Paralleling Residential Generators
Why this matters
A customer with a 36 kW load and a 22 kW generator has three options: live with reduced backup, buy a single larger generator, or parallel two smaller units. The two-generator parallel approach has historically been commercial-only because of the complexity (synchronizing two power sources, sharing load, handling failure of one unit). Generac in 2018 introduced parallel-capable residential generators that connect two units electronically; the residential parallel install is now achievable, expanding the upper end of residential backup. Knowing when paralleling makes sense (and when it doesn't) is the spec-sheet conversation that separates a contractor familiar with residential parallel from one who quotes a single oversized unit by default.
What paralleling solves
Paralleling two generators provides:
- Increased total capacity: 2x 22 kW = 44 kW, supports a load that would need a single 48 kW unit
- Redundancy: if one generator fails, the other delivers half the rated capacity (so partial backup continues)
- Easier installation in some configurations: two smaller units fit in spaces where one large unit would not
- Potentially lower cost: two mid-sized units sometimes cheaper than one very large unit, depending on brand and tier
- Future flexibility: customer can add a second unit later rather than commit to large unit upfront
What paralleling does NOT solve:
- Reliability during simultaneous failure (lightning hits both, common cause failures)
- Maintenance overhead (two engines to service)
- Initial install complexity
When paralleling makes sense
| Customer situation | Single large generator | Paralleled units |
|---|---|---|
| Load 22 kW or less | Single generator | Not needed |
| Load 22 to 48 kW | Single 36 to 60 kW | Possible parallel option |
| Load 48 kW or more | Single large unit if available | Parallel 2x 36 kW or 2x 48 kW typical |
| Redundancy priority | Single is single point of failure | Parallel provides redundancy |
| Maintenance simplicity | One engine | Two engines |
| Future expansion | Replace generator | Add second unit |
For a customer who values redundancy (hospital, critical home with medical equipment, server room), parallel is often justified even when single would work.
Brand support for residential paralleling
Not all generators support paralleling. As of 2025:
| Brand | Parallel-capable residential models |
|---|---|
| Generac | Guardian Bi-Fuel and Generac Generac Parallel models (specific PWY series) |
| Kohler | Select industrial-residential models; check current literature |
| Cummins | Limited residential parallel; mostly commercial |
| Briggs and Stratton | Not standard for residential |
| Cummins QuietConnect | Commercial-style; can parallel |
Generac is the most common residential parallel platform. Brand-specific approach varies.
How parallel synchronization works
Two generators connected in parallel must synchronize:
- Voltage: both must be at the same voltage (typically 240V single-phase or 208/480V three-phase)
- Frequency: both must be at the same frequency (60 Hz US)
- Phase: voltage waveforms must be in phase
Achieving sync:
- First generator starts and stabilizes (the "master")
- Second generator starts independently
- Second generator adjusts voltage and speed to match the master
- Synchronizer monitors both: voltage match, frequency match, phase match
- Phase angle drift to zero or near-zero, then close contactor
Most paralleling systems use:
- Active synchronizer: monitors and adjusts second generator's speed to match
- Auto sync check: ensures phase angle is acceptable before contactor close
- Load sharing: after parallel, both generators share load proportionally
Manual paralleling (where the operator watches a sync scope and closes the contactor at zero phase angle) was historical. Modern parallel systems are automatic.
Generac residential parallel example
The Generac PWY (parallel ready) series:
- Two generators of the same model (must be matched - two 24 kW or two 36 kW, not mixed sizes)
- A parallel controller / panel handles synchronization
- ATS rated for total output
- Specific Generac monitoring (Mobile Link) supports dual-unit monitoring
Wiring:
- Both generators feed into a parallel bus
- Parallel bus feeds the ATS
- ATS feeds the home panel
Cost premium (parallel system + second generator vs single oversized unit):
- Two Generac 24 kW + parallel kit + larger ATS: roughly a substantial cost
- Single Generac 48 kW: roughly a substantial cost
The parallel approach is more expensive than single, but provides the redundancy benefit.
Load sharing in parallel operation
When two generators run in parallel under load:
- Total load is distributed between the two units
- Distribution is typically 50/50 (each carries half) if both are sized equally
- If one generator's speed control is off, distribution shifts (the slower-running generator carries less load)
Modern systems handle this automatically. Older or simpler systems may require manual speed adjustment.
For the customer:
- "Both generators run during operation; if one fails, the other continues at full output"
- "Load is automatically shared between them"
- "Maintenance is twice as much (two engines)"
When one unit fails (during parallel operation)
If one generator faults during parallel operation:
- The fault is detected by the parallel controller
- The failed unit's contactor is opened
- The remaining unit continues to carry load (within its capacity)
- If the load exceeds the remaining unit's capacity, the load shed module trips lower-priority loads
- Customer continues to have power, possibly with reduced functionality
This is the redundancy value: a single-generator install would have no power; parallel install has partial.
Installation considerations
Space
Two generators require two pads. Total footprint typically 2x single-generator footprint.
Fuel
For propane: tank sizing for both generators combined. A 22 kW generator pair (2x 22 kW = 44 kW total) at full load consumes 6 to 7 gph; a 1000-gallon tank lasts 140 to 165 hours at full load.
For natural gas: gas line and meter must support the combined draw. Verify with utility.
Electrical
- Two separate generator output feeds to parallel bus
- Parallel bus sized for combined output
- ATS rated for combined output (typical 200A ATS handles 48 kW; 400A ATS for 96 kW)
Control wiring
References
- NFPA 70 (NEC) 2023, Article 705 (Interconnected Electric Power Production Sources).
- NFPA 110 (Standard for Emergency and Standby Power Systems).
- IEEE 1547 (Standard for Interconnection and Interoperability of Distributed Energy Resources).
- IEEE 446 (Recommended Practice for Emergency and Standby Power for Industrial and Commercial Applications).
- Manufacturer literature: Generac PWY series, Kohler residential parallel options, Cummins QuietConnect parallel.
- Manuall internal: Generator Whole House Generator Install, Generator Generator Sizing, Generator Transfer Switch Installation.