Enter your electricity rate and choose the heaters to compare from the presets (or enter their wattage and COP). The hourly, daily and monthly cost and the heat output of each heater are shown side by side. Enter the hours of use per day for each heater.
The preset wattages and COPs are typical values. For a heat pump, COP = heating capacity ÷ power input (in the same units, for example BTU/h ÷ (W × 3.412)), and it drops as the outdoor air gets colder. Electric resistance heaters, such as space heaters, oil-filled radiators, baseboard heaters and electric blankets, have a COP of 1 (a blank COP is treated as 1). Leave all fields blank for a heater you do not use. If days per month is blank, 30 days are used.
Result and graph
Choose the heaters you want to compare on the left, enter your electricity rate and press "Calculate". A comparison table and a graph of the cost of each heater will appear here.
Table of Contents
-
What you can do on this page
-
What is this calculation used for?
-
How to Use
-
Formula
-
Symbols and terms
-
Good to know before you start
-
How to calculate it in Excel
-
How to calculate it in Google Sheets
-
How to calculate it in Python
-
How to write it in LaTeX and other math languages (copy and paste)
-
How to have ChatGPT do the calculation
-
DataChef Features
-
Related Features
-
NumberChef Calculators List
What you can do on this page
- Choose up to 5 heaters to compare, such as a mini-split heat pump, a ceramic space heater, an oil-filled radiator, a baseboard heater or an electric blanket, and see the hourly, daily and monthly cost of each side by side
- Pick a preset and a typical wattage (W) and COP (coefficient of performance) are filled in for you. You can change them to the values on your own heater
- It also shows the heat output (wattage × COP) and the cost of the same amount of heat (1 kWh of heat), so you can compare the cost per unit of warmth too
- The heater with the lowest monthly cost is highlighted, and a bar chart lines up the monthly cost of each heater
- A plain-language explanation of why a heat pump usually costs less to run than a space heater (the heat pump and COP), and copy-and-paste formulas for Excel, Google Sheets and Python are all on this page
The wattage and COP of the presets are only typical values. Real values depend on the model, the room size, the thermostat setting and the outdoor temperature, so change them to the values for your heater. Heaters that turn their output up and down to hold a temperature, such as heat pumps and oil-filled radiators, spend much of the time below their rated wattage. Also, a heat pump warms the whole room while an electric blanket warms only your body, so the cost alone does not tell you which is "best". Electricity rates vary by utility, plan and state, so enter the rate from your own bill. To work out the cost of one appliance in more detail (with duty cycle, or per week and per year), use the sister page, the electricity cost calculator.
What is this calculation used for?
Settling "heat pump or space heater: which is cheaper?" with numbers
To heat a bedroom for 8 hours from evening to bedtime, a mini-split drawing 600 W with a COP of 3.5 costs about $24.48 a month, while a 1,500 W space heater costs about $61.20 ($0.17 per kWh, 30 days). And the heat pump delivers 2,100 W of heat, while the space heater delivers 1,500 W. Getting the same 2,100 W of heat from resistance heaters would cost about $85.68 a month (a standard 120 V outlet circuit supports only about 1,500 W, so one plug-in heater cannot actually do this; it is a "same warmth" calculation).
For heating a whole room for many hours, a heat pump usually wins on cost over resistance heaters, and COP is the reason. But a space heater warms you the moment it is on, so for "a short time, just where you are" the picture changes.
Saving with personal heating when you sit alone at a desk
When you work from home or study alone, keeping yourself warm may be enough without heating the whole room. An under-desk heater panel (200 W) for 8 hours a day costs about $8.16 a month, an electric blanket (100 W) about $4.08, and a space heater on low (750 W) for 5 hours about $19.13 ($0.17 per kWh, 30 days).
A heat pump that heats the room and a panel or blanket that heats you work differently, so neither is simply "better". But using personal heating when you are alone and the heat pump when the family gets together makes it easier to keep the bill down.
