Design Considerations for Running Pumps and HVAC in Off-Grid with Encharge Energy Storage

Summary

This document outlines key design considerations for integrating pumps and HVAC units into off-grid energy systems utilizing Enphase Encharge Energy Storage. It addresses the challenges of high startup current (inrush current) for these loads and provides solutions to ensure reliable operation.

• When most HVAC units and electric motors and pumps turn on, they draw 4-8 times their normal operating current for a short duration, making off-grid startup challenging.
• Enphase's proprietary Power Start™ technology significantly reduces the power needed to start pumps and HVACs off-grid.
• Enphase recommends installing a soft starter on HVAC units for off-grid operation, reducing startup current by 35%.
• Hyper Engineering SureStart soft starters are suggested for AC units. These parts are also available in the Enphase online store as accessories for the Encharge energy storage system.
• A table summarizes the minimum number of Encharge base units recommended for HVAC units with soft starters and a 10A (2.4kW) base load in off-grid mode.

*If the FLA of the AC unit is close to 16A, SS1B16-32SN OR SSTART-16-32A-1P-230V is suggested.

1. Introduction

The addition of a multimode, grid-agnostic Energy Storage System (ESS) to a PV system offers benefits like backup power during grid outages. Proper ESS sizing is crucial for off-grid operation to prevent blackouts. Encharge 3 and Encharge 10 storage systems are multimode ESS units capable of off-grid operation and grid-forming functionality. An Encharge 10 provides 3.84kW continuous power and 5.76kW surge power for ten seconds, delivering 24A of surge current via 12 IQ8X-BAT microinverters. Enphase Ensemble Technology includes proprietary control features like primary control, Power Start™, and black start.

AC power dynamics, including power factor and inrush current, are critical for sizing backup systems. Resistive loads are simpler to design for, but motor loads (e.g., HVAC, refrigerators, pumps) have a power factor less than one and require additional current during startup (inrush current). Load specifications are often expressed in Amps or Watts, with voltage assumed constant (P = VI).

2. Theory of Operation and Control in Off-Grid Operation

2.1 Primary Control

The Enphase Ensemble system uses a distributed primary control system to balance instantaneous generation and load. Inverters automatically increase power injection as voltage and frequency drop, a process known as "droop" control, managed by proprietary Application-Specific Integrated Circuit (ASIC) chips in each microinverter.

2.2 Power Start™

Power Start™ is a proprietary brownout control algorithm that helps manage large inrush loads. It detects when the inverter approaches its maximum power limit and actively reduces voltage and frequency to maintain the inverter within its limits. This temporary reduction, or brownout, allows equipment like pumps and HVACs to start with significantly less power. This "ride-through" capability prevents system shutdown. A typical grid-tied compressor startup completes in 150ms with current peaks near 200A. In off-grid operation, Power Start™ triggers a brownout response, limiting current and extending startup time (~600ms), but ultimately restoring normal voltage and frequency.

A visual representation of Power Start™ brownout control shows how the IQ8X-BAT microinverters reduce system voltage and frequency to aid motor and compressor startup. Another graph illustrates voltage and current of an AC unit with a soft starter in grid-tied and off-grid operation modes, demonstrating how Power Start™ technology reduces voltage in off-grid operation to enable compressor startup.

2.3 Black Start

While Power Start™ improves the system's resilience to temporary surges, a sufficiently large surge can cause a "blackout" (system shutdown). This shutdown limits the extent and duration of brownout conditions, protecting inverters and loads from premature wear. IQ8X-BAT microinverters shut down if voltage and frequency drop below pre-defined limits for a set duration (ride-through conditions).

If a system shutdown occurs and no grid is present, the Enpower smart switch (if installed) attempts an automatic "black start." This involves disconnecting Encharge branch circuits, allowing IQ8X-BAT microinverters to restart and reach full voltage under no-load conditions. Enpower then recharges its backup power source from the Encharge branch circuit(s) before attempting to re-energize the home system. If the load is too high and the system collapses, the process repeats automatically after a few minutes.

3. Electric Motors and Pumps in Off-Grid Operation

Electric motors, such as well, pressure, and booster pumps, have an inrush current (Locked Rotor Amperage - LRA) that can be 4-8 times higher than their Running Load Amperage (RLA) or Full-Load Current (FLC). LRA lasts for a fraction of a second. Power ratings are often expressed in Horsepower (HP), where 1 HP equals 745.7 Watts. Electric motors typically have a power factor of 0.7 to 0.9 during normal operation, and 0.5 to 0.6 during startup.

The following table provides estimated FLC and LRA values for single-phase electric motors (1/5 HP to 7 1/2 HP) across 110V-120V and 220V-240V voltage ranges, based on NEC 2017 tables 430.248 and 430.251(A). Actual nameplate values should be used if available.

