The model, stated exactly
Energy per meal = power in kW × minutes ÷ 60. Microwave: 0.9 × 3 ÷ 60 = 0.045 kWh. Oven: 2.4 × 15 ÷ 60 = 0.600 kWh, where the 15 minutes are 10 minutes of preheat plus 5 minutes of reheating — the split is stated, not hidden, because preheat is two-thirds of the oven's time and two-thirds of its energy. Cost per meal = energy × electricity price: $0.008 for the microwave (0.045 × $0.17 = $0.00765, rounded to three decimals) and $0.102 for the oven (0.600 × $0.17). Annual cost = meals per week × 52 × cost per meal, computed from the unrounded per-meal cost and then rounded to cents: 260 × $0.00765 = $1.99 for the microwave and 260 × $0.102 = $26.52 for the oven. The annual saving ($24.53) is the difference of the unrounded annual costs, rounded to cents; in every example on this page it also equals the difference of the rounded annual totals, so the printed lines add up exactly. Time saved per meal = oven minutes minus microwave minutes = 12 minutes; annual time saved = 260 × 12 ÷ 60 = 52 hours. The comparison is electricity cost only: it excludes buying either appliance, and it assumes one meal reheated alone — if the oven is already hot for something else, the preheat argument disappears.
The worked example, line by line
Setup: microwave 0.9 kW for 3 minutes, oven 2.4 kW for 15 minutes (10 preheat + 5 reheat), $0.17 per kWh, 5 meals a week.
Line 1 — microwave energy. 0.9 × 3 ÷ 60 = 0.045 kWh per meal.
Line 2 — microwave cost per meal. 0.045 × $0.17 = $0.008 per meal.
Line 3 — oven energy. 2.4 × 15 ÷ 60 = 0.600 kWh per meal.
Line 4 — oven cost per meal. 0.600 × $0.17 = $0.102 per meal.
Line 5 — meals per year. 5 × 52 = 260 meals a year.
Line 6 — annual costs. 260 × $0.00765 = $1.99 microwave; 260 × $0.102 = $26.52 oven.
Line 7 — the saving. $26.52 − $1.99 = $24.53 a year, plus 12 minutes per meal and 52 hours a year saved.
Try these
Just 2 meals a week. 104 meals a year cost $0.80 for the microwave and $10.61 for the oven — a saving of $9.81 a year and 20.8 hours of waiting saved.
Electricity at $0.30 per kWh. The microwave costs $0.014 per meal and the oven $0.180; annual totals are $3.51 and $46.80, a saving of $43.29 a year.
A longer oven: 25 minutes total. The oven uses 1.000 kWh and costs $0.170 per meal, or $44.20 a year — the microwave saves $42.21 a year and 22 minutes per meal (95.33 hours a year).
A smaller microwave: 0.6 kW. It uses 0.030 kWh and costs $0.005 per meal, or $1.33 a year — saving $25.19 a year over the oven, though it may need longer than 3 minutes for the same meal.
10 meals a week. 520 meals a year cost $3.98 for the microwave and $53.04 for the oven — a saving of $49.06 a year and 104 hours of waiting saved.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Microwave energy per meal | kW × minutes ÷ 60 | 0.045 kWh at the defaults. |
| Oven energy per meal | kW × minutes ÷ 60 | 0.600 kWh at the defaults, preheat included. |
| Microwave cost per meal | kWh × $/kWh | $0.008 at the defaults. |
| Oven cost per meal | kWh × $/kWh | $0.102 at the defaults. |
| Meals per year | Meals per week × 52 | 260 at the defaults. |
| Annual cost (each method) | Meals per year × cost per meal | $1.99 / $26.52 at the defaults. |
| Annual saving | Oven annual − microwave annual | $24.53 at the defaults. |
| Time saved per meal | Oven minutes − microwave minutes | 12 minutes at the defaults. |
| Time saved per year | Meals per year × minutes saved ÷ 60 | 52 hours at the defaults. |
Who uses this
- The weekday leftover reheater: enters 5 meals a week and learns the microwave saves $24.53 a year and 52 hours of waiting — the oven habit costs more in time than in money.
- The household on expensive electricity: enters $0.30 per kWh and sees the oven at $0.180 per meal versus $0.014 for the microwave, which settles the argument for everyday reheating.
- The cook who insists on oven texture: enters a 25-minute oven total and prices that preference honestly at $44.20 a year — then decides which meals deserve the oven and which do not.
- The renter with a small microwave: enters 0.6 kW and learns reheating costs $1.33 a year — and that a longer microwave time, not a bigger appliance, is the only real trade-off.
Accuracy & limitations
Three assumptions deserve daylight. First, the microwave is assumed to run at full rated power for the whole time: many microwaves cycle their magnetron on and off at lower power settings, so a meal reheated at 50% power for 6 minutes uses roughly the same energy as 3 minutes at full power, not half — the calculator prices the minutes you enter at the power you enter, so enter effective full-power minutes. Second, oven preheat varies a lot: a small oven or a toaster oven preheats in 5 minutes, a large double oven can take 15, and an oven that cycles its element does not draw full power continuously once it reaches temperature — the 2.4 kW figure is a planning rating, not a meter reading. Third, food mass is ignored: a dense, frozen casserole needs far longer in either appliance than a plate of rice, and the microwave's advantage shrinks when the meal itself dominates the time rather than the preheat. Gas ovens are not modeled here — a gas oven's cost depends on your gas price per therm, not your electricity price. Error policy: missing, non-numeric, zero or negative inputs, or inputs above their caps (10 kW, 180 minutes, $2 per kWh, 50 meals a week) are reported as errors rather than calculated, so no result ever shows NaN.
Cite this calculatorBright Side Kit. “Microwave vs Oven Reheat: Cost and Time Calculator.” brightoolbox.com, 2026.Bright Side Kit. "Microwave vs Oven Reheat: Cost and Time Calculator." https://brightoolbox.com/microwave-vs-oven-reheat/ Accessed October 24, 2026.
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