How Water vs Cold
Monday July 9, 2012: Updated 1/31/2021
Recently, my brother Billy asked me the age old question, “Which one is better? Boiling a pot of cold water or running the faucet until the water is hot and then boiling that up in a pot?”
This is a complicated question and to answer it correctly, many factors must be considered:
- How much water does the hot water pipe hold? How much water must be flushed through the pipes until the water (and the water pipe) gets hot?
- How efficient is the hot water heater? Fuel type? Gas or Electric?
- How efficient is the appliance that will ultimately boil the hot/cold water? Is it Gas or Electric?
- How much does natural gas and electricity cost in your particular area?
- What is the water temperature of the cold water entering the house?
- What is the temperature of boiling water at your particular elevation? What is the mineral content of the water? At sea level, water boils at 212 °F but at 4200 feet, it boils at 204 °F. The mineral content of the water will lower the boiling temperature further.
- How much energy does it take to raise the temperature of a given amount of water?
To start, I turned on the kitchen sink hot water faucet and collected the water until the water coming out of the faucet was hot. For my particular faucet, it took 11 cups of water before the water reached a temperature of 121 °F. That is 0.6875 gallons. It took 40 seconds.
I measured the temperature of the cold water before and the hot tap water after. 66 °F cold and 121 °F hot.
I then filled up a pyrex bowl with 1 quart of cold tap water and heated it in the microwave oven. I timed how long it took to start boiling vigorously. I repeated the same experiment with a bowl of hot tap water. Using TED, I measured the power consumption of the microwave oven to be 1533 watts on high.
It takes an extra 30 seconds to boil a quart of water in an uncovered bowl vs. a covered bowl in the microwave oven.
I then filled up a pot with 1 quart of cold tap water, put it on the stove and timed how long it took to start boiling vigorously. I repeated the same experiment with a pot with hot tap water. Both experiments used covered pots. Using TED, I measured the power consumption of the small stove burner to be 887 watts.
I calculated the energy required to raise the temperature of a given volume of water. Knowing this figure, I can calculate the efficiency of an appliance by measuring the amount of energy it required to heat up the same volume of water.
| Heating 1 Quart of Tap Water | BTU | Watt-hours | Efficiency | Cost @ 10¢/kWh |
|---|---|---|---|---|
| Theoretical Energy to heat 1 qt of water 66°F - 187°F | 251.1 | 73.6 | 100.0% | $0.007 |
| Electric Kettle | 273.0 | 80.0 | 92.0% | $0.008 |
| Stove Top -- Hot Water covered (burner already hot) | 303.0 | 88.8 | 82.9% | $0.009 |
| Induction Cooktop -- Covered | 409.4 | 120.0 | 61.3% | $0.012 |
| Microwave Oven Hot water Covered | 445.0 | 130.4 | 56.4% | $0.013 |
| Induction Cooktop -- UnCovered | 477.7 | 140.0 | 52.6% | $0.014 |
| Stove Top -- Cold Water covered | 514.5 | 150.8 | 48.8% | $0.015 |
| Microwave Oven Cold water -- Covered | 695.7 | 203.9 | 36.1% | $0.020 |
| Microwave Oven Cold water -- Uncovered | 744.2 | 218.1 | 33.7% | $0.022 |
| Energy to run tap water until hot | 1785.4 | 523.3 | 14.1% | $0.052 |
Energy Costs, Is It Worth It?
While it takes less time to boil tap water that is already hot, it takes more energy because of the wasted energy in letting the water run that was once hot but has cooled in the water pipe. It cost about 2-5 cents in water heating costs to run the tap until hot. In my opinion, if you have to boil more than a few cups of water, the time savings justifies running the tap water until it gets hot. But do what works for you, your house and your family.
Electric Water Heater 90% - 98% Efficient
Gas Water Heater 50% - 86% Efficient
Electric Kettle 90% - 94% Efficient
Induction Cook-top 51% - 59% Efficient
Electric Range 40% - 49% Efficient
Microwave Oven 33% - 36% Efficient
It is apparent that most water heaters are more efficient at heating water than a stove top or a microwave oven. The microwave oven is very fast but is the least efficient appliance overall. It does however fill an important niche in the kitchen. My microwave oven also has a fan that draws 53 watts and a light that draws 23 watts. That 76 watts consumes an extra 5% power, reducing the efficiency by 2%.
Update 10/17/2015:
I recently bought an single pot, induction cook-top on a deal at Newegg. It works great in an emergency if the power were to go out. We simply plug it into the off-grid outlet on our PV solar system. That way we can cook dinner, off-grid battery-less.
I ran some tests to determine its energy efficiency and it is around 51-59%. That beats all other kitchen cooking appliances. It would be even more efficient if it was used with an insulated pan that holds the heat better.
Want to know how I measured the energy efficiency of a kitchen appliance? Watch this video How to Measure the Efficiency of an Induction Cook-Top
Time Savings vs. Energy Savings
Using the microwave oven, it takes 8-minutes to boil a quart of cold water and less than 5-minutes to boil a quart of hot tap water.
Using a small burner on the stove, it takes longer to boil a quart of water; 10 minutes 15 seconds for cold water and 8 minutes for hot.
Appliance Cold Tap Water Hot Tap Water Time Difference
Electric Kettle 3:38 minutes n/a n/a
Induction 5:22 minutes n/a n/a
Microwave 8:00 minutes 4:50 minutes 3:10 minutes
Stove Top 10:15 minutes 8:00 minutes 2:15 minutes
In my home, I have a fairly long run from the water-heater in the basement to the kitchen sink on the main floor. It takes 40 seconds of running the water before the tap water gets all the way hot.
If you only have to microwave a single cup of water, cold tap water will be well on its way toward boiling in the microwave oven before the tap water even gets hot.
Induction cook-tops are the exception and will boil water faster than in a microwave oven.
Updated: 1/31/2021: An electric kettle beats them all. It is more efficient at heating water and faster than any other cooktop or microwave oven in the kitchen. Even if you are heating water to cook some spaghetti on the stove, pre-heat the water in an electric kettle first.
Cooking Tips --
- If you have to boil a lot of water (2 or more quarts) run the tap until hot, fill up the pot, cover it and heat it up to boiling.
- If you are using the stove top, to save energy, match the pot with the size of the burner.
- A covered pot will boil faster and use less energy than an uncovered one.
- If you only have to boil a few cups of water, use an electric kettle. It is the fastest and most efficient.
- In the summer time, after it has served its purpose, dump out the used cooking water. This will prevent the kitchen from heating up as much and the air conditioner from working as hard.
- In the summer time, after using a hot oven, crack the oven door and vent the heat outside the house through the hood vent until the oven cools down. ~10 minutes.
- Induction cook-tops are the exception and will boil water faster than in a microwave oven.