A variable frequency drive (VFD) starter reduces electric motor startup current by lowering voltage and frequency. While VFDs reduce inrush current, they can introduce harmonics that are difficult for inverters to support in off-grid mode. However, Enphase IQ8X-BAT microinverters can handle these demanding current waveforms due to fast control loops and advanced power management features, including Power Start™.

3.1 Case Study: 3 HP Pump with VFD

A 3 HP electric motor with VFD was successfully supported by two Encharge 10 and two Encharge 3 units (eight Encharge base units). The combined continuous current rating is 42.6A, with a peak current capability of 64A. The 3 HP pump with VFD drew a peak current of 8A (5.7A RMS) during startup. This demonstrates that even two Encharge 3 units could manage the load. A waveform analysis shows voltage and current waveforms of an electric motor with VFD, noting distortions on the current waveform.

3.2 Case Study: 3 HP Pump without VFD

A 3 HP pressure pump (booster pump) was successfully run off-grid without a VFD using seven Encharge 3 batteries. The pump typically consumed 64A to 80A (RMS) surge during startup. In off-grid mode, two Encharge 10 and one Encharge 3 (total of seven Encharge base units) provided up to 58.8A of startup current. The Encharge storage system consistently started the pump. Ensemble Power Start™ allowed the voltage to drop from 240V to approximately 186V during startup, enabling the pump to run off-grid via the smart brownout capability of the Encharge IQ8X-BAT microinverters. A nameplate for a Three HP Pump without VFD shows details such as HP: 3, RPM: 3450, Volts: 208-230, F.L.A.: 14.0-13.0, S.F.: 1.0, S.F.A.: 16.0-14.0, Made in Mexico, and various certifications.

3.3 Summary of Enphase Ensemble Advantages for Pumps

The Ensemble system can start electric motors and pumps with or without VFD due to its fast response time, proprietary algorithms, and Power Start™ technology. This allows for appropriately sized installations, avoiding the need for excessively large storage units often required by competitors.

4. HVAC Systems in Off-Grid Operation

HVAC systems, particularly their compressor motors, are a primary consideration for sizing. Most common compressors are scroll type, but rotary or piston types also exist. Compressors can be single-stage, multi-stage, or variable speed. Non-variable speed HVAC units typically have an increased startup current (Locked Rotor Amperage - LRA) that can be 4-8 times higher than Running Load Amperage (RLA) or Full Load Amperage (FLA). LRA usually lasts a few seconds, after which only RLA is needed. Variable speed HVAC systems generally lack inrush current but have other considerations as nonlinear loads.

Providing startup current is the main challenge for running ACs in backup/off-grid mode. Encharge storage systems provide surge current via IQ8X-BAT microinverters and the advanced power management feature in Ensemble, including Power Start™.

4.1 Reading an HVAC Nameplate

Properly reading an HVAC nameplate and understanding RLA and LRA definitions is crucial for designing storage systems that support HVAC in backup/off-grid operation. An example HVAC nameplate shows parameters like SERIAL: 5017E12619, MODEL: 24AAA548A300, FACTORY CHARGED R410A: 8.31 LBS, POWER SUPPLY: 208-230 VOLTS AC, 60 HZ, COMPRESSOR RLA: 15.5, LRA: 105.5, FAN MOTOR HP: 1/4, FLA: 1.40, MAX FUSE/CKT-BKR: 35 A. Full Load Amps (FLA), also known as Running Load Amps (RLA), is the continuous current expected under steady load. Locked Rotor Amps (LRA) relates to the expected inrush current during startup. The ESS peak current output should ideally exceed the LRA and the base load during HVAC startup.

4.2 Using a Soft Starter to Reduce Inrush Current

Large LRA values can be reduced with a soft starter. Examples of Hyper Engineering SureStart soft starters include: SS1B08-16SN (230V, 60/50Hz, 08-16 FLA) for scroll compressors, SS1B16-32SN (230V, 60/50Hz, 16-32 FLA) for scroll compressors, and SS0B12-20SN (115V, 60/50Hz, 12-20 FLA) for rotary/piston compressors. More details and data sheets are available at hypereng.com. Enphase also offers these parts in its online store. The table from the summary section (also on page 10) shows typical LRA before and after installing a soft starter.

4.3 Sizing Storage for running HVAC in Off-Grid

2017 NEC 710.15 (A) requires standalone power sources to support the largest load, often the HVAC system, which is a non-resistive load with high startup current. To support AC unit startup with Encharge in backup/off-grid mode, follow these steps:

1. Install a soft starter on the AC unit, such as SureStart from Hyper Engineering (hypereng.com). Use the provided tables and manufacturer guides to select the correct model.
2. Measure the reduced LRA of the unit after soft starter installation while connected to the grid. Ensure the surge power rating of Encharge units can support the HVAC unit's LRA. Each Encharge 3 provides 8A RMS of surge current for ten seconds via its four internal IQ8X-BAT microinverters. PV generation during HVAC startup can ease the process.
3. If your specific HVAC unit's startup current, even after soft starter installation, requires additional units to meet surge current requirements, leverage Encharge system modularity to optimize battery count.
4. Allow the soft starter to "learn" your AC behavior by starting it about ten times in grid-connected mode after installation.

