Calculator inputs
Formula used
The calculator estimates annual energy for cooking and standby, then converts energy into cost using your electricity rate.
- Cooking hours/year = (minutes/day / 60) x days/year
- Standby hours/year = (plugged hours/day x 365) - cooking hours/year
- Annual kWh = (input watts / 1000) x cooking hours + (standby watts / 1000) x standby hours
- Annual cost = (annual kWh x rate) + annual maintenance
- Year t rate = base rate x (1 + escalation%)^(t-1)
- Discounted savings = savings / (1 + discount%)^t
- NPV = sum(discounted savings) - net upfront cost
How to use this calculator
- Enter your typical cooking minutes and annual cooking days.
- Add old and new input watts from product labels or manuals.
- Include standby watts if the display stays active.
- Set your electricity rate, escalation, and analysis lifetime.
- Optional: add rebates, resale value, and maintenance differences.
- Press Calculate Savings to view results above.
Example data table
These sample scenarios show how usage and wattage changes affect annual savings.
| Scenario | Minutes/day | Days/year | Old W | New W | Rate | Est. annual savings |
|---|---|---|---|---|---|---|
| Light use | 10 | 260 | 1400 | 1100 | $0.16 | $3-$8 |
| Typical use | 18 | 330 | 1500 | 1200 | $0.18 | $8-$20 |
| Heavy use | 30 | 365 | 1600 | 1100 | $0.22 | $25-$55 |
Energy profile of typical microwave use
A 1,500 W unit used 18 minutes daily for 330 days runs about 99 hours yearly. Cooking energy equals 148.5 kWh. At $0.18 per kWh, that portion costs about $26.73. Use input watts from the label because it reflects the wall draw that your meter bills. For heavy users, efficiency upgrades can meaningfully cut energy and reduce kitchen heat buildup.
Standby draw and plugged time assumptions
If standby is 2 W while the unit stays plugged in 24 hours, standby adds roughly 17.3 kWh, lifting annual energy to 165.8 kWh. Cutting standby from 2 W to 1 W saves about 8.7 kWh, or $1.57 at $0.18 per kWh. Unplugging for eight hours nightly trims standby by 2,920 hours, saving about 5.8 kWh at 2 W.
Rate escalation and discounting effects
The model escalates the electricity rate each year and discounts future cashflows. With a $0.18/kWh base rate and 3% escalation, year ten is about $0.235/kWh. A 6% discount rate values a year ten dollar at roughly $0.56 today, so earlier savings weigh more heavily in NPV than later savings.
Payback drivers beyond energy
Net upfront cost equals purchase plus installation plus disposal, minus rebates and old resale value. For example, a $220 purchase with a $40 rebate and $0 resale gives $180 net cost. If first year savings are $15, simple payback is 12.0 years. If rates rise or the wattage gap widens, annual savings increase and payback shortens.
Interpreting the projection table
Each row combines energy cost and annual maintenance for old and new units. Savings can be negative when the new model draws more power, has higher upkeep, or when rates are very low. Use cumulative discounted savings to see when discounted payback occurs. The sensitivity panel shows savings at rates plus or minus 10%, and the CO2 estimate scales from your chosen emissions factor.
FAQs
Does cooking power equal the electricity draw?
Not always. Use the input wattage listed on the rating label or manual, because that is the power drawn from the outlet. Cooking output watts are lower and do not match your billed consumption.
How can I estimate standby watts accurately?
Check the specification sheet for standby or clock power, or measure with a plug-in watt meter. If you only know that the display stays on, values between 0.5 and 3 W are common.
Why might annual savings look small?
Microwaves run for short periods, so total kWh can be modest. If cooking minutes are low, or the new model’s input watts are similar, savings shrink. Standby reductions can still help when the unit stays plugged in.
What discount rate should I use?
Use a rate that reflects your alternative return or borrowing cost. For household decisions, 3–8% is a typical range. A higher discount rate reduces the present value of long-term savings and can push payback later.
Can I evaluate inverter or sensor models here?
Yes. Enter the best available estimate of average input watts during typical cooking. If you have only a maximum rating, use it for a conservative comparison, or reduce it slightly if the model modulates power frequently.
How do the CSV and PDF downloads work?
After you calculate, the file links export the same projection table and key metrics shown on screen. Exports use a temporary session token, so refresh the page and recalculate if you open the tool in a new browser session.