Calculator inputs
Example data table
| Scenario | Plugs | Off hours/day | Standby saved (W) | Rate | Annual net savings | Payback (years) |
|---|---|---|---|---|---|---|
| Home entertainment setup | 4 | 4.0 | 5.0 | 0.20 | 18.40 | 2.4 |
| Office peripherals bundle | 10 | 2.0 | 3.0 | 0.17 | 31.10 | 3.1 |
| Retail kiosks overnight shutdown | 18 | 8.0 | 2.0 | 0.22 | 172.60 | 1.1 |
Formula used
- Off-time savings per plug (kWh/yr) = (Device_W * Off_Hours_Day * Days_Year) / 1000
- Standby savings per plug (kWh/yr) = (Standby_Saved_W * Standby_Hours_Day * Days_Year) / 1000
- Plug standby added (kWh/yr) = (Plug_Standby_W * 24 * Days_Year) / 1000
- Net kWh saved total = Plugs * max(0, Off + Standby - Plug_Standby)
- Annual bill savings = Net_kWh * Rate * (1 + Fees%)
- Annual net savings = Annual bill savings - Annual maintenance
- Capex = (Plugs * Cost_per_plug) + Install - Rebates
- Payback (years) = Capex / Annual net savings
- NPV = -Capex + sum( AnnualNet*(1+Esc)^(t-1) / (1+Disc)^t )
How to use this calculator
- Enter how many smart plugs you plan to buy.
- Estimate how many hours per day devices turn OFF.
- Add standby watts avoided and standby hours removed.
- Include the smart plug standby draw for net savings.
- Enter electricity rate and any bill add-on fees.
- Set lifespan, escalation, and discount rate for NPV.
- Press Calculate savings to see results above.
- Download CSV or PDF for reporting and records.
Baseline standby losses add up
Small always-on loads accumulate into meaningful annual energy use. A typical 4 W standby draw running 12 hours daily equals about 17.5 kWh per year per device. At 0.18 per kWh plus 8% fees, that is about 3.40 per device annually. Multiply this across 20 devices and the standby portion alone can exceed 68 per year, before considering scheduled shutdown savings.
Scheduling impact on annual kWh
The off-time model converts watts and hours into kWh: (W × hours/day × days/year) ÷ 1000. If an 80 W entertainment stack is shut down 3 hours per day, the off-time savings is about 87.6 kWh per year. With 10 controlled outlets, portfolio savings becomes 876 kWh, which provides a clear baseline to evaluate whether the plug’s standby draw materially reduces net impact.
Tariff and fee sensitivity
Rates and bill adders change the same kWh into different financial outcomes. A 500 kWh annual reduction is 75 at 0.15 per kWh, but 108 at 0.20 per kWh, before fees. If your utility adds 10% in riders and taxes, the same energy savings becomes 82.50 or 118.80. This sensitivity is why the calculator separates the base rate from percentage add-ons.
Payback and NPV interpretation
Payback is a quick screen: capex ÷ annual net savings. If 12 plugs cost 150 net after rebates and save 75 per year, payback is 2.0 years. NPV then checks value over time using escalation and discounting. With 3% price growth and an 8% discount rate over five years, stable savings can remain positive NPV, supporting approvals even when ROI is moderate.
Reporting and verification tips
Use the CSV export to document assumptions by load group: watts, off-hours, avoided standby, and plug standby draw. Verify one representative device per group with a plug meter for 24–72 hours, then update averages. Track outcomes as monthly kWh and bill deltas, not only device-level readings. For emissions reporting, multiply verified net kWh by your selected kg/kWh factor to maintain consistency across audits.
FAQs
1) What if my devices are not always on at the same wattage?
Use an average wattage from a plug meter or a conservative estimate. The tool is for planning and comparison, not minute-by-minute simulation.
2) Should I include demand charges?
This calculator focuses on energy and percent add-on fees. If demand charges matter, model them separately or approximate with a conservative extra percentage in “Utility fees.”
3) How do rebates affect results?
Rebates reduce net upfront cost, improving payback, ROI, and NPV immediately. Enter the total incentives you expect to receive.
4) Why can payback show N/A?
If annual net savings is zero or negative after maintenance and plug standby, payback is undefined. Re-check off-hours, avoided standby watts, plug standby draw, and the electricity rate.
5) What does NPV mean here?
NPV is the present value of future net savings minus upfront cost, with escalation and discounting. A positive NPV suggests the project beats your chosen discount rate.
6) What is a realistic plug standby draw?
Many modern plugs fall between 0.4 and 1.2 W. If unsure, start with 0.7 W and refine later using a meter or manufacturer data.