One of the largest cost drivers for data centers and customers is power. It takes an unbelievable amount of electricity to power and cool an entire data center. According to Computerworld, “It takes 34 power plants, each capable of generating 500 megawatts of electricity, to power all of the data centers in operation today.” Knowing how to calculate and forecast power utilization and costs are important. Knowing how to improve efficiency is critical.
In this article, we focus on the fundamentals of electricity including power units, definitions, and equations. We examine techniques for calculating power consumption of servers, racks, and cabinets. We look at the differences between metered and unmetered power and how geography plays into the cost of electricity in kilowatt hours. Several examples included throughout to help you run the calculations. Make sure to comment at the end of this article with any tips or tricks that you use to calculate power usage and costs for data center colocation.
Power Units, Definitions, and Equations
You need to know basic terminology before learning to calculate power usage and costs. It is important to know watts (W), kilowatts (kW), and kilowatt hours (kWh). It is also important to understand how volts (V) and amps (A) factor into the equation.
The first term is watt (W). A watt is a delivered unit of power in the International System of Units (SI) and is used to express the rate of energy conversion or transfer with respect to time. Watts can be seen on the labels of household appliances and electronics – everything from refrigerators to televisions and laptops. Watts (W) equal volts (V) multiplied by amps (A). It is commonly expressed as (W = V x A).
The standard unit of measure for data center power is the kilowatt (kW). A kilowatt is equal to 1,000 watts.
Kilowatt Hours (kWh)
The kilowatt hour (kWh, kW-h, kW h) is the standard for data center power usage and billing. A kilowatt hour represents power in kilowatts and the time in hours.
A kilowatt hour is equal to 1,000 watt hours. Kilowatt hours are used by utility companies as well as data centers as a billing unit for the energy delivered to the customer. Most data centers do not markup electricity prices to the customer.
kWh Electricity Prices in United States
Electricity prices in kilowatt hours for each state and region can be found on a monthly basis by visiting the U.S. Energy Information Administration. The EIA is a great source of information for comparing electricity prices between two or more locations. It can be helpful in determining what geographic location to collocate your servers, switches, and IT hardware.
Power Units Summary:
Watts (W) = Volts (V) x Amps (A)
1 Kilowatt (kW) = 1,000 Watts (W)
1 Kilowatt Hour (kWh) = 1,000 Watt Hours
Calculating Server Power Requirements
Now that you have an understanding of terminology and equations, how do you calculate server power usage? As a rule of thumb, an individual server uses approximately 0.5 – 2.0 amps and 200-450 watts per hour.
Use a Watt Meter or Volt Meter
For more accurate results, test the watt utilization off one server with all of the cores fully loaded with a watt meter or amp clamp volt meter.
Read Manufacture Specifications
You can determine power requirements by researching your server manufacture specifications online. For example, an HP ProLiant DL380 G7 Server has several power configurations with maximum rated wattage of 460W, 750W, and 1200W. Watts are partially determined by volts including: 100V, 120V, 208V, 200V, 220V, 230V, and 240V – the higher the volts, the higher the watts.
Calculating Circuit Power
If you are unsure of your specifications or do not want to test your servers using a watt meter, you can use watts (W) equals volts (V) multiplied by amps (A) equation from above (W = V x A). For example, 230V multiplied by 20A equals 4,600W. In this example, 4,600W would likely power a half rack to full rack of servers.
Make sure to take into consideration the power factor. The power factor should be approximately 80% of maximum watts. For example, multiply .80 times 4,600W to get 3,680W. Use 3,680W instead of the maximum 4,600W in figuring your costs. This is also called sustained draw.
Calculating Kilowatt Hours (kWh)
Now that you have figured out watts, it is time to calculate the monthly kilowatt hour usage for your servers. Make sure to include the power usage of switches and related hardware.
The first step is to solve for the average amount of hours in a month. To do this, simply take 24 hours a day and multiply by 30 days in a month. The answer is 720 hours in a month.
Next, you will want to solve for kilowatts. To do this, you will need to take your watts and divide by 1,000. For example, take 3,680 watts and divide by 1,000 to get 3.68kW.
To get kilowatts per hour, simply take kilowatts and multiply by hours during a time period. For example, take 3.68kW and multiply times 720 hours in a month to get 2,649.60 kWh. The 2,649.60 kWh is the amount of power your servers and related hardware use in a month.
Watts (W) = Volts (V) x Amps (A)
Power Factor (PF) x Watts (W)
Monthly Hours = 24 Hours x 30 Days
Kilowatt x Hour = Kilowatt Hour (kWh)
You have done all of the heavy lifting in determining server power requirements. All that is left is pricing per kilowatt hour and calculating total power costs. There are two primary pricing models for data center power – metered and unmetered.
The most common pricing model is referred to as metered power. In this model, you are charged for the power you use similar to how a utility company charges a residential customer. Metered power is a pass through cost from the utility company to the data center to the customer. There is rarely a markup on electrical prices from a data center. However, certain data centers may require a monthly minimum of 40-50% of the breakered power total. There may also be annual increases charged by the data center to account for the increases they are charged by the utility company.
Make sure to take into consideration the electricity cost per kWh in different geographic areas as this can drive power costs substantially. See the example below of how the differences in kilowatt hour prices can impact monthly power costs.
Metered Power Examples:
California: 2,649.60 kWh x .1763/kWh = $467.12/ Month
New York: 2,649.60 kWh x .1583/kWh = $419.43/ Month
Texas: 2,649.60 kWh x .0813/kWh = $215.41/ Month
In addition to metered power, unmetered power is a pricing model where the data center or colocation provider charges a flat rate for the circuit(s). For example, a provider could offer 32A or 7,360W of power included in a bundled offering with space, power, bandwidth, IP, and remote hands.
It is no secret that power is one of the most important considerations in data center site selection. Knowing how to calculate power can help you forecast costs and make important decisions about where to collocate IT infrastructure. It can also be helpful in determining server refreshes or whether virtualization makes sense.