6 Best Alternatives to the FSP 400W Gold Flex ATX (2026)
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The FSP 400W Gold Flex ATX Mini ITX Power Supply delivers 80 Plus Gold efficiency in a compact 1.6-inch form factor. If you are looking for fsp group mini itx solution flex alternatives, you may need different wattage or connector options. This unit provides 400 watts and a single PCIe 6+2 pin connector, which limits its use for high-power graphics cards.
Some builders require more power for modern components. Others need standard ATX dimensions to fit larger cases. The alternatives listed here were chosen because their specs overlap with the original. They share the 80 Plus Gold efficiency rating but offer higher wattage or different physical sizes. You can find options ranging from 600W SFX-L units to 1000W full-size ATX supplies. Each choice balances capacity, form factor, and connectivity to suit your specific build requirements.
How these alternatives compare
Corsair RM850x Shift 850W White ATX Power Supply
The Corsair RM850x Shift offers 850W output, doubling the wattage of the FSP 400W unit. It uses standard ATX dimensions, measuring 6.3 inches long, which requires significantly more case space than the 1.6-inch Flex ATX form factor. You gain ATX 3.1 compatibility and PCIe 5.1 support for modern graphics cards. This unit is fully modular, providing flexible cable routing. It suits builders who have upgraded to a larger case and need higher power capacity. You sacrifice the compact footprint that makes the FSP unit ideal for small NAS or Mini-ITX servers. The larger size allows for better airflow and higher sustained power delivery.
Corsair RM750x Shift White 750W ATX Power Supply
The Corsair RM750x Shift provides 750W of power, nearly double the 400W output of the original FSP unit. It features side-facing modular connectors, a design change from the rear-facing ports typical in standard units. Like the RM850x, it uses standard ATX dimensions, so it does not fit in compact Flex ATX enclosures. It supports ATX 3.1 and PCIe 5.1 standards. This power supply is fully modular, allowing you to connect only the cables you need. It is best for builders who need more than 400W but want a clean cable management solution. You give up the small form factor advantage. The side connectors can simplify wiring in cases with limited rear clearance.
Corsair RM1000e 1000W ATX Power Supply
The Corsair RM1000e delivers 1000W of power, significantly exceeding the 400W limit of the FSP Flex ATX unit. It is a standard ATX power supply, so it requires a case that supports full-size ATX dimensions. This unit is fully modular, offering extensive cable management options. It provides 80 Plus Gold efficiency, matching the efficiency rating of the original FSP unit. However, it lacks the compact size needed for Mini-ITX or NAS builds. This option is ideal for high-performance desktops or workstations that require substantial power. You trade the space-saving design for maximum wattage and standard compatibility. The larger physical size allows for robust power delivery and cooling.
be quiet! SFX L 600W Gold Modular Power Supply
The be quiet! SFX L 600W is designed for compact builds, but it uses the SFX-L form factor instead of Flex ATX. It offers 600W of output, which is higher than the 400W of the FSP unit. This power supply is fully modular and features a 120mm fan for quieter operation. It includes four PCIe connectors, providing more flexibility for graphics cards than the single connector on the FSP unit. It fits in cases that support SFX-L dimensions, which are larger than Flex ATX but smaller than standard ATX. This is a strong alternative for Mini-ITX gaming builds. You gain more power and connectivity but lose the specific Flex ATX footprint.
Montech Gamma II 750W Gold Power Supply
The Montech Gamma II 750W provides 750W of power, almost double the capacity of the FSP 400W unit. It adheres to ATX 3.1 standards, ensuring compatibility with modern components. This unit is not specified as a Flex ATX power supply, so it requires a standard ATX case. It features Japanese capacitors for reliability. The dimensions are larger than the compact FSP unit, measuring 10 inches in length. It is suitable for builders who need higher wattage and modern power delivery standards. You sacrifice the compact size for increased power and updated specifications. This option is best for standard desktop builds where space is not a primary constraint.
Redragon 650W 80 Plus Gold Fully Modular Power Supply
The Redragon 650W Fully Modular Power Supply offers 650W of output, significantly more than the 400W FSP unit. It uses a standard ATX form factor, requiring a case that supports full-size dimensions. This unit is fully modular, providing flat cables for easier routing. It carries an 80 Plus Gold efficiency rating, matching the efficiency of the original FSP unit. The physical size is much larger, measuring 9.45 inches in length. It is designed for larger cases that do not require compact power solutions. This alternative is suitable for builders who need more power and modular flexibility. You give up the space-saving benefits of Flex ATX for higher wattage and standard compatibility.
