To correctly size a solar pump inverter for an existing AC pump, you must match the inverter’s Rated Output Current to the motor’s Full Load Amps (FLA), typically applying a 1.15x safety margin. Sizing by kW alone is a common field error that leads to premature IGBT failure. Additionally, the PV array configuration must ensure the Vmp stays within the inverter’s MPPT window while the Voc never exceeds the maximum DC input voltage at the site’s lowest temperature.
Step 1: The “Current Over Power” Rule
In the field, we often see installers matching a 7.5kW inverter to a 7.5kW pump and wondering why it trips on overcurrent.
La vérité: Power ratings are nominal. The real work is done by current. If your pump’s FLA (Full Load Amps) is 15A, your inverter must be able to deliver that current continuously under solar fluctuations.
- The Formula: Inverter Output Current (A) ≥ Motor Rated Current (A) × 1.15
- Why? Solar inputs are variable. High-inertia starts or slight mechanical wear on the pump can cause current spikes. If your inverter doesn’t have the “headroom,” it will trigger protection or burn out.
Step 2: PV Array Voltage Windows (Voc vs. Vmp)
A solar pump inverter is not a battery charger; it’s a high-speed MPPT driver. Efficiency depends entirely on voltage matching.
- Vmp (Voltage at Maximum Power): Your string’s combined Vmp must be at the center of the inverter’s MPPT range (e.g., 530V-580V for a 380V AC system) to ensure 100% pump frequency even during passing clouds.
- Voc (Open Circuit Voltage): Never design your string to hit more than 90% of the inverter’s max DC input. If the inverter max is 800V, keep your Voc below 720V. Cold mornings in regions like the Middle East or Andes can spike Voc by 10-15%, instantly frying the input capacitors.
Step 3: The Site-Specific “Correction Factors”
Standard data sheets assume 25°C and sea level. Real-world engineering doesn’t. HOBER inverters are built for extreme environments, but physics still applies.
- High Altitude (>3000m): Thin air means poor heat dissipation. In cities like La Paz or high-altitude Peru, you must over-size the inverter by one power rating (e.g., use an 11kW inverter for a 7.5kW pump) to compensate for derating.
- Extreme Heat (50°C+): If your site hits 50°C (Pakistan, Sudan), fan-cooled units will fail. You need a Architecture à triple refroidissement (Sealed IP65 + External Heatsinks + Forced Air) to maintain 50Hz output.
Quick Sizing Reference Table (B2B Standard)
| Puissance de la pompe | Motor Phase | Onduleur recommandé | Min. PV Power |
|---|---|---|---|
| 2.2kW / 3HP | 3-Phase 220V | 2.2kW – 3.7kW | 3.3kWp |
| 7.5kW / 10HP | triphasé 380 V | 7.5kW – 11kW | 11.2kWp |
| 15kW / 20HP | triphasé 380 V | 15kW – 18.5kW | 22.5kWp |
Step 4: The Long Cable Hazard
If your borehole is deeper than 100m, the voltage drop and reflected wave phenomenon will destroy your motor’s insulation.
- Engineering Rule:
- 100m – 200m: Install an Output Reactor.
- >200m: Install a Sine Wave Filter.
HOBER IP65 inverters are designed to handle these inductive loads, but protection is non-negotiable for system longevity.
Looking for a Professional System Selection?
The logic above is for preliminary sizing. For complete Bill of Materials (BOM), string configurations, and dealer-exclusive pricing for your specific project, visit our Academy.
Visit AC Pump Solarization Academy
FAQ for Solar Pump Inverter Sizing
Q: Can I use a single-phase inverter for a three-phase pump?
A: No. The output phase and voltage of the inverter must match the motor nameplate. Using HOBER hybrid inverters allows you to convert single-phase PV input into three-phase output for standard pumps.
Q: How do I handle 500HP+ loads?
A: For large-scale irrigation (citrus orchards, etc.), we use industrial control cabinets with 1130A+ input capacity and multi-drive synchronization. Contact our engineering team for high-power technical packs.
