estimating KW use

I'm considering installing electric baseboard heating in a building I just finished, because I all ready have the heaters and adequate electrical service to the building. The building is 20x30 and attached on the 30ft end to an exiting building. The new building has R19 walls, R40 ceiling, and on a slab with a perimeter thermal break. The only heat loss calculator I've found online provided by a stand-alone heater manufacturer suggests I need around a 4500kw heater. My area has around 5000 heating degree days. How do I derive the electrical usage dollar estimate at .07/kwh.

On Mon, 12 May 2008 11:29:48 -0500 Jeff D. wrote: | I'm considering installing electric baseboard heating in a building I just | finished, because I all ready have the heaters and adequate electrical | service to the building. The building is 20x30 and attached on the 30ft end | to an exiting building. The new building has R19 walls, R40 ceiling, and on | a slab with a perimeter thermal break. The only heat loss calculator I've | found online provided by a stand-alone heater manufacturer suggests I need | around a 4500kw heater. My area has around 5000 heating degree days. How do | I derive the electrical usage dollar estimate at .07/kwh.

Whatcha heating? An indoor football stadium?

The 4500 KW is a mistake. Even in interior Alaska you could heat that building with 4 each 1500 watt baseboard electric heaters. That comes to 6 KW. At $0.07 per kwhr that would be 0.07 x 6 =3D 42 cents per hour. But they would not be on all the time. I would estimate your average monthly bill at $200 in the winter. I don't know where you get a rate like 0.07/kwh. Here, it is more like 0.20/kwh. We don't heat with electricity in the Interior of Alaska because it is way too expensive, but use fuel oil. But fuel oil has doubled in price in the last year and is now at over $4.00 a gallon. Many people are switching to wood stoves or coal. Coal is the cheapest at 25 percent the cost of fuel oil.

Oops that's 4500W not 4500kw as you-all caught. I live in northern Indiana and we get our power through and electric co-op utility. I'm trying to understand how degree days and heat loss relate to power consumption. I also have LPG to this building, but would have to purchase a heater of some type. I think the recommendation I found for a LPG heater was in the 30,000btu range. Unless cost is prohibitive I'd prefer the baseboard heat over LPG because of the cleanliness and I don't have to punch a hole for a flue.

The 4500 KW is a mistake. Even in interior Alaska you could heat that building with 4 each 1500 watt baseboard electric heaters. That comes to 6 KW. At $0.07 per kwhr that would be 0.07 x 6 = 42 cents per hour. But they would not be on all the time. I would estimate your average monthly bill at $200 in the winter. I don't know where you get a rate like 0.07/kwh. Here, it is more like 0.20/kwh. We don't heat with electricity in the Interior of Alaska because it is way too expensive, but use fuel oil. But fuel oil has doubled in price in the last year and is now at over $4.00 a gallon. Many people are switching to wood stoves or coal. Coal is the cheapest at 25 percent the cost of fuel oil.

You can easily look up the BTUs per KW, I don't recall the value offhand. Also keep in mind that most combustion heaters are 80%, or more recently 90% efficient, so the BTU output will be lower than the rated input. You could always install multiple baseboard heaters, put in one or two, then if you find you need more heat, install additional heaters.

I heat a 22 by 22 foot cabin with a half 2nd floor and dormer with a

72, 000 BTU Toyo Laser 73. The cabin has 3 inches of sprayed foam in the walls and 5 inches in a cathedral roof that is on a 12 x 12 pitch. When it is 55 F below zero outside it runs on high all the time, and we sometimes add a 1500 watt electric heater. The Toyo stove has a great reputation in Alaska as being very efficient, but they have to use No. 1 fuel oil and cost about $1700. They last and last and do not require a flu, just a little 2 inch hole through the wall. The reason they are so efficient is because they suck combustion air in through a pipe around the exhaust pipe so the combustion air is preheated. But I am putting in a coal stove this summer, because No.1 diesel is now about $4.50 a gallon and goes up about 50 cents every two months. We burn about 90 gallons a month in the winter. No one here can afford electric heat, because it too has risen in price and probably will continue to increase. Coal is the answer. It seems we are going back 60 years in time because of the state of the economy. Incidentally, all the stores around here are out of rice and they say flour is next.

