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QSFP+ v. SFP+ v. 10GBASE-T testing shows that optical SFP+ and QSFP+ connections are generally more power-efficient than 10GBASE-T, but QSFP+ normally delivers 40GbE while SFP+ and 10GBASE-T normally deliver 10GbE. Passive SFP+ DAC uses the least absolute power; QSFP+ often provides the best watts-per-gigabit result.
The comparison is not a simple contest between three connector labels. SFP+ describes a 10GbE-capable pluggable form factor, QSFP+ usually describes a four-lane 40GbE form factor, and 10GBASE-T describes Ethernet signaling over twisted-pair copper. The cable, transceiver, NIC, switch ASIC, negotiated speed, reach, and traffic load all affect the final result.
The practical decision is straightforward once those differences are separated: use passive SFP+ DAC for short compatible 10GbE links, optical SFP+ for longer or EMI-sensitive 10GbE links, QSFP+ for 40GbE or four-link breakout, and 10GBASE-T when existing copper cabling or RJ-45 compatibility is more valuable than lower port power.
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- Passive SFP+ DAC is the lowest-power option in the cited Cisco figures at approximately 0.1 W per assembly.
- Cisco lists approximately 1 W for cited 10GBASE-SR and LR SFP+ optics, compared with 2.5 W maximum for its 10GBASE-T SFP+ module.
- A QSFP+ module can consume more absolute power than one SFP+ module while using less power per gigabit because QSFP+ normally carries 40GbE.
- The January 23, 2017 ServeTheHome test measured complete NIC and platform behavior above a 155 W system baseline, not isolated transceiver power.
- 10GBASE-T remains a sensible choice when Cat6A or better cabling and RJ-45 interoperability already exist, despite its higher PHY power.
What exactly are QSFP+, SFP+, and 10GBASE-T?
QSFP+, SFP+, and 10GBASE-T describe different aspects of a network connection, so they should not be treated as interchangeable speed standards.
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| Term | What it describes | Typical use in this comparison | Common cable or module |
|---|---|---|---|
| SFP+ | A single-lane pluggable interface family | 10GbE | Passive DAC, AOC, or removable SR/LR optical module |
| QSFP+ | A four-lane pluggable interface family | 40GbE, or four 10GbE breakout links | QSFP+ DAC, SR4 optic, or LR4/ER4 optic |
| 10GBASE-T | An Ethernet signaling standard over twisted-pair copper | 10GbE, often with RJ-45 connectivity | Native RJ-45 port or an SFP+ RJ-45 transceiver |
| DAC | A direct-attach copper cable with attached transceivers | Short, low-power links | Usually passive for short distances |
| AOC | An active optical cable with permanently attached transceivers | Longer or more flexible preterminated links | Optical cable assembly |
| Optical transceiver | A removable electro-optical module | Fiber-based 10GbE or 40GbE | SR, LR, ER, or ZR optic plus fiber patch cable |
An SFP+ cage can accept several physically different connection types. A passive DAC, optical SR module, AOC, and RJ-45 10GBASE-T module can all occupy an SFP+ port while having very different power and distance characteristics. The cage label alone therefore does not identify the connection’s power consumption.
QSFP+ is similarly broader than one particular cable. A QSFP+ port may carry one 40GbE connection or operate as a breakout source for four 10GbE connections. A comparison must state whether it is measuring one 40GbE link, one 10GbE breakout leg, or the complete four-link group.
What did the original QSFP+ v. SFP+ v. 10GBASE-T power test measure?
The original test, published by ServeTheHome on January 23, 2017, compared Intel network adapters installed in an ASUS 2U RS520 server. The test found the dual-port Intel XL710-QDA2 40GbE adapter substantially more efficient per gigabit than the tested 10GbE alternatives, with 10GBASE-T the least favorable option in that comparison. The original ServeTheHome test report is historical platform evidence rather than a universal measurement of every QSFP+, SFP+, or 10GBASE-T product.
