Watts on a spec sheet don't mean anything until you turn them into a number on your bill. Here's the watts-to-kWh-to-money formula, why the answer changes if you're on a time-of-use (peak/off-peak) rate plan, and how to have the math done for you hour by hour instead of guessing with one flat rate.
Quick answer: kWh = (watts ÷ 1000) × hours running, then cost = kWh × your price per kWh. A PC averaging 150 W for 8 hours a day at $0.15/kWh runs about $65.70/year. If your utility charges different rates at different times of day (time-of-use / TOU), you need to split those hours apart first — a flat multiplication will be wrong. Read on for the TOU math and how to automate it.
Electricity bills are priced in kilowatt-hours (kWh), not watts, so the first step is always a unit conversion. Three numbers, in order:
kWh = (watts ÷ 1000) × hours running. A PC drawing 150 W for 8 hours uses (150 ÷ 1000) × 8 = 1.2 kWh that day.cost = kWh × price per kWh. At a flat $0.15/kWh, 438 kWh ≈ $65.70/year. Swap in your own wattage, hours, and rate and the number moves accordingly.The watts figure matters most — a gaming PC pulling 350 W under load costs roughly 2.3× as much per hour as one idling at 150 W, so where that number comes from (a plug-in meter is most accurate; software CPU+GPU estimates run a bit low since they miss motherboard, RAM, storage, and PSU conversion loss) changes the answer more than the rate does.
The formula above assumes one price per kWh no matter when you used it. That's true for a flat-rate electricity account — most residential plans work this way. But a growing number of utilities, and most Taiwan residential accounts on Taipower's time-of-use (TOU) plans, charge a different rate depending on the hour: more during peak demand, less off-peak. On a TOU plan, the exact same 1.2 kWh can cost noticeably more or less purely based on when your PC drew that power — the single-multiplication formula above will be wrong unless you split the hours apart first.
| Rate structure | How it's priced | What you need to calculate cost |
|---|---|---|
| Flat rate | One price per kWh, all day, every day | Total kWh × the one rate |
| Time-of-use (TOU) | A higher rate during peak hours (e.g. weekday 10:00–17:00), lower off-peak | kWh used in each time band × that band's rate, then added together |
| Automated hour-by-hour | PowerDoze applies your rate schedule automatically | Nothing — it multiplies every hour of measured usage by that hour's rate for you |
台電 (Taipower) residential time-of-use plans commonly split the day into a peak window on weekdays — a common example is 10:00–17:00 at a higher rate — with everything else billed off-peak: nights, early mornings, weekends, and national holidays. The exact hours and per-kWh prices depend on which Taipower plan you're enrolled in, so check your own bill or Taipower's site for your specific schedule — the numbers below are just a common example, not a universal rate.
Most TOU plans bill all day Saturday and Sunday at off-peak, and the same goes for national holidays (New Year's Day, Lunar New Year, Peace Memorial Day, Children's Day, Tomb Sweeping Day, Dragon Boat Festival, Mid-Autumn Festival, National Day, and others). That means the exact same weekday-10am usage that costs peak-rate money on a Tuesday costs the lower off-peak rate if that Tuesday happens to fall on a holiday — a detail that's easy to forget when estimating a whole year by hand.
Best for: Anyone estimating annual cost who wants to know why a rough "weekday = peak" assumption overcounts. Note: Holiday dates shift every year (especially the lunar-calendar ones), so a fixed list needs updating annually to stay accurate.
If your PC runs 24 hours a day at a steady 150 W on a plan with a 10:00–17:00 peak at NT$4.44/kWh and an off-peak rate of NT$1.85/kWh, weekday cost looks like this:
Peak hours (7 hours, 10:00–17:00): (150 ÷ 1000) × 7 × 4.44 ≈ NT$4.66. Off-peak hours (17 hours): (150 ÷ 1000) × 17 × 1.85 ≈ NT$4.72. Total for that weekday: ≈ NT$9.38. On a weekend or holiday, the whole 24 hours falls to off-peak: (150 ÷ 1000) × 24 × 1.85 ≈ NT$6.66 — noticeably less for the identical usage.
Best for: A one-off estimate or sanity-checking a bill by hand. Note: This gets tedious fast for a variable load (a PC's wattage swings by workload, not a flat 150 W), which is where doing it per actual hour of measured usage instead of an assumed average pays off.
Manually splitting peak/off-peak hours for a PC whose wattage changes all day (idle vs. gaming vs. a render job at 2am) isn't realistic to do by hand every day. PowerDoze's electricity rate settings let you enter either a single flat rate or a full time-of-use schedule: an off-peak base rate (defaults to a common NT$1.85/kWh example) plus your own custom peak windows (e.g. 10:00–17:00 at NT$4.44/kWh), with weekends and national holidays automatically treated as all-day off-peak. It then multiplies every hour of your PC's measured or estimated power draw by that hour's rate and adds it up — instead of one flat number applied to your whole day. Works in 7 currencies (TWD, USD, EUR, JPY, KRW, CNY, GBP), so it's not Taiwan-only even though the default rate schedule is tuned for a typical Taipower residential plan.
Honest note: This is a simulation of the rate structure you enter, not a live read of your Taipower account — your actual bill is the source of truth, and PowerDoze's number is for tracking and comparison, not a substitute. Requires Pro. Free users still see PC power usage; the rate-schedule math (fixed and TOU) is a Pro feature.
Convert watts to kilowatt-hours, then multiply by your rate: kWh = (watts ÷ 1000) × hours running, then cost = kWh × price per kWh. A PC averaging 150 W for 8 hours a day at $0.15/kWh works out to (150 ÷ 1000) × 8 × 365 × 0.15 ≈ $65.70 a year. Swap in your own wattage, hours, and rate.
Multiply the kWh figure by your price per kWh from your electricity bill. If you don't know your rate, it's printed on your bill as a price per kWh (or per 度 in Taiwan) — flat-rate accounts have one number; time-of-use accounts have a different rate for each time band, so the same kWh costs more or less depending on when it was drawn.
Only if you're on a time-of-use (TOU) rate plan. Flat-rate accounts charge the same price no matter the hour, so timing doesn't change the bill. TOU accounts charge more during peak hours and less off-peak, so the exact same kWh can cost noticeably more if it's drawn at 2pm instead of 2am.
Taipower's residential time-of-use plans commonly price weekday daytime hours (for example 10:00–17:00) as peak, with everything else — nights, weekends, and national holidays — billed at the lower off-peak rate. The exact hours and prices depend on which Taipower rate plan you're enrolled in, so check your bill or the Taipower site for your specific schedule.
Yes, if it tracks your PC's power draw over time and can apply a rate per hour rather than one flat number. PowerDoze does this: enter either a single flat rate or a time-of-use schedule with peak/off-peak bands, and it multiplies each hour's measured usage by that hour's rate automatically, in 7 currencies.
No — it's a calculation based on the rate schedule you enter and PowerDoze's own measured or estimated power draw, not a read of your utility account. Treat it as a close simulation for tracking and comparison, not a substitute for your actual Taipower (or other utility) bill.
Want your PC's real cost calculated hour by hour instead of estimated with one flat number? PowerDoze Pro applies your own flat or time-of-use rate schedule automatically, in 7 currencies.
Download free for Windows 10/11See also: How to measure your PC's actual power consumption · How real power savings are tracked · All features