4.4 Case Study: Half-Ton (0.5) AC

Make and model: LG LW6017R. HVAC unit data sheet: lg.com/us/air-conditioners/lg-LW6017R-window-air-conditioner. FLA: 4.9 A. LRA: not captured on data sheet. Size: 6000 BTU / 0.5-ton. This installation used two Encharge 3 units, providing a total rated capacity of 6.6kWh, 2.56kW continuous power, and 3.84kW surge power for ten seconds. A scope capture shows the startup of this AC unit in an off-grid condition, successfully supported by two Encharge 3 units. The AC unit did not have an inrush current, and the peak current was 7.4A (5.2A RMS). Even one Encharge 3 (8A RMS peak current for ten seconds, 5.3A continuous current) could support these half-ton AC units. The graph shows voltage (V) on the left Y-axis and current (A) on the right Y-axis over time (s). The voltage waveform is a clean sine wave, while the current waveform shows a peak during startup before settling.

Summary of Enphase Ensemble Advantages for Small AC Units

Encharge Power Start™ technology and primary control enable supporting small AC units in off-grid mode with minimal battery storage. The modularity of Encharge allows customization of storage size to fit site needs, avoiding unnecessary battery purchases.

4.6 Case Study: Four-Ton AC with Soft Starter

This case study demonstrates how a soft starter reduces inrush current on a four-ton HVAC system. HVAC unit details: Make: Carrier Corporation. Model: 24AAA548A300. HVAC unit data sheet: carrierenterprise.com/24aaa548a003. FLA: 15.5A. LRA: 105.5A. Size: 48,000 BTU / 4-ton (model number '48' refers to 48kBTU/four-ton capacity). The installation used five Encharge 3 units, with a total rated capacity of 16.6kWh, 6.4kW continuous power, and 9.6kW surge power for 10 seconds.

A scope capture of a 4-ton HVAC startup without a soft starter shows an inrush current of 110A RMS in grid-connected mode, lasting approximately 12 cycles (200ms). Both fan and compressor start simultaneously. An unsuccessful attempt with an 8A to 16A soft starter (SS1B08-16SN or SSTART-08-16A-1P-230V) failed to reduce inrush current, as the AC unit's FLA was 15.5A and the soft starter's recommended LRA design limit was 90A, lower than the nameplate 105.5A AC-RMS LRA. A successful installation with a 16A to 32A soft starter (SS1B16-32SN or SSTART-16-32A-1P-230V) reduced the inrush current. With this soft starter, compressor startup occurred approximately 500ms after fan startup, with an inrush current of 37.99A RMS. Another scope capture shows the four-ton AC startup with the soft starter installed, zoomed in to show a reading of 37.99A.

4.7 Soft Starter Wiring Diagrams

When wiring the soft starter, identifying fan motor (OFM) and compressor wires was key. Wires were traced from the AC unit's wiring compartment. Compressor power conductors were larger than fan conductors. The diagram shows connections for the compressor (COMP), fan motor (OFM), capacitor (CAP), and SureStart soft starter, with various windings and terminals labeled (RUN WINDING (R), RUN CAPACITOR (RC), ACTIVE (T2), START WINDING (S), COMPRESSOR COMMON (C)). The wiring diagram legend defines: CONT - Start contactor included with AC; COMP - AC compressor; OFM - Outdoor fan motor; CAP - Start capacitor included with AC; *CH - Crankcase heater (did not exist in this AC); *CHS - Crankcase heater switch (did not exist in this AC). Actual wiring observed: Larger yellow conductor from run (R) terminal of compressor to run winding (R) terminal of SureStart; Larger black conductor from common (C) terminal of compressor to contactor terminal 21; Blue conductor from start (S) terminal of compressor to herm (H) terminal of capacitor; Smaller yellow conductor from fan to common (C) terminal of capacitor; Smaller black conductor from fan to contactor terminal 21; Brown conductor from fan to fan (F) terminal of capacitor. An image shows the air conditioner wiring compartment with the soft starter installed, highlighting the internal wiring.

4.8 Case Study Conclusion

The Hyper Engineering SureStart SS1B16-32SN soft starter reduced the AC's inrush current from 105.5A to 37.99A, a 64% reduction. This allowed the AC to start in off-grid mode with five Encharge 3 units, which are rated to provide 40A of current for up to ten seconds.

5. Summary and Sizing Recommendations

Enphase recommends a minimum number of Encharge base units to support HVAC systems with a soft starter and a 10A (2.4kW) base load. The actual number of Encharge units may vary based on the typical base load before startup and the soft starter's effectiveness for specific HVAC make, model, and age. If multiple HVAC systems are backed up, a safety margin should be added for additional base load when sizing the energy storage system based on inrush.