| Test component | Reported configuration | Why it matters |
|---|---|---|
| Server | ASUS 2U RS520 | Power included the host platform as well as the adapter behavior. |
| Baseline | Approximately 155 W | Measurements were interpreted above a relatively stable system baseline. |
| 40GbE NIC | Intel XL710-QDA2, dual QSFP+ | Represents 40GbE-class hardware, not a single 10GbE port. |
| 10GBASE-T NIC | Intel X550-T2, dual RJ-45 10GbE | Uses copper PHYs integrated into a network adapter. |
| 10GbE SFP+ NICs | Intel X520-DA2, X710-DA2, and X710-DA4 | Shows that adapter generation and port count also affect results. |
| Cabling | 3 m DACs and 3 m Cat6A patch cables | Short passive copper and short twisted-pair copper were tested, not every possible reach. |
| Traffic load | Three hours of iperf3 before measurement |
Provides a sustained-load condition rather than an instantaneous reading. |
| Environment | 19.4 °C and 53% relative humidity | Preserves the environmental context of the test. |
The test measured power above the static server baseline and included NIC and platform behavior. It did not isolate the electrical consumption of a removable transceiver from the host adapter, PCIe interface, cage, and other components.
The accessible report presents the main results as charts, but it does not provide a complete numerical table in its text. Exact chart values should therefore not be reconstructed from visual estimates. The defensible conclusion is the reported ranking and the methodology, supplemented by current vendor specifications.
The XL710-QDA2 result also needs a bandwidth qualification. The dual-port 40GbE adapter did not sustain two full 40GbE links because its PCIe 3.0 x8 host interface could not provide enough host bandwidth for 80Gbps aggregate traffic. That limitation does not invalidate the adapter comparison, but it makes achieved throughput important when calculating watts per gigabit.
How much power do current modules use?
Current manufacturer documentation preserves the same broad order of magnitude: passive DAC is usually lowest, optical 10GbE modules are generally next, and 10GBASE-T SFP+ modules usually consume more. The figures below are product-specific ratings, not guaranteed values for every module sold under the same technology name.
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|---|---|---|---|
| Passive SFP+ DAC | Approximately 0.1 W | Approximate | Short links and compatible SFP+ cages are assumed. |
| 10GBASE-SR SFP+ | Approximately 1 W | Product-family rating | Short-reach multimode optical module. |
| 10GBASE-LR SFP+ | Approximately 1 W | Product-family rating | Longer-reach optic; fiber plant and optical budget still matter. |
| 10GBASE-T SFP+ | 2.3 W typical; 2.5 W maximum | HPE rating | Comparable HPE module; exact behavior varies by implementation. |
| Cisco SFP-10G-T-X | 2.5 W maximum at 10Gbps | Maximum | Specified for up to 30 m over Cat6A/Cat7 or better. |
| QSFP+ copper or SR4 | Approximately 1.5 W | Approximate product-family figure | Normally associated with a 40GbE-class module or assembly. |
| QSFP+ LR4/ER4 | Approximately 3.5 W | Approximate product-family figure | Longer optical reach increases module complexity and power. |
The Cisco Nexus 5600 documentation lists approximately 0.1 W for passive SFP+ copper DAC, 1 W for cited SFP+ SR/LR optics, approximately 1.5 W for several QSFP+ copper or SR4 configurations, and approximately 3.5 W for QSFP+ LR4/ER4 modules. Cisco’s 10GbE transceiver documentation lists 1 W for cited SR and LR optics and a 2.5 W maximum for the SFP-10G-T-X at 10Gbps.
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HPE lists 2.3 W typical and 2.5 W maximum for a comparable 10GBASE-T SFP+ module in its official QuickSpecs documentation. The HPE figure supports the approximate 2–2.5 W class for 10GBASE-T SFP+ modules, but it should not be presented as a standard-mandated value.
Power can change with operating mode. Cisco documents a lower maximum figure for the cited copper module when operating at 100 Mbps or 1 Gbps than when operating at 10 Gbps. A 10GBASE-T module therefore should not be represented by one fixed number across every negotiated speed.
Third-party modules can differ materially. A vendor-produced comparison of 10GBASE-T SFP+ modules reported approximately 2.0 W for some Marvell- and Realtek-based models and approximately 2.1–2.5 W for another model. That comparison is useful evidence of implementation variation, but it is not an independent laboratory standard.
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Is QSFP+ more efficient than SFP+?
QSFP+ is often more efficient per gigabit than SFP+ when QSFP+ carries 40GbE, but QSFP+ is not necessarily lower in absolute watts than one 10GbE SFP+ link.
