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Old 03-21-12, 01:50 AM   #11
MN Renovator
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"Power is a rate, but watts can also be a quantity. A machine might use up 100 watts in one hour, or in one minute. Knowing how many watts are involved doesn't tell you how much time it took to use them up, just that 'x' watts is equivalent to 'x' joules for 'x' seconds. Your 21 megawatt power plant doesn't take all winter to generate 21 MW - it is pumping out 21 megawatts when it is running at full capacity"

No, if its using 100 watts, it's using 100 watts at a given point in time. If it used 100 watthours, then you could ask how long it was doing that for. You have the 21 megawatts part right though.

"MN Renovator - No, the calculation i did doesn't yield kWh, it's kilowatts."
I'm quite certain it didn't. I'll explain why your calculation didn't somewhere below this.
"Kilowatts are equivalent to Btu/hr - 3412 Btu/hr is 1 kW. They don't have an equivalent in Btu's."
Yes, they do. 1kWh is 3412 BTU. If you run a 1kW heater for an hour you have 3412
BTU of output. If you run the 1kW heater for two hours, it's 2kWh or 6824 BTU.

"U values in metric are stated as W/m2 x K. One of those is equivalent to 5.678 Btu/hr x ft2 x deg F. There is no time unit mentioned in the metric version."
Yes, there is! W/m2 * K for two hours at the same temperature will be whatever the result of W/m2 mulitiplied by 2 and the answer will be in Wh. When you take wattage and have time as a factor, it is watthours.

"That follows the same logic, but yields a number that can't be compared to Btu's. Or kWh. But it doesn't really matter, because what i need to compare it to is joules. My thermal mass energy storage has been tallied up in joules."
Yes, you can. 1 kWh is 3412 BTU. 1 kW is 3412 BTU/hr. If you can turn kWh into joules, then there you go. With the multiplier to convert that into joules, there you go. You can do the math directly.

21,359 kWh of heat is what it seems you have calculated for your winter heating needs, I'm sure a great quantity of that will be reduced from that heating needs through your glazing. If you already factored glass gains in, you've got a house that is colossal, not passive, or your math is fouled somewhere.

From what I'm seeing with my information, the metric definition of a Passive house is an annual heating/cooling requirement of under 15kWh/m2/year.

If you still think that your yearly heating needs are 21,359 kW all winter and lets say you theoretically used an electric furnace(bad idea, but it's an example) you'll be running enough electricity to where you would instantly pop your main circuit breaker. You would need to have a very large uninsulated house made of 30 gauge sheet metal with a -40 degree temperature outside, I'd imagine such a building would have to be the size of an office building to lose that much heat constantly over the course of a winter.

Please learn about the difference of the two units: watts and watthours.

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conversion, heat loss, joules, thermal mass, watts

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