Energy Savings Calculator

How to use the energy savings calculator

Enter the old device’s power in watts, the new device’s power in watts, how many hours a day it runs, and your electricity rate, and you get the savings from the swap — the dollars saved a year, the saving a month, and the kilowatt-hours saved over the year. The defaults price the classic upgrade: a 100 W incandescent bulb replaced by a 15 W LED, on five hours a day. The rate is the input that swings the answer most: the US residential average is about 17.5¢/kWh as of 2026, but rates vary widely by state, so use the figure from your own bill.

The saving comes entirely from the gap in wattage, so getting both numbers right is what makes the answer real. The old wattage is whatever the device draws today — the nameplate on an incandescent bulb, the sticker on an old fridge, the label on a window air conditioner. The new wattage is what the replacement pulls; LED packaging lists it plainly, and Energy Star appliance tags give a yearly kWh figure you can work back from. If the two devices do the same job at the same brightness or cooling, the difference between them is pure saving.

Hours per day is the multiplier that decides whether a swap is worth doing. The same 85 watts saved is worth far more on a porch light that burns ten hours a night than on a closet bulb flicked on for a minute. Be honest about real runtime, not how long the device is plugged in: a fridge runs around the clock, a living-room lamp a few hours an evening, a rarely-used basement bulb almost never. Set the hours to match the device you’re actually pricing.

Use the tool to compare candidates before you buy, not just to confirm a swap you’ve already made. Run the always-on, high-hours loads first — an old refrigerator, a porch or hallway light that never goes off, a pool pump — because those return the most for the watts saved. Then check the rarely-used stuff, and you’ll usually find it isn’t worth the trouble. The calculator turns a vague “LEDs save money” into a specific yearly figure you can rank one device against another with.

Once you have the yearly saving, you can work out the payback period — how long the more efficient device takes to pay for the price premium you paid over the cheaper, thirstier option. Divide that extra purchase cost by the annual saving and you get the years to break even; anything sooner than the device’s lifespan is money ahead. A few dollars of LED over an incandescent pays back in months on a light that’s on for hours a day, while a pricey efficient appliance on a low-hours load can take years — which is exactly why runtime, not just the wattage gap, decides where to spend first.

The formula

The saving is the power you no longer draw, turned into energy and then into money. Subtract the new wattage from the old to get the watts saved, divide by 1,000 to reach kilowatts so the units match the kilowatt-hours your utility bills, multiply by daily hours for the energy saved each day, then by 365 days and your rate for the yearly figure:

daily kWh saved = ((old watts − new watts) ÷ 1000) × hours per day
annual saving = daily kWh saved × 365 × rate

Worked example with the defaults — a 100 W incandescent bulb replaced by a 15 W LED, five hours a day at $0.175/kWh: old 100 − new 15 = 85 W saved; 85 ÷ 1000 × 5 h = 0.425 kWh saved per day; × 365 = 155 kWh a year; × $0.175 = about $27.15 a year, roughly $2.23 a month.

The saving is bounded by watts-saved × hours, so replacing an always-on device that runs 24 hours a day saves far more than an occasional one — and a replacement that uses as much or more power than the old device saves nothing, which is why the figure is floored at zero. That bound is also why the high-hours, high-draw loads — an old fridge, a light that never goes off — are the ones to swap first.

Frequently asked questions