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Energy Comparisons and Notes

    Does Natural Gas or Electric cost you more?

    Incremental cost vs. Total cost?
    When comparing costs there are several issues to settle first. If you are planning to buy a new clothes dryer and add it to the current energy load of the house, then you must use the incremental cost when asking 'How much will it cost if I buy a gas dryer vs. an electric dryer?'. If you are planning a new house and comparing energy costs for an 'all electric house', then you must use the Total cost.

    This system is common in many areas including taxes - if you incrementally make $100 more, the taxes may be less than your average taxable amount over your other earnings. With energy, if your supplier includes a base charge or a sliding scale of usage, then your average cost will be different than the incremental cost of more energy.

    Some billing systems have a 'base charge' plus a 'per energy unit used' charge. Most all energy suppliers use this system whether it is explicitly described on your bill or not. Some just lump all the charges into one final charge. Others may use a sliding scale with similar numbers. Either way, that very first kilowatt/hour or therm you used cost you a fortune. That's because you had to pay the relatively fixed base charge plus the charge for energy unit, so that one energy unit could have cost you $20-30 or more!

    An example: In December 2001, the cost of natural gas from one supplier in Atlanta was '72.9 cents per therm + Base Charge'. The Base Charge was $25.41 and the gas usage charge for 65 therms was $47.39, totaling $72.80. So the incremental cost for one more therm was 72.9 cents, yet the total cost was $1.12 per therm ($72.80 / 65 therms).

    Switching Energy Sources?
    The Total cost is useful for monthly budgets and playing 'what if' I switched energy sources. Such as, what if I started using Solar Energy? If you plan to switch or add a few items then you need the Incremental costs you are paying now, yet if you plan to replace all your electric usage with solar energy or perhaps wind power, then you should use the Total cost.

    Further down this page you'll find some practical information about the energy used by various household appliances, which will force you to reconsider your current costs and any potential savings from any changes.

    An All Electric Example
    For December 2001, Atlanta:
    Natural Gas = 65 therms @ $1.12/therm (72.9 cents/therm + base) Total = $73
    Electric = 543 Kwh (kilowatt/hours) @ 8.3 cents/kwh Total = $45
    Combined Total = $118

    Costs, for all electric:
    The conversion is 1 therm = 29.3 Kwh (Use your 'Back' button to return here.)

    So, 65 therms of gas was used. And, 65 X 29.3 = 1905 Kwh, the equivalent electrical power, but when directly comparing, we have to add in the efficiency factor. Electric is usually 100% efficient and gas is about 75% efficient (although newer gas appliances can reach 95%!!) So, take 1905 Kwh X .75 = 1429 Kwh actually needed. Each Kwh costs 8.3 cents each, or $118.61. The original electric charge was $45, now the total energy cost is $45 + $119 = $164. That's $164 for all electric vs. the original combined total of $118 of gas and electric.

    Note: some electric suppliers allow a discount for all electric homes. That discount would need to be greater than 39% to be more cost efficient. Are you hot?

    Also, Heat Pumps sometime have an additional factor. When the outside temperature is moderate, say 50° F, it 'pumps' latent outside heat inside (the exact opposite of normal 'Air Conditioning'). And because the heat pump's compressor is only moving heat, not generating the heat directly, it takes only about 1/3 the power. That is, it only takes 1 Kwatt of power to move 3 Kwatts of heat inside. That's quite efficient, but only with moderate outside temperatures. When the outside temperature is closer to freezing, there is much less latent heat for the heat pump to bring inside and it becomes inefficient. To provide adequate heat, the heat pump resorts to using electric heating strips, and at this point the electricity is directly used to provide the heat. Then the example above applies.

    Another factor with gas utilities is suppling the air needed for combustion. A 100,000 BTU furnace using 100 cubic feet of natural gas (ccf) per hour, needs an absolute minimum of 1000 cubic feet of air to burn the gas and another 250 cubic feet of air to assure complete combustion, with little carbon monoxide. So let's say 1250 cubic feet is needed.

    Every gallon of oil used by a furnace needs about 2000 cubic feet of air for combustion.

    This air must come from somewhere. Some systems have an external venting system to bring outside air to the furnace. If your system doesn't have this, your furnace is using the air inside your house. Naturally that air must be replaced, or your house would suck the walls together, so the air is pulled in from the outside around doors and windows - even in tightly sealed, efficient homes! Worse, that cold outside air must then be heated while your heated air is used for combustion and goes up the exhaust flue.