For sleeping, an electric blanket costs far less
An electric blanket (100 W) for the 8 hours you sleep costs about $0.14 a night, or about $4.08 a month. Running a mini-split (600 W) for the same 8 hours costs about $0.82 a night, or about $24.48 a month ($0.17 per kWh, 30 days).
When you only need to keep the bed warm, a low-wattage device like an electric blanket is a good fit. Follow the safety instructions in the manual about timers and heat settings.
For older units and cold mornings, estimate with a lower COP
The rated COP is measured under set conditions (47°F outdoors), so on very cold mornings, in cold climates or with a unit that is 10 or more years old, the real COP is lower. Even if the COP drops to 2, the cost per kWh of heat is \(0.17 \div 2 = 0.085\) dollars, half of a space heater's $0.17 ($0.17 per kWh).
Checking that "even with a low COP it is still cheaper than a space heater" gives you an estimate that covers bad days too. When it is very cold, a heat pump also pauses heating to defrost its outdoor coil, which the formula does not include, so use the result as a guide.
A resistance heater makes sense for short heating in a bathroom
Using a 1,500 W space heater in the bathroom for a total of 30 minutes a day (morning and evening) costs about \(1.5 \times 0.5 \times 0.17 \approx 0.13\) dollars a day, or about $3.83 a month ($0.17 per kWh, 30 days). Even with a high wattage, a short time of use keeps the cost low.
A heat pump takes time to warm a room and often cannot be installed in small spaces, so where you need "quick heat for a short time", a resistance heater with a COP of 1 is a sensible choice. Since the cost is "wattage × time", the difference in COP matters little when the time is short.
Formula
Cost per day and per month ($)
Standard notation
(the usual math form)
\(C_{d}\)
\(=\)
\(P\)
\(\div\)
\(1000\)
\(\times\)
\(t\)
\(\times\)
\(u\)
\(C_{m}\)
\(=\)
\(C_{d}\)
\(\times\)
\(n\)
In words
(symbols replaced with words)
⑤
\(C_d\): cost per day ($)
\(=\)
①
\(P\): wattage (W)
\(\div\)
②
1000 (W to kW)
\(\times\)
③
\(t\): hours of use per day
\(\times\)
④
\(u\): electricity rate ($/kWh)
⑦
\(C_m\): cost per month ($)
\(=\)
\(C_d\): cost per day ($)
\(\times\)
⑥
\(n\): days per month
The formula in words
①
Take the
\(P\): wattage (W)
②
divide it by
1000
to turn it into kW (kilowatts)
③
multiply by the
\(t\): hours of use per day
to get the electricity used per day (kWh)
④
multiply by the
\(u\): electricity rate ($/kWh)
⑤
and you get the
\(C_d\): cost per day ($)
⑥
Multiply that by the
\(n\): days per month
⑦
and you get the
\(C_m\): cost per month ($)
Quick example
The cost of running a 1,500 W space heater 8 hours a day for 30 days at $0.17 per kWh is
\(C_d\): cost per day ($)
\(=\)
wattage (1,500 W)
\(\div\)
1000
\(\times\)
hours (8)
\(\times\)
rate ($0.17/kWh)
\(C_m\): cost per month ($)
\(=\)
cost per day ($2.04)
\(\times\)
days (30)
\(1500 \div 1000 \times 8 \times 0.17 = 2.04\)
\(2.04 \times 30 = 61.2\)
Key idea
Electricity cost is basically "power (kW) × hours used × rate ($/kWh)". Power is usually given in watts (W), so first divide by 1,000 to turn it into kilowatts (kW). For the cost of one hour, set the hours \(t\) to 1 (for a 1,500 W space heater, \(1.5 \times 0.17 = 0.255\), about $0.26 an hour).
This formula is the same for a space heater and a heat pump. Only the wattage and the time decide the cost. "Warmth" does not appear in the formula. So, to compare how much heat you get for the same cost, the next two formulas look at the heat output and the COP.
