |
| Quartz or carbon infrared | 1,000–1,500 W | $0.24 | 100% | Warming a person at a desk | Lets you tolerate a cooler room, so run time can drop |
| Micathermic panel | 1,000–1,500 W | $0.24 | 100% | Slim spaces, no fan | Wall-mount versions must be installed per the manual |
| Low-wattage panel heater | 250–450 W | $0.04–$0.07 | 100% | Tiny rooms, supplemental only | Can’t hold a cold room on its own |
| Heated throw or seat pad | Check the label (well under a space heater) | ~$0.016/hr per 100 W | 100% | Personal comfort while seated | The cheapest way to feel warm |
| Portable or mini-split heat pump | 400–900 W input | $0.06–$0.14 | COP 2–4 | Rooms that need long hours of heat | Only category that beats resistance per BTU; costs more upfront |
| Unvented propane or kerosene | Fuel-fired | Varies with fuel price | Burns nearly all its fuel energy, but dumps moisture and combustion gases indoors | Outdoors, workshops, emergencies | Needs a working CO alarm and the manual’s ventilation; never use an outdoor-only model inside, and never in a bedroom, bathroom, or closet |
Note the bottom two rows. A heat pump is the one category that changes the math, because it delivers 2–4 BTUs per BTU of electricity (with output and COP falling as it gets colder outside). Combustion heaters change the risk profile instead: they produce carbon monoxide and water vapor indoors. Gas piping and appliance connections are regulated locally — check your jurisdiction’s rules and hire a qualified pro rather than improvising, and never modify a heater to burn a fuel it wasn’t built for.
Step 7: How much room can 1,500 watts actually hold?
A lot of overspending starts with a 5,120 BTU/hr appliance in a 300 sq ft room, running 24 hours a day at 100% duty cycle and never reaching setpoint. Size with a watts-per-square-foot rule of thumb instead.
| Room condition | Watts per sq ft | Area held by 750 W | Area held by 1,500 W |
|---|---|---|---|
| Interior room, good insulation, newer windows | 8 | ~94 sq ft | ~188 sq ft |
| Average room, one exterior wall, double-pane | 10 | 75 sq ft | 150 sq ft |
| Corner room, two exterior walls | 12 | 63 sq ft | 125 sq ft |
| Drafty room, single-pane, uninsulated wall | 15 | 50 sq ft | 100 sq ft |
| Sunroom, garage, converted porch | 20+ | 38 sq ft | 75 sq ft |
Ceilings above 8 feet, or outdoor temperatures well below freezing, push you toward the higher figures. If your heater runs nonstop and the room still feels cool, you don’t need a bigger heater — you need to stop the heat from leaving, which is the cheapest step of all (Step 9).
Step 8: Electrical limits and safety you can’t skip
A 1,500 W heater draws 12.5 amps at 120 volts. That’s not a trivial load — it’s the biggest continuous load most renters ever plug in.
| Circuit | Total capacity | Safe continuous load (80%) | What fits |
|---|---|---|---|
| 15 A / 120 V | 1,800 W | 12 A / 1,440 W | One 1,500 W heater and nothing else — and 12.5 A already exceeds the 80% figure, so run it on low (750–1,000 W) if the breaker warms or trips |
| 20 A / 120 V | 2,400 W | 16 A / 1,920 W | One 1,500 W heater plus small loads (lamp, laptop) |
| Two 1,500 W heaters | 25 A combined | — | Must be on two separate circuits |
The non-negotiables, straight from heater manuals and fire-code requirements:
- Plug the heater directly into a wall outlet. No extension cords, power strips, surge protectors, or cube taps — they’re rated for lower continuous loads and are a documented ignition point.
- Keep 3 feet (0.9 m) of clearance on all sides: bedding, curtains, laundry, sofas, paper, pet beds, holiday trees.
- Buy only UL-, ETL-, or CSA-listed units with both a tip-over switch and an overheat cutoff.
- Turn it off when you leave the room or go to sleep. Heating equipment is a leading cause of US home fires — NFPA puts it at roughly 37,000 home fires and about 417 deaths a year (2020–2024 averages, about 11% of reported home fires). Space heaters and heating stoves, stationary and portable together, account for around 30% of those fires but roughly 73% of the deaths.
- Damp rooms: a heater used in a bathroom must be rated for bathroom use and plugged into a GFCI-protected outlet. Unplug it and let it cool fully before cleaning or moving it.
- Wall-mounted panel heaters: follow the installation manual’s fastening instructions to the letter — typically anchoring into framing or manufacturer-specified anchors, not bare drywall. Skip any install that needs a tall ladder; a fall is a bigger risk than a cold room.
- Timers and smart plugs: many manuals tell you not to put any accessory between the heater and the outlet. Read yours; if it does allow one, its own amp rating has to comfortably exceed the heater’s 12.5 A continuous draw. Check the rating stamped on the device, not the claims on the box.
- Fuel-burning heaters: install a working CO alarm, follow the manual’s ventilation requirements exactly, and never run a camping, patio, or unvented shop heater in a bedroom, bathroom, or closet.
- Renters: read your lease. Some prohibit portable heaters outright, and some older buildings’ 15 A circuits genuinely can’t handle two of them.
