How To Speak Data Center: kWh to BTUs
November 01, 2012
In my first post on how IT can better communicate with their data center facility counterparts, I discussed the distinction between power and energy, and why that distinction matters when IT talks about its power and cooling needs with data center professionals.
The next step in building your data center credibility is your proficiency in converting between units of power and energy. Miscommunication over these measurements can lead to errors in power, cooling and business continuity. Take the time to learn each unit of measurement and how it is translated into other units.
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Common units of electricity include volts, amps, VoltAmps, watts, and British Thermal Units (BTUs). This post will address Kilo-Volt-Amps (kVA), Kilowatts (kW) and BTUs. IT professionals should be conversant in the correct use of these terms.
kVA to kW
The electricity that IT equipment uses is measured in VoltAmps (VA). VA is a measure of power. IT equipment is also rated for watts, which is also a measurement of power. However, watt-hours is a measure of energy, which may cause some people to stumble in their calculations. As a quick reminder, power is the rate at which energy can be expended, while energy is the total quantity of work performed.
Generally speaking, the VA (power) for IT equipment can be considered almost the same as watts (power). Most IT power supplies have a 0.96-0.99 "power factor" which is used to convert VA to watts. So for instance, 1000VA converts to 960 to 990 watts. Data center facilities folks won't scoff at you if you generally round up to a 1-to-1 ratio in VA-to-watts conversion when talking about IT equipment. (Note that most other types of equipment don't work this way. A motor, for example, might only have a power factor of 0.80, which means 1,000VA becomes 800 watts.)
Why should IT care about kVA and kW? Because IT equipment loads are most often expressed as kWs, while most of the electrical distribution capacity is described in VA. Your ability to describe the capacity of a UPS or Power Distribution Unit (PDU) in terms of the IT equipment requirements is important. For example, a UPS manufacturer may try to say you need a larger UPS due to your power factor. If you don't understand the role power factor plays in conversion, and you don't understand your power factor, the vendor can talk you into a larger UPS than you need. Conversely, if the vendor isn't converting correctly, you could run out of UPS capacity far sooner than expected.
Don't confuse the power factor with the efficiency rating of the power supply. Many power supplies now have Power Factor Correction, or PFC. A perfect power factor of 1.0 is very good, but it could still be very inefficient at converting the AC energy into DC energy used by the computer.
kWh to BTU
Similar to VA and watt conversion, you should be comfortable converting IT kW (power) use into heat loads (energy), which is necessary to understand the needs of your cooling systems. In this case, a facility's capacity is expressed in BTUs or "tons" of cooling (more on these units in a later article). IT equipment is really efficient at converting watt-hours into heat. This means that, similar to converting approximately 99% of VA to watts (power to power), the IT equipment converts most of the energy it consumes into heat. For estimation purposes, you can use a 1-to-1 conversion without losing credibility.
Before you get out your calculator, don't forget that BTU is a unit of energy, not power. This means that you will look like a n00b if you convert kW (power) to BTU (energy). You must convert kilowatt hours, or kWh, (energy) to BTU (energy) or BTU hours (BTUh).
One watt-hour is about 3.412 BTU. 1kWh of energy will generate 3,412 BTU of heat over one hour (or 3,412 BTUh). Be aware that not all the energy consumed by the IT equipment is converted directly into heat; some energy is consumed by fans. IT equipment that consumes 1kWh won't produce exactly 3,412 BTUh, but it's close enough for capacity planning and forecasting.
Why should you care? If you can't calculate the heat load increases from your energy consumption, how can you know if your cooling system can meet the demands? What if you are running over the rated capacity of your cooling systems at this moment? Would you be able to tell?
Learn the conversions and practice them. Your ability to quickly convert between measurements will be useful for capacity planning and emergency preparedness, and ensure that you and your data center facilities counterparts are speaking the same language.
Ken Miller is data center architect with the IT Infrastructure and Operation Services division of Midwest ISO, developing mission-critical facilities.