Using the cited Cisco approximations, a 1.5 W QSFP+ SR4 or copper assembly carrying 40Gbps works out to approximately 0.0375 W/Gbps. A 1 W 10GbE SFP+ optical module carrying 10Gbps works out to approximately 0.1 W/Gbps. The calculation is illustrative: it compares module figures only and does not include the NIC, switch ASIC, host interface, cooling, or actual achieved throughput.
| Comparison | Absolute module power | Nominal bandwidth | Illustrative module watts per gigabit |
|---|---|---|---|
| Passive SFP+ DAC | Approximately 0.1 W | 10GbE | Approximately 0.01 W/Gbps |
| 10GBASE-SR/LR SFP+ | Approximately 1 W | 10GbE | Approximately 0.1 W/Gbps |
| 10GBASE-T SFP+ | Up to 2.5 W for cited Cisco module | 10GbE | Up to approximately 0.25 W/Gbps using the maximum figure |
| QSFP+ SR4/copper | Approximately 1.5 W | 40GbE | Approximately 0.0375 W/Gbps |
| QSFP+ LR4/ER4 | Approximately 3.5 W | 40GbE | Approximately 0.0875 W/Gbps |
The table does not mean every QSFP+ installation beats every SFP+ installation. A long-reach QSFP+ module can use more power per gigabit than a short passive DAC, and four separate SFP+ links may have different switch and NIC overhead than one QSFP+ port. The fair comparison is the complete configuration delivering the required throughput.
Why does 10GBASE-T usually consume more power?
10GBASE-T generally consumes more power because a copper PHY must drive and recover a high-speed signal through a demanding twisted-pair channel while compensating for attenuation, noise, crosstalk, and channel variation.
The copper implementation uses substantial equalization and digital signal processing. The PHY also has to support link training and, depending on the product, backward-speed negotiation. Those functions require more active silicon than a short passive DAC and commonly create more heat inside a small SFP+ housing.
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Fiber optics are not automatically low-power. Optical transmitters and receivers consume energy, and longer-reach modules may require more sophisticated lasers and signal conditioning. The cited Cisco figures show 10GbE SR/LR modules near 1 W, while longer-reach ER/ZR families can rise to approximately 1.5–2 W. The physical reach and optical design matter as much as the word “fiber.”
10GBASE-T power also depends on the PHY chipset, firmware, temperature, cable length, negotiated speed, and vendor thermal design. The product data sheet is more useful than a generic claim that all RJ-45 or all SFP+ copper modules consume the same amount.
Which comparison metric is fairest?
No single power metric answers every deployment question. A network architect should report module power, port power, throughput, and system power separately.
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- Port power at idle: shows the cost of maintaining a link without traffic.
- Port power under sustained traffic: captures traffic-dependent PHY, NIC, and switch behavior.
- Incremental power above baseline: isolates the added network hardware from an otherwise identical system.
- Watts per active port: the most useful headline metric for a fixed number of 10GbE connections.
- Watts per gigabit: the better metric when comparing one 40GbE connection with one or more 10GbE connections.
- Joules per gigabyte: useful for traffic-heavy systems because it relates energy to transferred data volume.
- Rack-level power: includes switches, NICs, fans, power-supply losses, cooling, breakout cables, and any additional infrastructure.
For a short server-to-switch link, the cost and power of a passive DAC may matter more than optical efficiency. For an aggregation layer, watts per delivered gigabit and port density may favor QSFP+. For an existing copper building plant, the energy difference may be less important than avoiding a cabling replacement.
What should you deploy?
