Here is the thing: the math is trivial. It is three multiplications. The hard part is getting honest numbers for the watts, so this post covers both, with a script you can run against your own setup.

Quick answer

A small home server or mini PC that idles at 15 W costs about $2 a month at the U.S. average residential rate of 18.31 cents per kWh (EIA Electric Power Monthly, Table 5.6.A, July 2026 data). A 4-bay NAS plus a mini PC, around 50 W combined, costs about $6.60 a month. A repurposed gaming PC idling at 90 W with a GPU is closer to $12 a month, and that is before it does any work. The formula: watts × hours ÷ 1000 = kWh, then kWh × your rate = cost.

The formula (and why "watts" is the only thing that matters)

Your meter bills you in kilowatt-hours. A kilowatt-hour is 1,000 watts drawn for one hour, or 100 watts for ten hours, or 10 watts for 100 hours. The only input that is actually yours to measure is the average draw in watts. Everything after that is arithmetic:

kWh per month = watts × 24 × 30 ÷ 1000
cost per month = kWh per month × price per kWh

In code, because that is how most of us think:

Output for that rack:

About $258 a year, and two thirds of it is the gaming PC doing nothing. That is the pattern in almost every home lab I have seen: one legacy box eats the budget.

If you would rather not run code, the kWh calculator on VoltFlow does the same thing in a form: watts in, hours in, kWh and cost out.

Where to get honest watt numbers

The label on the power supply is not the draw. A 650 W PSU in a machine idling at 70 W pulls 70 W plus conversion losses, not 650. Three ways to get the real number, best first:

1. A plug-in power meter. Twenty dollars, reads actual wall draw, and it is the only method that includes PSU inefficiency, fans, and drives spinning up. Measure idle, measure under your normal load, and average by how many hours you sit in each.

2. A smart plug with energy monitoring. Same idea, but it logs over days, which catches the backup job at 3 a.m. that you forgot about. Pull the kWh total after a week and multiply by 4.3 for a month.

3. Software readings. power top or the BMC on a server shows package power, not wall power. Add 10 to 20 percent for the PSU, more for an old or oversized unit.

Typical wall-draw ranges I see reported from plug meters, as rough ranges rather than specs (your hardware will differ, so measure):

The pattern worth noticing: idle draw dominates. Your server is idle 95 percent of the time, so a box that idles at 90 W costs more than one that idles at 15 W and peaks at 100 W, even if the second one works harder.

Amps, breakers and the UPS question

Two follow-up questions come up every time.

"Can my UPS handle this?" A UPS is rated in VA and watts, and the watt figure is the one that matters for a rack. Add up the measured watts of everything behind it. Runtime drops fast as you approach the rating, so a 600 W rack on a 900 W UPS gets minutes, not hours. Size the UPS for the longest outage you want to survive gracefully, which for most people means "long enough to shut down cleanly".

"Am I overloading the circuit?" A 120 V, 15 A household circuit can carry about 1,800 W, and in the U.S. the continuous-load rule means you should plan for 80 percent of that, around 1,440 W. A homelab rarely gets near it, but a space heater on the same circuit does. To convert your rack's watts to amps, the watts to amps calculator handles the voltage and power factor for you.

Your rate is not the national average

18.31 cents is the U.S. average. It hides a range from about 13 cents in the cheapest states to roughly 48 cents in Hawaii, and time-of-use plans change the picture again: a rack that runs 24/7 pays peak rates for a third of its hours. Pull the exact number from your bill (look for "price per kWh" or divide the energy charge by the kWh used), then put it into the script. If you want a state-level starting point, VoltFlow has rate pages for each state, for example Texas electricity rates, and a monthly bill estimator if you want to see the rack's share of the whole bill.

Three cheap wins

1. Retire the legacy tower. In the example above, replacing the idle gaming PC with a 15 W mini PC saves about $10 a month, $120 a year. The mini PC pays for itself in two to three years, and sooner if your rate is above average.

2. Spin down drives you do not read. A 3.5-inch drive draws 5 to 8 W spinning and under 1 W asleep. Eight drives spinning for media you watch twice a week is 40 to 60 W of nothing.

3. Check the power plan. Linux boxes with no governor set and desktop Windows machines with sleep disabled idle 20 to 40 W higher than they need to. powertop --auto-tune is a five-minute test.

Putting it together

The whole thing fits in your head now: measure watts at the wall, multiply by 720 hours, divide by 1,000, multiply by your rate. Run the script above with your real numbers, then run it again after you have retired the biggest idle draw. The difference is usually the price of a nice mini PC within a year.

If you want the same math for a specific appliance, a dryer, an AC unit, an EV charger, the appliance energy calculator and the electricity cost calculator on VoltFlow cover those with the same formula and current rates.

Electricity prices: U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A, July 2026 data. Device figures are typical ranges reported from plug meters, not manufacturer specs. Circuit figures follow the U.S. 120 V / 15 A convention and the 80 percent continuous-load guideline; check your local code.

Munir Afridi builds free electrical and energy calculators at VoltFlow.net and financial calculators at FreeFinCalc.net.