Our position: we have not personally or laboratory tested StopWatt. We have not found independent evidence supporting the dramatic StopWatt-specific savings claims in its advertising.
How your electric bill is calculated
Most U.S. homes are billed for energy in kilowatt-hours (kWh). Your meter totals the energy your appliances actually consume over the billing period, and the utility multiplies that by its rates, then adds fixed charges, taxes and fees. To lower the energy part of the bill, something has to make your appliances consume fewer kWh.
That is the key test for any plug-in saver. Plugging a unit into one outlet does not change how long your air conditioner runs, how hot your water heater keeps the tank, or how many loads your dryer handles.
What the engineering evidence says
The seller explains its device in terms of smoothing current and reducing wasted power, ideas related to power factor. The National Institute of Standards and Technology examined power-factor correction for homes in Technical Note 1654. In summary, it concludes that correcting power factor does not by itself reduce what a typical homeowner pays, because residential customers are billed on real energy (kWh) rather than on reactive power. Any reduction in wiring losses inside a home is described as very small.
This is general engineering analysis of the technique. It is not a test of StopWatt, and we are not claiming NIST evaluated this product.
Why the green light does not prove savings
An LED on the unit tells you it is receiving power. It does not measure anything about your home's consumption. The only way to know whether energy use changed is to measure energy, either with your utility meter over a careful comparison or with a plug-in or whole-home energy monitor.
How to compare bills fairly
If you want to judge any energy product yourself, compare like with like:
- Use kWh, not dollars. Rates and fees change, so the dollar total can move even when use does not.
- Normalize by billing days. Divide kWh by the number of days in each period to get average daily use. A 33-day bill will look higher than a 28-day bill.
- Account for weather. Heating and cooling dominate many bills. Compare the same month a year apart, or look at heating and cooling degree days if your utility lists them.
- Note household changes. Guests, a new appliance, working from home, an electric vehicle or a pool pump can swamp any small effect.
- Check your rate plan. Time-of-use or tiered pricing can change the total without any change in kWh.
- Look over several months. One month is noise. A consistent change across comparable periods is more meaningful.
Compare daily electricity use between two bills
Billing periods are rarely the same length, so a lower kWh total can appear even when your home used energy at the same rate. Divide each bill's kWh by its billing days:
Daily use = kWh used / days in billing period
Change (%) = (later daily use - earlier daily use) / earlier daily use x 100
Example: 900 kWh over 30 days is 30 kWh per day. 840 kWh over 28 days is also 30 kWh per day. The second total is 60 kWh lower, yet daily use is unchanged (0%).
A change in daily use is not proof that any device caused it. Weather, guests, new appliances and routines differ between periods. This tool does arithmetic only; it does not predict savings and is not a laboratory result.
Things that do reliably reduce kWh
- Adjusting thermostat set points and using a schedule.
- Replacing remaining incandescent bulbs with LEDs.
- Sealing air leaks and adding insulation where needed.
- Lowering water heater temperature to a safe, comfortable setting.
- Unplugging or switching off idle electronics with power strips.
The U.S. Energy Information Administration breaks down where household electricity typically goes, which helps you target the biggest loads first.
So, does StopWatt really work?
Based on the evidence we could find, there is no reliable reason to expect a noticeable reduction in a typical home's kWh use from this kind of device. We cannot rule out every individual situation, and we did not test it. If you choose to try it, measure your kWh before and after using the method above and keep within the seller's return window.