| Situation | Best default | Reason | Check before buying |
|---|---|---|---|
| Same-rack 10GbE | Passive SFP+ DAC | Very low power and usually low complexity over short distances. | Port compatibility, cable length, vendor coding, and bend/route requirements. |
| 10GbE over tens to hundreds of meters | Optical SFP+ | Lower module power than cited 10GBASE-T and good reach with appropriate fiber. | SR/LR optical budget, fiber type, transceiver compatibility, and patching. |
| Existing Cat6A infrastructure | 10GBASE-T | Reuses installed twisted-pair cabling and RJ-45 patching. | 30 m-class distance, module power limits, heat, and switch release notes. |
| 40GbE aggregation | QSFP+ | Provides four-lane 40GbE bandwidth and high port density. | NIC and switch support, PCIe capacity, optics, DACs, and FEC/breakout behavior. |
| Four 10GbE links from one uplink | QSFP+ breakout | One QSFP+ port can fan out to four 10GbE connections where supported. | Breakout mode, switch configuration, cable type, and endpoint compatibility. |
| Mixed 1/2.5/5/10GbE copper environment | 10GBASE-T | RJ-45 copper can provide multirate negotiation where the equipment supports it. | Supported rates, cable category, distance, and power/thermal limits. |
| Maximum rack efficiency | QSFP+ or optical SFP+ | Choice depends on whether the requirement is aggregate bandwidth or individual 10GbE links. | Complete system power, not only module ratings. |
Choose passive SFP+ DAC when the run is short, both devices have compatible SFP+ cages, and the lowest link power and cost are priorities. Cisco lists passive SFP+ DAC at approximately 0.1 W in the cited Nexus documentation.
Choose optical SFP+ when the link extends beyond practical DAC distances, the network already uses fiber, or electromagnetic-interference immunity matters. Cisco lists cited 10GBASE-SR and LR modules at approximately 1 W.
Choose QSFP+ when the network needs 40GbE, four 10GbE breakout links, or high aggregate bandwidth density. QSFP+ is not automatically the right answer for a single 10GbE connection.
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Choose 10GBASE-T when existing Cat6A/Cat7 cabling, RJ-45 interoperability, or multirate copper negotiation outweighs additional PHY power. Cisco and HPE cite a 30 m class for comparable 10GBASE-T SFP+ modules at 10Gbps over appropriate cabling.
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What are the main 10GBASE-T deployment risks?
The principal risk is not that 10GBASE-T is unusable; the risk is underestimating its heat and power budget in a dense switch.
HPE warns that some switches limit the number of high-power 10GBASE-T SFP+ transceivers that can be installed. Cisco also documents deployment restrictions associated with a 2.5 W per-port maximum in its 10GBASE-T module interoperability and power guidance. Check the exact switch model’s release notes, transceiver matrix, temperature range, and per-port power allowance before filling a chassis with copper modules.
Compatibility is another failure mode. A module may fit physically but fail vendor validation, coding checks, firmware requirements, or multirate negotiation. Compatibility must be checked at both endpoints, especially for third-party SFP+ RJ-45 modules.
Distance and speed also matter. A 10GBASE-T module may consume less at 1Gbps than at 10Gbps, while cable length and channel conditions can affect PHY behavior. Do not use the 2.5 W maximum of one Cisco module as a universal value for every speed, cable, or vendor.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should an independent retest be designed?
A useful retest should measure both isolated module behavior and end-to-end platform power. A module’s diagnostic reading cannot replace a calibrated wall or inline measurement because module telemetry, when available, does not necessarily represent whole-port or system power.
Hardware matrix
- 10GBASE-SR SFP+ with short OM3 or OM4 fiber.
- 10GBASE-LR SFP+ when long-reach optics are relevant.
- Passive SFP+ DAC at 1–3 m.
- Active SFP+ copper or AOC, if available.
- 10GBASE-T SFP+ RJ-45 module.
- Native 10GBASE-T NIC or switch port.
- 40GBASE-SR4 QSFP+ optic.
- 40G QSFP+ passive DAC.
- QSFP+ breakout to four 10GbE links if breakout is part of the deployment decision.
Use the same vendor and switch family where possible, then repeat with a second vendor. That separates technology effects from vendor implementation, PHY chipset, firmware, and thermal-design effects.
Test controls
- Keep the host, CPU, memory, PCIe slot, operating system, drivers, and firmware constant.
- Use the same switch chassis and port configuration where possible.
- Record no-link, link-up-idle, and sustained-traffic states.
- Use an appropriate cable for each technology while documenting cable type and length.
- Record negotiated speed, FEC status, autonegotiation, link training, and error counters.
- Allow switch, NIC, and transceiver temperatures to stabilize before recording.
- Measure at the wall, at the server input, and at the module or switch telemetry layer when available.
- Run each condition multiple times and publish average, minimum, maximum, and variance.