Heat output and COP
Standard notation
(the usual math form)
\(Q\)
\(=\)
\(P\)
\(\times\)
\(\mathrm{COP}\)
In words
(symbols replaced with words)
③
\(Q\): heat output (W)
\(=\)
①
\(P\): wattage (W)
\(\times\)
②
COP (coefficient of performance)
The formula in words
①
Take the
\(P\): wattage (W)
②
multiply it by the
COP (coefficient of performance)
③
and you get the
\(Q\): heat output (W)
(for an electric resistance heater, COP = 1, so the heat output equals the wattage)
Quick example
The heat output of a mini-split heat pump drawing 600 W with a COP of 3.5, and of a 1,500 W space heater (COP 1), is
heat output of the heat pump \(Q\) (W)
\(=\)
wattage (600 W)
\(\times\)
COP (3.5)
heat output of the space heater \(Q\) (W)
\(=\)
wattage (1,500 W)
\(\times\)
COP (1)
\(600 \times 3.5 = 2100\)
\(1500 \times 1 = 1500\)
Key idea
Space heaters, oil-filled radiators, baseboard heaters and electric blankets are all electric resistance heaters: they turn electricity into heat. They can only give off as much heat as the electricity they use, so 1,500 W in means 1,500 W of heat out, a COP of 1.
A heat pump, such as a mini-split, does not "make" heat from electricity. It collects heat from the outdoor air and "moves" it into the room. Moving heat takes less electricity than making it. If 600 W of electricity moves 2,100 W of heat, the COP (coefficient of performance) is \(2100 \div 600 = 3.5\). This is why a heat pump usually costs less to run than a space heater.
You can find the COP from the spec sheet: heating capacity ÷ power input (in the same units; BTU/h ÷ 3.412 gives watts). The rated value is measured under set test conditions (in the US, 47°F outdoors), and the colder it is outside, the less heat there is to collect, so the COP drops. In real use the unit also slows down as the room nears the set temperature, so the COP changes with the time of day. On very cold mornings or in cold climates, it is safer to use a lower COP than the rating.
Cost of the same heat (1 kWh of heat)
Standard notation
(the usual math form)
\(c\)
\(=\)
\(u\)
\(\div\)
\(\mathrm{COP}\)
In words
(symbols replaced with words)
③
\(c\): cost per kWh of heat ($/kWh)
\(=\)
①
\(u\): electricity rate ($/kWh)
\(\div\)
②
COP (coefficient of performance)
The formula in words
①
Take the
\(u\): electricity rate ($/kWh)
②
divide it by the
COP (coefficient of performance)
③
and you get the
\(c\): cost per kWh of heat ($/kWh)
Quick example
At $0.17 per kWh, the cost of the same heat (1 kWh of heat) from a heat pump with a COP of 3.5 and from a space heater (COP 1) is
heat pump, cost per kWh of heat ($/kWh)
\(=\)
rate ($0.17/kWh)
\(\div\)
COP (3.5)
space heater, cost per kWh of heat ($/kWh)
\(=\)
rate ($0.17/kWh)
\(\div\)
COP (1)
\(0.17 \div 3.5 \approx 0.049\)
\(0.17 \div 1 = 0.17\)
Key idea
Comparing "$ per hour" is not fair on its own, because heaters with different wattages give different amounts of warmth. This formula compares "how much the same amount of heat costs" instead. The price of 1 kWh of electricity turns into COP times as much heat, so the cost per kWh of heat is the rate divided by the COP.
In the example, the heat pump gets the same heat for \(\dfrac{1}{3.5}\) of the space heater's cost, a little under 30%. But this is about putting the same amount of heat into the room. Heaters that warm only your body, such as an electric blanket or an under-desk heater, do not need to heat the whole room at all, so they often end up cheaper than a heat pump that heats the room. Think about how each one heats as well.
The electricity cost of any heater is "wattage (W) ÷ 1,000 × hours of use × electricity rate". But an electric resistance heater, such as a space heater, gives off only as much heat as the electricity it uses (COP = 1), while a heat pump moves heat from outdoors and delivers about 2 to 4 times its wattage as heat (its COP). To compare "the cost of the same warmth", divide the rate by the COP.