Quick picks
Our 7 picks
FSP 400W Gold Flex ATX Mini ITX Power Supply
Best for: compact Mini-ITX and NAS server builds
- Wattage
- 400W
- Efficiency Rating
- 80 Plus Gold
- PCIe Connector Type
- 6+2 pin
- Fan Size
- 4 cm
- Form Factor
- Mini ITX
- Dimensions
- 5.9 × 1.6 × 3.25 inches
View on Amazon: FSP 400W Gold Flex ATX Mini ITX Power Supply
The FSP 400W Gold Flex ATX Power Supply delivers 400 watts of continuous power in a compact Flex ATX form factor measuring 5.9 by 1.6 by 3.25 inches. It features 80 Plus Gold efficiency, an internal 4-centimeter cooling fan, and a single 6+2 pin PCIe connector. This unit suits you if you build high-density Mini-ITX systems, compact network-attached storage servers, or small industrial setups requiring efficient power delivery.
Pros
- 80 Plus Gold certified with over 90% efficiency
- Compact Flex ATX dimensions of 5.9 by 1.6 by 3.25 inches
- Includes one 6+2 pin PCIe connector for dedicated graphics
- Rated for 100% burn-in under 50°C ambient temperature
Cons
- Small 4 cm cooling fan
- Total output limited to 400 watts
- Single PCIe 6+2 pin connector restricts dual-cable GPUs
Good fit if
- You build in small form factor Mini-ITX, server, or NAS cases designed for Flex ATX units.
- You need an efficient 80 Plus Gold supply for hardware requiring 400 watts or less.
Skip if
- You need more than 400 watts of continuous power for high-draw components.
- You require standard ATX mounting dimensions or multiple PCIe power cables.
Verdict: FSP 400W Gold Flex ATX Power Supply provides efficient 80 Plus Gold power in a specialized, space-saving format. Its 400-watt output and single PCIe connector make it a reliable choice for compact servers and modest Mini-ITX builds.
Corsair RM850x Shift 850W White ATX Power Supply
Best for: ATX upgrades needing 850W capacity
- Wattage
- 850W
- Efficiency Rating
- 80 Plus Gold
- Modularity
- Fully Modular
- ATX Version
- ATX 3.1
- PCIe Connector Type
- PCIe 5.1
- Dimensions
- 6.3 × 5.91 × 3.39 inches
View on Amazon: Corsair RM850x Shift 850W White ATX Power Supply
This Corsair unit is a full-size ATX power supply delivering 850W with 80 Plus Gold efficiency. Unlike compact Flex ATX units, it measures 6.3 inches long and requires a case that fits standard ATX hardware. It provides fully modular cabling, ATX 3.1 compliance, and PCIe 5.1 support. You get Zero RPM fan mode for low-load operation. It suits you if you switch from mini-ITX to an ATX enclosure needing higher power.
Pros
- 850W capacity with 80 Plus Gold efficiency
- Fully modular cable system
- ATX 3.1 and PCIe 5.1 compliance
- Zero RPM fan mode
Cons
- Standard ATX size does not fit Flex ATX cases
- Length of 6.3 inches exceeds compact standards
Good fit if
- You need 850W with ATX 3.1 and PCIe 5.1 support.
- You use an enclosure that accommodates standard ATX power supplies.
- You prefer fully modular cables to simplify build management.
Skip if
- You need a small Flex ATX power supply for a compact chassis.
- You have a case that cannot clear a 6.3-inch power unit.
Verdict: Corsair RM850x Shift 850W ATX Power Supply delivers 850W and modern ATX 3.1 connectivity for systems that fit standard ATX dimensions. It provides high wattage and fully modular routing if you move away from compact Flex ATX limits.