    If the outside temperature is 0° F, it takes about 14,000 BTU just to warm the new air every hour! And let's calculate, from the energy chart, 100,000 BTU = 1 therm. So, 100,000 / 14,000 = 0.14 therm, and if each therm costs $1.00, then for each hour of use, the furnace wastes 14 cents, or 14% of your gas, heating outside air.

    At least that's not nearly as bad as using a fireplace!

    Now Much Does It Cost To Heat Your Hot Water?

    The Formula:
    Cost = Fuel Unit Cost X (Gals X Wt per Gal X Temp Rise) / (Fuel BTU X Efficiency of Water Heater)

    First, we'll figure for natural gas then for electric. Assume a 40 gallon water tank - and you've either used 40 gallons over the period of a day or perhaps you've taken a long shower that used all 40 gallons at once. How much will it cost to reheat that water?

    For the example, assume 40 gallons, heated from 60° F to 120° F. For gas, the Fuel BTU is 1 Therm = 100,000 BTU And 1 gal. of water = 8.34 lbs.
    The efficiency of an older gas water heater is 70% (a lot of heat is lost going up the flue with a gas heater). Finally, we will use a Fuel Unit Cost of $1.00 per therm.

    So, $1 X (40 X 8.34 X 60°) / (100,000 BTU X 0.70) = 0.28 or 28 cents. Hmmm. That's not very much.

    For electric, three factors change. Efficiency is 100% for electric as no energy goes up an exhaust flue, the Fuel BTU is 1 Kwh = 3413 BTU, and the Fuel Unit Cost is 8.3 cents per Kwh.

    So, 0.083 X (40 X 8.34 X 60°) / (3413 BTU X 1.00) = 0.49 or 49 cents.

    Expressed differently, to heat 1 Gallon of Water 1° F:
    8.34 BTUs raises 1 Gal 1° F -- about $0.00008
    To do it with electricity:
    1 KWH = 3413 BTUs
    So, 8.34 / 3413 = 0.00244 KWH X 8 cents = $0.00019

    Your costs may vary.

    The Water:
    I assume you know that 40 gallons of water isn't free, so we must also figure in the cost of the water.

    Your water bill may be similar to the gas and electric bills, they may have base charges, may have sliding scales based on usage, and may have minimum charges for months you use very little water. For those households with municipal sewage systems, most will have an additional charge for handling it, and that charge will also be based on your total water usage.

    For simplicity, take your total charge and divide by the gallons billed. This is the cost per gallon. An example might be $15.40 / 1200 gal = 1.29 cents per gallon.

    Using the water heater example above, that adds about 52 cents to the energy bill. Notice the water costs more than the cost of the energy to heat it!

    Your costs may vary.

    Water Heater Wonders

    I wonder...

    I wonder if you know some little-understood details about your water heater, that have drastic effects on the efficiency. Those same details may indicate your water heater is trying to kill you or perhaps, vice versa, you are trying to kill it.

    Too high a pressure inside the tank and you may well be trying to end the life of your water heater, likewise that same pressure may be the end of you. Water heaters are required to have a "pressure relief" valve, which is usually preset to 150 lb./sq.in. If you don't know, 150 lb./sq.in. is a lot of pressure! That's 150 pounds of pressure mashing on each square inch! A pair of size 9 men's shoes totals around 80 square inches, and if a man stood on an 80 square inch plate with 150 lbs per sq in pressure on the other side, it could lift a man weighing 12,000 lb. (80 X 150 = 12,000) Well, that's not too realistic, let's just use one foot and say a 40 sq. in. plate could lift a 6000 lb. man standing on one foot. Hmmm, these examples are too difficult to visualize.

    A typical car tire has at least 20 sq. in. of rubber on the ground, so 150 lb. of pressure beneath four tires, equally pushing upwards, could lift a 3000 lb. car! ...a 3000 lb. car, with 3 other 3000 lb. cars on top of it!

    OK, now we're talking! You can begin to see what force could be lurking inside your water heater tank! And consider the SIZE of the outside of that tank, then remember 150 lb. would be pressing outward on every square inch!

    But, because your water system has a "pressure reducer" that reduces the street water pressure down to a comfortable 40 lb./sq. in. or so, you don't have to worry near as much about your water heater! Or is your reducer set very high? Or don't you have one?

    Gauges for checking water pressure cost from about $3 to $8 and they screw onto a standard hose fitting, like those on your washing machine. Not one device in your house likes high water pressure - not the water heater, the dishwasher, the clothes washer, nor any of the faucets. Everything is unnecessarily stressed and will need replacement sooner if you force high pressure on them . Well, except for your lawn sprinkler. With super high pressure, you can water a lawn two houses away.