Symbols and terms
Symbols
| \(P\) | P | The wattage - how much power the heater uses, in W (watts), from the first letter of "power". For a heat pump, use the power input for heating from the spec sheet. |
| \(t\) | t | The hours of use per day, from the first letter of "time". |
| \(u\) | u | The electricity rate - what you pay for 1 kWh ($/kWh), from the first letter of "unit price". |
| \(n\) | n | The number of days per month, from the first letter of "number". If blank, this calculator uses 30 days. |
| \(C_d\) | C sub d | The cost per day ($). \(C\) is for cost, and the small \(d\) below the line is for day. |
| \(C_m\) | C sub m | The cost per month ($). The small \(m\) below the line is for month. |
| \(Q\) | Q | The heat output - how much heat the heater gives off per second, in W (watts). In physics, \(Q\) is the usual letter for an amount of heat (said to come from "quantity of heat"). |
| \(\mathrm{COP}\) | C-O-P | The coefficient of performance - how many times its wattage a heater delivers as heat. It is 1 for electric resistance heaters and about 2 to 4 for heat pumps. |
| \(c\) | small c | The cost per kWh of heat ($/kWh) - the cost of the same warmth, the rate \(u\) divided by the COP. It is written in lowercase to tell it apart from the cost \(C\). |
Terms
| wattage | The power (W) a heater uses while it runs. The value on the label or spec sheet is usually the maximum (rated) wattage, so for heaters that turn their output up and down to hold a temperature, the real average is lower. |
| kWh (kilowatt-hour) | The unit for an amount of electricity used. Using 1 kW (1,000 W) for 1 hour is 1 kWh. Electric bills are based on kWh. |
| COP (coefficient of performance) | A number that shows how many times the electricity used comes out as heat. For a heat pump, it is the heating capacity divided by the power input (in the same units). Space heaters and other electric resistance heaters can only give off as much heat as the electricity they use, so their COP is 1. The colder it is outside, the lower a heat pump's COP. |
| heat pump | A system that does not "make" heat from electricity but collects heat from the air (or the ground) and "moves" it somewhere else. It pumps heat uphill the way a pump lifts water, hence the name. Moving heat takes less electricity than making it, so heat pumps, including mini-splits and heat pump water heaters, give the same warmth for less electricity than resistance heaters. |
| heating capacity | How much heat a heat pump can deliver to the room, shown on the spec sheet as "Heating Capacity 7,200 BTU/h". It is a separate line from the power input. The ratio of the two (in the same units) is the COP. |
| heat output | How much heat a heater gives off per second (W). On this page it is calculated as wattage × COP. For a resistance heater it equals the wattage, and for a heat pump it means the same as the heating capacity (in watts). It is not the same as the heating value of a fuel (such as BTU per therm). |
| electric resistance heater | Any heater that warms by passing electricity through a wire or element that gets hot. Space heaters (ceramic, infrared, oil-filled), baseboard heaters, electric blankets and heated panels are all in this group, and their COP is 1. |
| electricity rate | The price of 1 kWh of electricity. The examples on this page use $0.17 per kWh, close to the US average residential price in recent years (US Energy Information Administration). The real rate depends on your utility, plan and state, so use the rate from your bill. |
| HSPF2 | Heating Seasonal Performance Factor 2 - the heat a heat pump delivers over a whole heating season (BTU) divided by the electricity it uses (Wh). While the COP is the efficiency at one moment, HSPF2 is the average over the season, including cold days. HSPF2 ÷ 3.412 gives roughly the average COP over the season (an HSPF2 of 8.5 is an average COP of about 2.5). |
| rated wattage | The wattage the maker states for the heater running at full power. The "wattage" on the label or spec sheet is usually this value, so for heaters that lower their output with a thermostat, such as oil-filled radiators, the real average is lower. |
| thermostat | An automatic control that switches the heater off when the set temperature is reached and on again when it drops. Heat pumps, oil-filled radiators and many space heaters turn their output up and down this way, so "wattage × hours" can overestimate the real cost. |
| radiant heating | Heating that warms people and objects in front of it directly with infrared, as an infrared (quartz) heater does. You feel warm as soon as it is on, and it is best for heating a spot, unlike convection heating, which warms the whole room. Choose by use, not by cost alone. |
| convection heating | Heating that warms the whole room with warm air, as heat pumps, baseboard heaters and fan heaters do. It is best for the whole room, unlike radiant heating, which warms what is in front of it. Choose by use, not by cost alone. |
Good to know before you start
Here is what helps you use the calculation on this page with real understanding, not just by pressing the button.
