Server Rack Power Budget
Add up server and device watts to get the rack load in kW, its heat in BTU per hour, a branch circuit size and the monthly and yearly energy cost. How it works
Use the Server Rack Power Budget
About the Server Rack Power Budget
Ten servers drawing 500 W each plus two 50 W switches make a 5.10 kW rack, which gives off 17,401 BTU/hr of heat and costs $446.76 a month at $0.12 per kWh around the clock. The rack power calculator adds the watts, converts them to BTU per hour (W × 3.412), prices the energy and suggests a 208 V branch circuit sized by the 80% continuous-load rule: 5.10 kW is over the 4,992 W a 30 A circuit carries, so it needs 50 A.
Rack load, heat, cost and circuit (two 50 W devices, $0.12 per kWh, 730 h)
| Servers | Total load | Heat | Energy cost a month | Circuit suggested |
|---|---|---|---|---|
| 4 at 350 W | 1.50 kW | 5,118 BTU/hr | $131.40 | 30 A, 208 V |
| 8 at 600 W | 4.90 kW | 16,719 BTU/hr | $429.24 | 30 A, 208 V |
| 10 at 500 W | 5.10 kW | 17,401 BTU/hr | $446.76 | 50 A, 208 V |
| 16 at 400 W | 6.50 kW | 22,178 BTU/hr | $569.40 | 50 A, 208 V |
| 20 at 450 W | 9.10 kW | 31,049 BTU/hr | $797.16 | 55 A or more, 208 V |
Above 8,320 W, the continuous limit of a 50 A circuit, the tool states the amps needed and says to contact facility power.
How to use this tool
- Enter the number of servers and the watts each one actually draws, from a power meter, the PDU readout or the vendor’s typical-load figure.
- Add other devices such as switches and storage with their watts, then the PSU rating and the redundancy (N, N+1 or N+2).
- Enter your price per kWh and the hours per month (730 is a full month). The totals update as you type.
- Set the supply voltage (120, 208 or 240 V single-phase, or your own) and the power factor from the PSU datasheet. The page shows the current per server, per other device and in total, and sizes the circuit on VA.
Frequently asked questions
- How do you convert watts to BTU per hour?
- Multiply by 3.412. A 5,100 W rack gives off 17,401 BTU/hr, which is about 1.45 tons of cooling at 12,000 BTU/hr per ton. Every watt a server draws ends up as heat in the room.
- Should I enter the PSU rating or the measured watts?
- The measured or typical draw. A power supply’s rating is the most it can deliver, not what the server takes from the wall. Entering nameplate ratings as the draw overstates the heat, the cost and the circuit size.
- What size circuit does a server rack need?
- A breaker must be rated at least 125% of a continuous load, so a load may use 80% of the breaker. At 208 V that is 4,992 W on a 30 A circuit and 8,320 W on a 50 A circuit. The tool sizes the circuit on VA (watts ÷ power factor) at the single-phase voltage you set: the default 5.10 kW draws 24.5 A at 208 V and needs 50 A, while at 240 V a 30 A circuit carries 5,760 W and is enough. Three-phase feeds are not modeled.
- What does N+1 mean for power supplies?
- Enough PSUs to carry the server, plus one spare. An 1,800 W server on 1,000 W supplies needs 2, so N+1 means 3, and with all three running each carries 60% of its rating. Redundancy changes the PSU count, not the power the rack draws.
- Does the energy cost include cooling?
- No. It prices only the IT load you entered, at a constant draw for the hours given. The cooling plant and power losses in UPS and distribution add to the real bill.
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