1 Watt Rule of Thumb
Posted by Luke Fishback
In the spirit of sanity checking my energy saving decisions, I like to keep simple technical estimations in mind. In this post, I’ll discuss one of the most useful. If you’re not in the mood for math, here’s the bottom line: 1 Watt of energy reduction equates to approximately $1 of electric bill savings per year. So, eliminating 10 Watts of phantom load from your house will save you about $10 per year, 100 Watts produces $100 of annual savings…
This is a gross estimation. As such, this rule should only be used for getting a ballpark idea of savings potential.
If 1 Watt of energy reduction produces $1 of savings per year, replacing a 60 Watt incandescent bulb with an equivalent 13 W compact fluorescent could provide as much as $47 (60-13=47) of annual electricity bill savings. BUT, most of us don’t keep our lights turned on 24/7. So, instead let’s say that that same bulb is turned on only 2.5 hours per day. Since 2.5 hours is about 1/10th of a day, our savings have dwindled to $4.70 per year. If it’s on just 15 min per day (1/100th of a day), the annual savings fall to 47 cents per year.
Let’s look at another example. A typical cell phone charger has a phantom load of about 2 Watts. If you leave it plugged in all the time the charger will waste $2 per year. S0, religiously unplugging it after charging your phone will save you about $2 per year. Cool. But if you forget to plug the charger back in just once you might find yourself with a dead cell phone (and that might cause > $2 of frustration). For that reason, I prefer to target the less risky phantom loads, like that TV in the guest room. A TV might use as much as 15 Watts of energy when it is turned off but in standby mode. Unplug the TV’s you seldom use and save as much as $15 per year per TV.
Here’s the math:
One kilowatt-hour is the amount of energy used by 1000 Watts running for one hour or 1 Watt running for 1000 hours. There are 8760 hours in a year (24 hours per day x 365 days = 8760). So, if an appliance uses 1 Watt of electricity constantly (24 x 7 x 365) it uses a total of 8760 Watt-hours or 8.76 kilowatt-hours of electricity in a year. With me?
The average cost of a kilowatt hour in the US is between 9 and 10 cents. We’ll round up and call it 10 cents. So, if one watt burns up 8.76 kwh in a year, it will cost you 87.6 cents on your electric bill every year. In the spirit of gross estimation (and in anticipation of rising electricity costs), I round that up to $1.
The graphic shows the approximate distribution of electricity rates around the country. The 1 Watt rule of thumb is close enough for most of us. If you’re in Hawaii though, where electricity is more than 20 cents per kwh, this estimation should be closer to $2 per Watt.
Happy energy saving!