Okay I know I'm being a bit pendantic here, but your heater is probably rated not in BTU but in BTU/hr. BTU is a measure of energy, and BTU/hr is a measure of power. Similarly W or kw is a measure of power.

Anyway, 1 kw is 3413 BTU/hr. So if you have an LPG heater rated at 30,000 BTU/hr, that's about equivalent to 8.8 kw.

The building is going to take about the same amount of energy to heat it regardless of the source, so the thing to look at is how much do you pay for a kw-hr of electricity (100% efficient heater) versus how much you would pay for the amount of LPG that would give you the same energy.

The heating value of LPG is about 20,000 BTU/lbm. I can buy a small amount for my outdoor grill at about $12 for 20 lbm or $0.60 for one pound. So that's $0.60 for 20,000 BTU which is the same as $0.60 for 5.86 kw-hr. So at that rate it works out to about $0.10 per kw-hr of energy from the LPG. But on top of this we have to figure that the gas heater isn't 100% efficient. So if it runs about 80% efficient, that works out to about $0.128 per kw-hr of gas heat delivered.

So if bulk LPG cost the same (I doubt it though), the LPG would cost more to heat with than the electric. But if you know the price you pay in your area for LPG you can probably figure this out with what I've given you.

daestrom

| Oops that's 4500W not 4500kw as you-all caught. I live in northern Indiana | and we get our power through and electric co-op utility. I'm trying to | understand how degree days and heat loss relate to power consumption. I also | have LPG to this building, but would have to purchase a heater of some type. | I think the recommendation I found for a LPG heater was in the 30,000btu | range. Unless cost is prohibitive I'd prefer the baseboard heat over LPG | because of the cleanliness and I don't have to punch a hole for a flue.

If I were building this from the ground up, I'd prefer to go with some kind of water or mineral oil based thermal distribution system. A central heating and cooling facility heats or chills the liquid. Then you can put the heat and chilling source at a central point, maybe even in a separate building.

| The 4500 KW is a mistake. Even in interior Alaska you could heat that | building with 4 each 1500 watt baseboard electric heaters. | That comes to 6 KW. At $0.07 per kwhr that would be 0.07 x 6 = 42 | cents per hour. But they would not be on all the time. I would | estimate your average monthly bill at $200 in the winter. | I don't know where you get a rate like 0.07/kwh. Here, it is more | like 0.20/kwh. We don't heat with electricity in the Interior of | Alaska because it is way too expensive, but use fuel oil. But fuel | oil has doubled in price in the last year and is now at over $4.00 a | gallon. Many people are switching to wood stoves or coal. Coal is | the cheapest at 25 percent the cost of fuel oil.

The heating power would dictate the level of insulation you need to maintain a given temperature, as well as the temperature rise time when heating it up cold (along with volume).

The size of the heaters depends on the minimum design temp your local codes require. You didn't state that, so I'm skipping it. A rough estimate of energy use is:

ceiling: 600 s.f. / R40 = 15 BTU/hr-°F unheated walls: 560 s.f. / R19 = 29.4 BTU/hr-°F slab*: 600 s.f. / R10 = 60 BTU/hr-°F ---- 104.4 BTU/hr-°F

104.4 BTU/hr-°F * 24 hr/day * 5000 degree-days = 1.25x10^7 BTU/year 1.25x10^7 BTU/year / 3413 kW-hr/BTU = 3899 kW-hr/year 3899 kW-hr/year * $0.07 = $273 year

This ignores radiation losses (not that much), infiltration (very significant, depends on wind speeds), windows and doors(very significant).

*This is how our state code handles slabs. If yours uses loss perlinear foot of slab perimeter and you know what figures your code requires, plug that in instead.