    ... With super high pressure, you can water a lawn two houses away.

    Realistically, your street water pressure is not likely to be more than about 60-100 lb./sq. in. Then why are we worried about the pressure in your water tank getting to 150? Because the water can get trapped in your house pipes! More and more municipalities require a "backflow preventer" in your water supply line. It prevents nasty water from being sucked out of your house and flowing into your neighbor's for them to drink. That's why all underground lawn sprinkler systems must have a backflow preventer. Should the street water pressure dip while the sprinklers are on, it could suck that water back into your house.

    ...car radiators and casseroles in the oven...

    With backflow devices, the water can get into your house, but can't go back. Surely you are familiar with the concept of things expanding when they are heated - that's why car radiators and casseroles in the oven boil over. Likewise, as your water heater raises the temperature of the incoming water, it increases the pressure above normal.

    After taking a nice long shower, you might need to reheat 1/2 to 3/4 a tank full - all of that water will try to increase about 3% in volume, but there's no where for it to go, so it increases the water pressure until someone opens a faucet, the dishwasher starts, or perhaps the refrigerator's ice maker cycles. Of course, that higher pressure may be captured for a while, should you immediately leave for work or go on vacation.

    Do you remember those hoses connecting your washing machine to your water supply? Whether you wash any clothes or not, those hoses constantly withstand the full house water pressure. The inexpensive ones are made of rubber - think of using flexible rubber pipes in other parts of your house! Are you really comfortable leaving your house alone with those rubber pipes lurking near your washer? Imagine someone standing near your washer, with a garden hose, spraying full force against the ceiling! Or, have you replaced the rubber hoses with the steel-braided type that very seldom develop a leak, and never develop the catastrophic leaks the rubber ones eventually do?

    We barely mentioned temperature, and that's the major purpose of a water heater, changing the temperature of water. Each water heater has a temperature dial, but it's seldom marked in degrees - usually it has arbitrary numbers, like 1 to 5, or "W" to "H" in several marks. Some years back if you had a dishwasher, they suggested setting your water temperature to about 130° F., or higher. Naturally, the lower the setting, the more energy you save - remember, the water heater must keep all that water at whatever temperature you select, ALL the time. Recent dishwashers include a self-contained booster heater, that operates only when washing dishes, and that lets the home water heater operate at a lower temperature. In addition to saving energy all year long, the lower water temperature is safer - much less chance of someone being scalded by the water.

    ...it can explode - but under very strange circumstances!!

    I wonder if you know what happens if the thermostat behind the temperature dial fails, and turns on all the time? Your water temperature will continue to increase, the pressure will continue to increase. If nothing intervenes, it can explode - but under very strange circumstances!!

    The intervening safety device is the same pressure regulator mentioned above. It has two functions, it releases the water if the pressure exceeds 150 lb./sq.in. as mentioned above, AND it also releases if the temperature exceeds 210° F. This temperature is very important. It's just under boiling and is certainly not to protect you from being scalded!! 130° F water can scald you.

    It prevents a terrible explosion. NO, I don't mean like a water balloon with steaming hot water inside, that makes you jump. I mean like a stick of dynamite that makes houses in the neighborhood jump! If the water were allowed to exceed 212° F. (boiling), the pressure inside the tank would keep if from boiling, so the temperature could keep rising. At some point, both the excessive temperature and pressure would cause the tank to crack and begin to leak. Immediately as the first trickle of water escapes, the tank's pressure is relieved, the pressure drops, and that lower pressure suddenly allows this super-heated water (water that is heated above the boiling point) to suddenly boil, or, more correctly, flash to steam. Suddenly converting 40 gallons of water to steam releases more power than a pound of nitroglycerin!! Such power makes 'ex-houses' and 'ex-neighbors'. That's why the pressure regulator and temperature release valve is required on all water heaters. You should hope your neighbor has one! - one that works!

    Your water heater manual tells you to periodically test your relief valve by gently lifting up on the handle to release some water, perhaps once every couple of months. This is an excellent suggestion if your heater is not too old, otherwise it may become an adventure. After many years without testing, debris may build up around the inside of the valve and after you open the value, some of the debris may not let the valve fully close when you release it. It becomes a difficult decision on an old water heater, if your valve will not fully re-close, you'll need to replace it, on the other hand, an old valve may be too corroded to operate and may not properly release when needed.

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    Copyright © 2003- Tom Scogin    Norcross, GA.    All rights reserved.