- Use both one-way and bidirectional traffic.
- Test several 10GBASE-T cable lengths because copper PHY behavior can vary with channel conditions.
Useful Linux commands
Run an iperf3 server and a four-stream client with:
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ethtool INTERFACE
ethtool -S INTERFACEFor a longer sustained run, use:
iperf3 -c SERVER_IP -P 8 -t 3600 --logfile iperf3.logRead module diagnostics where supported with:
ethtool -m INTERFACEethtool -m is not a substitute for a calibrated power meter. Not every module exposes power telemetry, and a DOM reading is not necessarily a measurement of whole-port or wall power.
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The original ServeTheHome test’s three-hour iperf3 load and controlled 19.4 °C, 53% relative-humidity environment provide a useful starting point for a repeatable test, but a modern retest should publish raw readings, meter location, achieved throughput, and hardware firmware versions.
What does the evidence actually prove?
The evidence supports an implementation-level conclusion: optical SFP+ and QSFP+ connections are generally more power-efficient than 10GBASE-T when equivalent network-interface hardware and sustained throughput are compared. The conclusion is consistent with the higher processing burden of a 10GBASE-T copper PHY and with the cited product ratings.
The evidence does not prove that every QSFP+ product consumes less power than every SFP+ product. QSFP+ can use more absolute watts than a single SFP+ optic, and long-reach QSFP+ optics can consume more than short-reach SFP+ optics. The evidence also does not prove that every current product behaves exactly like the Intel adapters tested in 2017.
The original test compared different Intel NIC generations and architectures. Newer silicon may reduce copper PHY power, optical modules may change, switch ASICs may become more efficient, and software or firmware may alter traffic behavior. The 2017 result is valuable historical primary evidence, not a timeless benchmark for current hardware.
Infrastructure can reverse a module-level advantage. Fiber patch panels, extra transceivers, breakout cables, additional switch ports, cooling, power-supply losses, and replacement cabling all belong in a rack-level calculation. A lower module rating does not automatically produce lower operating cost unless utilization, electricity price, cooling overhead, and port count are specified.
Frequently Asked Questions
Is QSFP+ lower power than SFP+?
QSFP+ is often lower power per gigabit when carrying 40GbE, but QSFP+ is not necessarily lower in absolute watts than one 10GbE SFP+ link. The cited Cisco figures are approximately 1.5 W for several QSFP+ SR4 or copper configurations and approximately 1 W for cited 10GbE SFP+ SR/LR optics.
How much power does a 10GBASE-T SFP+ module use?
The cited Cisco SFP-10G-T-X is rated at 2.5 W maximum at 10Gbps, while HPE lists 2.3 W typical and 2.5 W maximum for a comparable 10GBASE-T SFP+ module. Actual power varies with PHY chipset, cable length, negotiated speed, firmware, temperature, and vendor design.
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What is the lowest-power 10GbE connection?
A passive SFP+ DAC is generally the lowest-power 10GbE connection for a short compatible link; Cisco lists approximately 0.1 W in the cited Nexus documentation. Optical SFP+ is a practical low-power choice for longer links, while 10GBASE-T is usually higher-power but preserves RJ-45 copper compatibility.
Does 10GBASE-T work over existing Cat6A cable?
10GBASE-T can use existing Cat6A cabling within the supported channel distance. Cisco and HPE specify a 30 m class for the cited 10GBASE-T SFP+ modules at 10Gbps over Cat6A/Cat7 or better, but the exact channel, patching, and switch compatibility must be checked.
The Bottom Line
Bottom line: choose the complete connection that matches the required bandwidth and infrastructure, not the connector name alone. Passive SFP+ DAC has the lowest absolute power for short 10GbE links; optical SFP+ is usually the efficient longer-reach 10GbE option; QSFP+ is often best for 40GbE aggregate bandwidth and watts per gigabit; and 10GBASE-T is justified when existing copper, RJ-45 access, or multirate negotiation outweighs its higher PHY power and thermal burden.
For a purchasing decision, verify the exact module’s typical and maximum power, switch port limits, firmware compatibility, cable distance, and achieved throughput. For a benchmark, measure idle and loaded wall or port power across identical hardware and report watts per port, watts per gigabit, and total rack impact separately.
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