Step 9: Eight fixes that beat both options on cost per degree
Before you spend on either heating strategy, plug the leaks. These are landlord-friendly and removable, and they cut the watts and the therms you need from here on out.
| Fix | Ballpark cost | What it does | Renter-safe? |
|---|---|---|---|
| Door sweep or draft stopper | $10–$25 | Closes a 1/4–1/2 in. gap under the door | Yes (adhesive or slide-on) |
| Window insulation film kit | Varies by kit | Adds a still-air layer and kills the cold-glass draft | Yes — removes in spring |
| V-seal or foam weatherstrip on sashes | $8–$20 per window | Cuts perimeter air leakage | Yes |
| Rope caulk on rattling sashes | $5–$10 | Seasonal seal; peels off cleanly | Yes |
| Thermal-lined curtains, closed at dusk | $25–$70 per window | Reduces nighttime radiant loss | Yes |
| Programmable or smart thermostat setback | $25–$180 | Up to 10% annual heating and cooling savings at 7–10°F back for 8 hrs/day (US DOE) | Ask the landlord; low-voltage (24 V) work only |
| Foam gaskets behind exterior-wall outlet and switch plates | $5–$12 per pack | Small but cumulative — kill the circuit before removing plates | Yes |
| Close unused rooms; check register dampers | $0 | Redirects the central heat you’re already paying for | Yes |
One comfort-science note: at the same air temperature, you feel warmer when nearby surfaces are warmer and the air is still. That’s why film on a cold window or a closed thermal curtain can let you drop the setpoint a couple of degrees with no loss of comfort — worth more than any heater swap.
Step 10: Your 5-minute worksheet
Fill in six blanks with numbers from your own bills.
- Electricity price: total electric bill ÷ total kWh = $____/kWh (all-in).
- Gas price: total gas bill ÷ therms (or Ccf × 1.037) = $____/therm.
- Heater cost per hour: (watts ÷ 1,000) × line 1 × duty cycle = $____.
- Heater cost per month: line 3 × hours per day × days = $____.
- Central heating cost per month now: pull last winter’s usage for the same month = $____.
- Expected setback savings: line 5 × your setback estimate — DOE’s guidance of up to 10% a year for a 7–10°F, 8-hour setback is a sensible ceiling = $____.
If line 6 is bigger than line 4, zone heating saves you money. If it’s smaller, the space heater is a comfort purchase rather than a savings purchase — a perfectly good reason to own one, as long as you know which you’re buying. Track your next two bills against these estimates and adjust the duty-cycle figure; that single number is usually the source of any big error.
Frequently Asked Questions
Is it cheaper to run a space heater or turn up the central heat?
It depends entirely on your central system’s fuel. With natural gas or a working heat pump, an hour of central heat delivers many times more heat for only a few times the hourly cost, so per BTU it’s much cheaper — nudging the thermostat up 2°F usually beats running a 1,500 W heater. With electric baseboard or an electric furnace, heating one 130 sq ft room with a portable unit instead of the whole house can cut heating costs by well over half.
How much does it cost to run a 1,500-watt heater for 24 hours?
At full output it uses 36 kWh: $3.60 at 10¢/kWh, $5.76 at 16¢/kWh, $10.80 at 30¢/kWh. With a thermostat cycling it at a 60% duty cycle, figure roughly $2.16, $3.46, and $6.48. Over a 30-day month, that’s about $65 to $195.
Do oil-filled radiators use less electricity than fan heaters?
Not at the same wattage. Both are 100%-efficient resistance heaters, so 1,500 W costs the same per hour either way. Oil-filled units can seem cheaper because they keep releasing stored heat after the element shuts off, which can trim the duty cycle in a small, sealed room, and because many people run them on a 600–900 W setting. The savings come from the setting, not the technology.
Will a space heater trip my breaker?
It can. A 1,500 W heater pulls 12.5 A, which is 83% of a 15 A circuit’s rating all by itself. Add a hair dryer, a microwave, or a second heater on that circuit and the breaker will trip. Put heaters on separate circuits, use the low setting on older wiring, and never plug one into an extension cord or power strip. Repeated tripping or a warm outlet means stop and call a licensed electrician — don’t open the panel yourself.
Can I turn off central heat completely and just use space heaters?
Only if you can genuinely live in one or two rooms, and only if your central heat is electric resistance or an expensive delivered fuel. Two big caveats: a 1,500 W heater tops out around 100–150 sq ft, and letting the rest of the house get too cold invites frozen pipes plus condensation and mold on cold surfaces. Keep the central thermostat at 55°F minimum in freezing weather.
Are heat pump space heaters worth the extra money?
If you need long hours of heat in the same room every winter, yes. A unit running at a COP of 3 delivers the same heat on about a third of the electricity — roughly $0.08/hour instead of $0.24 at 16¢/kWh. Heat one room 10 hours a day for 120 days and that’s about $190 a season saved versus resistance heat, which pays back a portable or mini-split heat pump within a few winters. In very cold weather the COP drops and the gap narrows.
What’s the cheapest way to stay warm at night?
Personal heat beats room heat by a wide margin. A heated mattress pad or throw draws a small fraction of a space heater’s wattage — check the label, but at 100 W you’re looking at about 1.6¢ an hour at 16¢/kWh versus $0.24 for a 1,500 W heater. Pair that with a 62–65°F room, a door sweep, and film on the window and you get the same comfort for a fraction of the energy. Follow the manufacturer’s instructions: don’t fold heated bedding while it’s on, and replace units per the maker’s guidance.
How do I measure what my heater is really costing me?
Use a plug-in energy monitor rated for the load. Plug it into the wall, plug the heater into it, and read cumulative kWh after three cold days. Multiply those kWh by your all-in electricity rate, then divide by hours to get your true average cost per hour with the duty cycle already baked in. That measured number — not a spec sheet — is what belongs in the worksheet above.
Reviewed and edited by Jason Paik. Spotted an error? Tell us — we verify and correct. See how we create content.