If you get stuck, going back to these topics is the quickest way forward.
| Multiplying and dividing decimals (Grades 5–6) |
|
| Ratios and "how many times" (Grades 6–7) |
|
| Unit conversion (Grades 4–8) |
|
| Power, energy and heat (middle school science) |
|
How to calculate it in Excel
Copy the whole table below and paste it into cell A1 in Excel. It works as is.
Table to find the cost per day and per month
| Wattage (W) | 1500 |
| Hours of use per day | 8 |
| Electricity rate ($/kWh) | 0.17 |
| Days per month | 30 |
| Cost per day ($) | =B1/1000*B2*B3 |
| Cost per month ($) | =B5*B4 |
Table to find the heat output
| Wattage (W) | 600 |
| COP (coefficient of performance) | 3.5 |
| Heat output (W) | =B1*B2 |
Table to find the cost per kWh of heat
| Electricity rate ($/kWh) | 0.17 |
| COP (coefficient of performance) | 3.5 |
| Cost per kWh of heat ($/kWh) | =B1/B2 |
After pasting, the upper cells in column B are your inputs and the formula cells are calculated automatically.
The first table is a 1,500 W space heater used 8 hours a day for 30 days at $0.17 per kWh: B5 shows 2.04 and B6 shows 61.2 (dollars). To compare heaters, copy the table to the right once for each heater and change the wattage and hours in column B (C, D and so on).
The second table is the heat output of a mini-split drawing 600 W with a COP of 3.5: B3 shows 2100 (W). The third table is the cost of 1 kWh of heat from the same heat pump: B3 shows about 0.0486 ($/kWh). For a space heater, set the COP cell to 1 and you get the rate itself ($0.17 per kWh).
The first table is a 1,500 W space heater used 8 hours a day for 30 days at $0.17 per kWh: B5 shows 2.04 and B6 shows 61.2 (dollars). To compare heaters, copy the table to the right once for each heater and change the wattage and hours in column B (C, D and so on).
The second table is the heat output of a mini-split drawing 600 W with a COP of 3.5: B3 shows 2100 (W). The third table is the cost of 1 kWh of heat from the same heat pump: B3 shows about 0.0486 ($/kWh). For a space heater, set the COP cell to 1 and you get the rate itself ($0.17 per kWh).
How to calculate it in Google Sheets
Copy the whole table below and paste it into cell A1 in Google Sheets. It works as is.
Table to find the cost per day and per month
| Wattage (W) | 1500 |
| Hours of use per day | 8 |
| Electricity rate ($/kWh) | 0.17 |
| Days per month | 30 |
| Cost per day ($) | =B1/1000*B2*B3 |
| Cost per month ($) | =B5*B4 |
Table to find the heat output
| Wattage (W) | 600 |
| COP (coefficient of performance) | 3.5 |
| Heat output (W) | =B1*B2 |
Table to find the cost per kWh of heat
| Electricity rate ($/kWh) | 0.17 |
| COP (coefficient of performance) | 3.5 |
| Cost per kWh of heat ($/kWh) | =B1/B2 |
These formulas use only multiplication and division, so the same formulas as in Excel work as is. Copy the whole table, paste it into cell A1, and replace the numbers in column B with the values for your heater.