Corsair RM750x Shift White 750W ATX Power Supply
Best for: ATX builders wanting side-facing modular connectors
- Wattage
- 750W
- Efficiency Rating
- 80 Plus Gold
- Modularity
- Fully Modular
- ATX Version
- ATX 3.1
- Fan Size
- 140mm
- PCIe Version
- PCIe 5.1
View on Amazon: Corsair RM750x Shift White 750W ATX Power Supply
The Corsair RM750x Shift is a 750W fully modular ATX power supply featuring side-mounted micro-fit connectors. While the FSP Flex ATX targets ultra-compact builds, this standard ATX unit suits builders moving to larger enclosures that accommodate its 6.3-inch length. It provides 80 Plus Gold efficiency, ATX 3.1 compliance, and PCIe 5.1 support. You will find it ideal if you need higher capacity and streamlined side-panel cabling.
Pros
- Intel ATX 3.1 certified with PCIe 5.1 compliance
- Side-mounted fully modular micro-fit connector panel
- 140mm fluid dynamic bearing fan
- 100% 105°C-rated Japanese electrolytic capacitors
Cons
- Standard ATX form factor does not fit Flex ATX cases
- Side cable layout requires dedicated lateral clearance inside the case
Good fit if
- You have an ATX case with sufficient side clearance for modular cables.
- You need 750W of power with ATX 3.1 and PCIe 5.1 support.
- You want 80 Plus Gold efficiency with fully modular micro-fit cables.
Skip if
- You are building inside a dedicated Flex ATX or compact Mini-ITX chassis.
- You require a power supply with traditional rear-facing modular ports.
Verdict: Corsair RM750x Shift 750W ATX Power Supply delivers modern ATX 3.1 power delivery and convenient side-mounted cable sockets. It is a solid, high-wattage upgrade if your case supports standard ATX dimensions instead of Flex ATX.
Corsair RM1000e 1000W ATX Power Supply
Best for: high-wattage full ATX system upgrades
- Wattage
- 1000 Watt
- Efficiency Rating
- 80 PLUS Gold
- Modularity
- Fully Modular
- Form Factor
- ATX
- Size
- 1000W
- Dimensions
- 5.91 × 5.51 × 3.39 inches
The Corsair 1000W Fully Modular ATX Power Supply delivers high-capacity output with an 80 Plus Gold efficiency certification. Its fully modular design lets you attach only the cables you need, reducing clutter inside your case. It features dual EPS12V connectors and 105°C-rated capacitors within standard ATX dimensions measuring 5.91 by 5.51 by 3.39 inches. This unit suits you if you are migrating from a small Flex ATX setup to a larger build.
Pros
- 1000-watt power capacity
- 80 Plus Gold efficiency rating
- Fully modular cable configuration
- Dual EPS12V connectors
- 105°C-rated capacitors
Cons
- Full ATX size will not fit standard Flex ATX chassis
- Heavier unit at 3.66 pounds
Good fit if
- You need 1000 watts of power for demanding hardware.
- You have a chassis that fits standard ATX dimensions.
- You want fully modular cables to minimize internal clutter.
Skip if
- You need a compact Flex ATX supply to fit a slim chassis.
- You only need 400 watts for a basic low-power setup.
Verdict: Corsair 1000W Fully Modular ATX Power Supply provides high output and Gold efficiency for builders moving beyond the wattage limits of Flex ATX hardware.
be quiet! SFX L 600W Gold Modular Power Supply
Best for: Mini ITX gaming builds needing SFX-L power
- Wattage
- 600W
- Efficiency Rating
- 80 PLUS Gold
- Modularity
- Modular
- Fan Size
- 120mm
- Warranty
- 3-Year
- Form Factor
- Mini ITX
View on Amazon: be quiet! SFX L 600W Gold Modular Power Supply
This be quiet! unit is a 600W SFX-L power supply designed for Mini ITX and compact gaming builds. It features 80 Plus Gold efficiency, a temperature-controlled 120mm cooling fan, and full modular cabling to reduce clutter. With four PCIe connectors and DC-to-DC conversion, it suits you if you need dedicated graphics card power in a compact chassis that accommodates an SFX-L footprint.
Pros
- 80 Plus Gold certified efficiency up to 92%
- Four PCIe connectors for multi-GPU support
- Temperature-controlled 120mm cooling fan
- Modular cable management
- Includes SFX-to-ATX adapter bracket
Cons
- SFX-L depth of 5.12 inches exceeds standard Flex ATX dimensions
- Limited to a 3-year manufacturer warranty
- Maximum capacity in this line tops out at 600W
Good fit if
- You need four PCIe power connectors in a Mini ITX system.
- You have case clearance for an SFX-L chassis and want a 120mm fan.
