That space heater you run all winter. The gaming PC left on overnight. The pool pump running 12 hours a day. Each of these has a real, calculable monthly cost, and most people have never worked out what it is.
The calculation is simple: Watts times Hours divided by 1,000, multiplied by your rate. The insights from applying it to every device in your home are anything but simple.
The universal formula
Monthly cost = (Watts x Hours per day x 30) / 1000 x Rate
You need three numbers: the device wattage (on its label or in its manual), hours of daily use, and your electricity rate (on your bill, measured in dollars per kWh).
Example: A 200-watt desktop computer used 10 hours per day at $0.15/kWh.
(200 x 10 x 30) / 1000 x $0.15 = $9.00/month
That same computer in sleep mode (5 watts) for the remaining 14 hours:
(5 x 14 x 30) / 1000 x $0.15 = $0.32/month
Total: $9.32/month. Compared to leaving it fully on 24/7: $21.60/month.
The devices that cost the most to run
I went through every device in my home and calculated the monthly cost. The results reshuffled my assumptions:
| Device | Watts | Hours/day | Monthly cost |
|---|---|---|---|
| Central AC | 3,500 | 8 | $126.00 |
| Electric water heater | 4,500 | 3 | $60.75 |
| Electric dryer | 5,000 | 0.5 | $11.25 |
| Space heater | 1,500 | 6 | $40.50 |
| Refrigerator | 150 | 24 | $16.20 |
| Desktop computer | 200 | 10 | $9.00 |
| TV (65") | 100 | 5 | $2.25 |
| LED bulb | 9 | 8 | $0.32 |
| Phone charger | 5 | 3 | $0.07 |
The AC and water heater together account for more than all other devices combined. This is why insulating your home and upgrading to a heat pump water heater are the highest-impact energy efficiency investments.
Vampire loads add up
Devices that draw power while "off" or in standby are called vampire loads or phantom loads. Individually they are small. Collectively they matter.
A typical home has:
- 3 TVs in standby: 3 x 5W = 15W
- Cable/streaming box: 15W
- Game console in standby: 10W
- Microwave (clock display): 3W
- Printer in sleep: 5W
- 4 phone/tablet chargers (plugged in, not charging): 4 x 1W = 4W
- Smart home devices: 10W
- Total vampire load: ~62W
Running 24/7: (62 x 24 x 30) / 1000 x $0.15 = $6.70/month, or $80/year.
Smart power strips that cut power to devices in standby can eliminate most of this.
Seasonal variation
Your electricity cost is not constant. HVAC drives dramatic seasonal variation. A $150/month summer bill with AC can drop to $60/month in spring and fall. If you heat with electricity, winter bills may be the highest.
Understanding the seasonal pattern helps with budgeting and identifies when efficiency improvements have the most impact.
The calculator
For calculating the cost of running any device, I built an electricity cost calculator that takes wattage, hours, and your local rate, then shows daily, monthly, and annual costs. Compare multiple devices side by side to see where your money goes.
I'm Michael Lip. I build free developer tools at zovo.one. 500+ tools, all private, all free.
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