How to calculate it in Python
price_per_kwh = 0.17 # electricity rate ($/kWh)
days_per_month = 30 # days per month
# Heaters to compare: (name, wattage W, COP, hours per day). Resistance heaters have COP = 1
heaters = [
("Mini-split heat pump", 600, 3.5, 8),
("Ceramic space heater", 1500, 1, 8),
("Under-desk heater panel", 200, 1, 8),
]
for name, power_watts, cop, hours_per_day in heaters:
hour_cost = power_watts / 1000 * price_per_kwh # cost per hour ($)
day_cost = hour_cost * hours_per_day # cost per day ($)
month_cost = day_cost * days_per_month # cost per month ($)
heat_watts = power_watts * cop # heat output (W)
heat_cost_per_kwh = price_per_kwh / cop # cost per kWh of heat ($/kWh)
print(f"{name}: per hour ${hour_cost:.2f} / per day ${day_cost:.2f} / per month ${month_cost:.2f} / "
f"heat output {heat_watts:.0f} W / per kWh of heat ${heat_cost_per_kwh:.3f}")
# The heater with the lowest monthly cost
cheapest = min(heaters, key=lambda h: h[1] / 1000 * h[3] * price_per_kwh * days_per_month)
print("Lowest monthly cost:", cheapest[0])
Runs with the standard library only. Change the rate and days at the top and each line of heaters (name, wattage, COP, hours per day) to your own heaters and run it. Add lines to compare 4 or 5 heaters.
How to write it in LaTeX and other math languages (copy and paste)
Cost per day and per month ($)
C_d = P ÷ 1000 × t × u, C_m = C_d × n
C_d = \frac{P}{1000} \times t \times u, \quad C_m = C_d \times n
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<msub><mi>C</mi><mi>d</mi></msub>
<mo>=</mo>
<mfrac><mi>P</mi><mn>1000</mn></mfrac>
<mo>×</mo>
<mi>t</mi>
<mo>×</mo>
<mi>u</mi>
<mo>,</mo>
<msub><mi>C</mi><mi>m</mi></msub>
<mo>=</mo>
<msub><mi>C</mi><mi>d</mi></msub>
<mo>×</mo>
<mi>n</mi>
</mrow>
</math>
C_d = P/1000 * t * u, C_m = C_d * n
dayCost = power/1000*hours*price; monthCost = dayCost*days
C_d := P/1000*t*u; C_m := C_d*n;
C_d = P/1000*t*u; C_m = C_d*n;
C_d = P/1000×t×u, C_m = C_d×n
Heat output and COP
Q = P × COP
Q = P \times \mathrm{COP}
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>Q</mi>
<mo>=</mo>
<mi>P</mi>
<mo>×</mo>
<mi mathvariant="normal">COP</mi>
</mrow>
</math>
Q = P * COP
heat = power*cop
Q := P*COP;
Q = P*COP;
Q = P×COP
Cost of the same heat (1 kWh of heat)
c = u ÷ COP
c = \frac{u}{\mathrm{COP}}
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mi>c</mi>
<mo>=</mo>
<mfrac><mi>u</mi><mi mathvariant="normal">COP</mi></mfrac>
</mrow>
</math>
c = u / COP
heatCost = price/cop
c := u/COP;
c = u/COP;
c = u/COP
How to have ChatGPT do the calculation
You are an assistant for comparing heating costs. Do the following calculation by actually running Python code, and base your answer only on the numbers from the execution result (do not answer by mental math or guessing). The electricity rate is $0.17 per kWh, and a month is 30 days. Compare these three heaters: - Mini-split heat pump: 600 W, COP 3.5, 8 hours a day - Ceramic space heater: 1,500 W, COP 1, 8 hours a day - Under-desk heater panel: 200 W, COP 1, 8 hours a day For each heater, find: 1. The cost per hour, per day and per month ($): "wattage (W) ÷ 1000 × hours × rate" 2. The heat output (W): "wattage × COP" 3. The cost of the same warmth (1 kWh of heat, $/kWh): "rate ÷ COP" Finally, show which heater has the lowest monthly cost and which has the lowest cost per kWh of heat. Show the formulas you used and the numbers from the execution result.
How to Use
-
1Enter your numbersType the numbers you want to calculate with into the input fields
-
2CalculatePress the "Calculate" button
-
3Check the resultThe result appears on the spot. The same page also explains the idea behind the calculation and the formula
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