- You want modular cables to reduce clutter in compact enclosures.
Skip if
- You require a strict Flex ATX form factor.
- You need more than 600W of total output power.
- You want a warranty period longer than three years.
Verdict: be quiet! 600W SFX-L Modular Power Supply delivers 80 Plus Gold efficiency and four PCIe connectors for compact gaming rigs with SFX-L clearance. Its modular cables and 120mm fan offer dependable power delivery up to 600W.
Montech Gamma II 750W Gold Power Supply
Best for: modern builds needing ATX 3.1 power
- Wattage
- 750W
- Efficiency Rating
- 80 PLUS Gold
- ATX Version
- ATX 3.1
- PCIe Connector Type
- 12V-2x6
- Fan Size
- 135mm
- Warranty
- 10-year
You get a 750W power supply equipped with 80 PLUS Gold efficiency, LLC and DC-to-DC topology, and ATX 3.1 support via a 12V-2x6 cable. A 135mm fluid dynamic bearing fan operates in zero RPM mode during low loads. All internal capacitors use Japanese manufacturing. It suits builders shifting away from small Flex ATX supplies to a higher-capacity platform with a dedicated 10-year warranty.
Pros
- ATX 3.1 and PCIe 5.1 ready with a 12V-2x6 cable
- 135mm FDB silent fan with zero RPM mode
- 100% Japanese capacitors
- 10-year warranty with full OCP, OVP, UVP, SCP, OTP, and OPP protections
Cons
- 10-inch length exceeds compact case clearance limits
- Modular cabling not specified in manufacturer data
Good fit if
- You need a 12V-2x6 connector for modern PCIe 5.1 hardware.
- You want silent low-load operation via a zero RPM fan profile.
- You require 750W capacity with 80 PLUS Gold efficiency.
Skip if
- You must fit a native Flex ATX chassis form factor.
- You need modular cabling to manage case clutter.
Verdict: Montech Gamma II 750W Gold Power Supply delivers ATX 3.1 compliance and Japanese capacitors backed by a 10-year warranty. It fits systems needing higher wattage and modern PCIe power delivery.
Redragon 650W 80 Plus Gold Fully Modular Power Supply
Best for: larger cases needing modular 650W power
- Wattage
- 650W
- Efficiency Rating
- 80 Plus Gold
- Modularity
- Full Modular
- Dimensions
- 9.45 × 8.66 × 3.94 inches
- Weight
- 4.41 pounds
View on Amazon: Redragon 650W 80 Plus Gold Fully Modular Power Supply
This unit delivers 650W of output with an 80 Plus Gold efficiency certification and Active PFC. Its fully modular layout uses flat cables, letting you attach only the lines your system needs. At 9.45 by 8.66 by 3.94 inches, it is a larger option rather than a compact Flex ATX unit. It suits your build if you have chassis space for these dimensions and require more wattage.
Pros
- 80 Plus Gold efficiency certification with Active PFC
- 650W total power output
- Fully modular cabling with flat style cables
- Main capacitor manufactured in Taiwan
Cons
- Dimensions of 9.45 x 8.66 x 3.94 inches will not fit Flex ATX slots
- Weighs 4.41 pounds, heavier than standard Flex ATX units
Good fit if
- You have chassis space for a 9.45 x 8.66 x 3.94-inch power supply.
- You need 650W of capacity with 80 Plus Gold efficiency.
- You want flat modular cables to streamline internal routing.
Skip if
- You require a dedicated slim Flex ATX power supply.
- You need a lightweight power supply under 4 pounds.
Verdict: Redragon 650W Fully Modular Power Supply provides an 80 Plus Gold modular setup for systems requiring more output and flat cables. It serves builders who have standard enclosure space and do not require a dedicated Flex ATX form factor.
Specifications side by side
| Spec | FSP 400W Gold Flex ATX Mini ITX Power Supply | Corsair RM850x Shift 850W White ATX Power Supply | Corsair RM750x Shift White 750W ATX Power Supply | Corsair RM1000e 1000W ATX Power Supply | be quiet! SFX L 600W Gold Modular Power Supply | Montech Gamma II 750W Gold Power Supply | Redragon 650W 80 Plus Gold Fully Modular Power Supply |
|---|---|---|---|---|---|---|---|
| Wattage | 400W | 850W | 750W | 1000 Watt | 600W | 750W | 650W |
| Efficiency Rating | 80 Plus Gold | 80 Plus Gold | 80 Plus Gold | 80 PLUS Gold | 80 PLUS Gold | 80 PLUS Gold | 80 Plus Gold |
| Modularity | not specified | Fully Modular | Fully Modular | Fully Modular | Modular | not specified | Full Modular |
| ATX Version | not specified | ATX 3.1 | ATX 3.1 | not specified | not specified | ATX 3.1 | not specified |
| PCIe Connector Type | 6+2 pin | PCIe 5.1 | not specified | not specified | not specified | 12V-2x6 | not specified |
| Fan Size | 4 cm | not specified | 140mm | not specified | 120mm | 135mm | not specified |
| Warranty | not specified | not specified | not specified | not specified | 3-Year | 10-year | not specified |
| Form Factor | Mini ITX | ATX | ATX | ATX | Mini ITX | not specified | not specified |
| PCIe Version | not specified | PCIe 5.1 | PCIe 5.1 | not specified | not specified | not specified | not specified |
| Recommended Use | not specified | not specified | not specified | not specified | Gaming | not specified | not specified |
| Size | 400W_Gold | 850W | 750W | 1000W | 600W | not specified | not specified |
| Dimensions | 5.9 × 1.6 × 3.25 inches | 6.3 × 5.91 × 3.39 inches | 6.3 × 5.91 × 3.39 inches | 5.91 × 5.51 × 3.39 inches | 5.12 × 4.92 × 2.52 inches | 10 × 2 × 2.7 inches | 9.45 × 8.66 × 3.94 inches |
| Weight | 2 pounds | 3.75 pounds | 3.75 pounds | 3.66 pounds | 2.56 pounds | not specified | 4.41 pounds |
| Model | FSP400-60FGGBA | not specified | CP-9020273-NA | not specified | BN639 | GAMMA II | RGPS-650W |
| Color | not specified | White | White | Black | Black | not specified | not specified |
When you size a compact chassis for this form factor, examine silverstone technology mini itx slim small alternatives.
Frequently asked questions
Can I use an ATX power supply in a Mini-ITX case?
You can only use standard ATX units if your case supports that larger form factor. The Corsair RM850x Shift and RM750x Shift measure 6.3 inches long, which is much larger than the 1.6-inch FSP Flex ATX unit. You must verify your chassis dimensions before swapping to a standard ATX model.
What is the best high-wattage alternative for compact builds?
The be quiet! SFX L 600W Gold Modular is ideal for compact gaming rigs needing more power. It delivers 600W in an SFX-L form factor, which is larger than Flex ATX but smaller than standard ATX. This unit includes four PCIe connectors and a 120mm fan for reliable performance.
Which power supply supports the latest PCIe 5.1 standard?
The Corsair RM850x Shift and RM750x Shift both support PCIe 5.1 connectivity. They also adhere to the ATX 3.1 specification, providing modern power delivery for new graphics cards. These units are fully modular and designed for standard ATX cases rather than compact Flex ATX enclosures.
Is there a 1000W option for full ATX systems?
Yes, the Corsair RM1000e provides 1000W of output for high-wattage full ATX system upgrades. It features 80 Plus Gold efficiency and fully modular cabling. This unit is significantly larger than the FSP Flex ATX, requiring a standard ATX-compatible case with adequate internal space.
Does the Montech Gamma II fit in small cases?
The Montech Gamma II measures 10 inches long, 2.7 inches high, and 2 inches wide. This compact footprint allows it to fit in cases with limited space, though it differs from the specific Flex ATX dimensions of the FSP 400W unit. It offers 750W output with ATX 3.1 compliance.
What is a good modular choice for larger cases?
The Redragon 650W Fully Modular Power Supply is suitable for larger cases needing modular flexibility. It provides 80 Plus Gold efficiency and flat cables for better airflow management. This unit is much larger than the FSP Flex ATX, measuring 9.45 inches long, and is not intended for compact Mini-ITX builds.
How does the Corsair RM750x Shift differ from the RM850x Shift?
Both units share similar dimensions and features, including ATX 3.1 compliance and side-facing modular connectors. The primary difference is output capacity, with the RM850x Shift delivering 850W and the RM750x Shift providing 750W. Both are fully modular and designed for standard ATX cases rather than compact Flex ATX form factors.