How to Size a Solar Water Pump Inverter for Irrigation Projects

This is Andy. I've spent the last eight years in the solar pumping trenches, and one sizing mistake keeps showing up in irrigation projects: people match the inverter only by motor kW, then ignore the DC voltage window and field conditions.

That shortcut costs water. The pump may start late, trip at noon, or run below the flow rate the farmer expected. For EPC contractors and distributors, it also creates callbacks, warranty arguments, and lost trust.

For most irrigation projects, size the solar water pump inverter at the same kW level as the pump motor or one practical step above when the load is heavy, then check the PV string voltage, pump phase, MPPT range, and site conditions. Inverter kW is only the first filter. The real sizing happens in the voltage and operating window.

Quick answer: how to size a solar water pump inverter

Use this field rule:

Inverter output kW >= pump motor rated kW

Then check the solar side:

PV array power = pump power x 1.3 to 1.5

PV string voltage must sit inside the inverter MPPT input range

If the inverter kW is right but the PV voltage is wrong, the system still fails. Too low, and the inverter stays asleep. Too high, and you start risking over-voltage alarms or damaged components.

Step 1: start from the pump motor kW

The pump nameplate gives the first sizing number. Do not skip it. You need the motor rated power, rated voltage, phase, rated current, and frequency.

Pump motor	Typical inverter choice	Field note
2.2kW pump	2.2kW solar pump inverter	Common for small farms and shallow irrigation
4kW pump	4kW solar pump inverter	Check startup current and water head carefully
7.5kW pump	7.5kW solar pump inverter	Often needs stronger PV margin for stable flow
11kW pump	11kW solar pump inverter	Three-phase systems become more common here
18.5kW pump	18.5kW solar pump inverter	Thermal capacity and cabinet design matter

Do not oversize the inverter blindly. A much larger inverter will not fix a weak PV array, wrong string voltage, undersized cable, or poor pump selection. It only adds cost.

Step 2: match the pump voltage and phase

Motor type changes the inverter selection. A 220V single-phase pump and a 380V three-phase pump do not use the same output design.

Pump type	Typical use	What to watch
220V single-phase AC pump	Small farms, wells, remote irrigation points	Needs stable frequency and enough DC voltage to start early
220V three-phase AC pump	Medium irrigation or retrofit systems	Check phase output and motor current, not only voltage
380V three-phase AC pump	Large irrigation, deep wells, higher flow projects	Needs higher PV string voltage and stronger thermal design

For 220V pump projects, this 220V solar pump inverter page gives the product path. For 380V well and irrigation projects, see the 380V solar well pump systems guide.

Step 3: check the DC input voltage window

This is where many irrigation projects go wrong.

The inverter must receive enough DC voltage from the solar array before it can run the pump. Panel wattage matters, but voltage wakes the inverter up.

System type	Typical target Vmp	Typical target Voc	Common field result if wrong
220V single-phase pump inverter	Around 310V DC	Around 380V DC	Too low: late startup or sleep mode
380V three-phase pump inverter	Around 540V DC	Around 650V DC	Too high: over-voltage alarm risk

With many 540W to 600W panels, Vmp sits around 40V to 42V per panel. A 220V system often uses about 8 panels in series. A 380V system often uses 13 to 15 panels in series.

Do not copy these numbers without checking the actual panel datasheet. Cold weather raises Voc. High heat lowers Vmp. Long cable runs add voltage drop. Those details decide whether the system starts at 8 AM or waits until the sun is already high.

Step 4: size the PV array around daily water output

The inverter cannot create power that the panels do not provide. For irrigation, the target is not only peak operation at noon. The target is usable water over the day.

Use this rule:

Total PV power = pump power x 1.3 to 1.5

Pump power	Minimum PV array	Stronger irrigation design
2.2kW	2.9kW to 3.3kW	3.6kW+
4kW	5.2kW to 6kW	6.6kW to 7.2kW
7.5kW	9.8kW to 11.3kW	12kW+
11kW	14.3kW to 16.5kW	15.6kW+
18.5kW	24kW to 27.8kW	25.2kW+

For example, an 11kW irrigation pump usually needs around 15kW or more of PV power. With 600W panels, two strings of 13 panels give:

2 x 13 x 600W = 15.6kW

That layout can hit the voltage target and the power target at the same time, as long as the inverter input range accepts the string voltage.

Step 5: check load conditions before final selection

Pump kW alone does not tell the full story. A 7.5kW pump in a shallow canal project behaves differently from a 7.5kW submersible pump lifting water from a deep well.

Field condition	Why it changes inverter sizing
High head or deep well	Raises load pressure and startup demand
Long pipe distance	Adds friction loss and may require longer operating hours
Drip or sprinkler irrigation	Needs stable pressure and frequency control
High ambient temperature	Reduces inverter cooling margin inside cabinets
Dusty farm sites	Requires better enclosure, wiring, and maintenance planning
Weak morning sunlight	Makes low-voltage startup and MPPT behavior more important

If the project uses a 380V three-phase pump, compare the requirement with the SP4 solar pump inverter range before confirming the final model.

Undersized vs oversized inverter: what happens in the field

Sizing mistake	What the buyer sees	What it costs the EPC
Inverter too small	Overload trips, unstable flow, failed startup	Commissioning delays and service calls
PV voltage too low	Pump starts late and stops early	Lower daily water output and client complaints
PV voltage too high	Over-voltage alarm or component stress	Risk of damaged inverter and warranty disputes
Inverter too large	No clear increase in water output	Higher upfront cost without real gain
Poor cooling or cabinet design	Trips during hot hours	Lost irrigation time at the worst part of the day

The best system is not the largest inverter. It is the inverter that matches the pump, PV string, site heat, and irrigation schedule.

Hidden cost: panels, wiring, and crew time

Inverter sizing also affects the rest of the system. If the PV array design is weak, the buyer may try to fix the project later by adding panels, changing strings, or rewiring the combiner box. That costs more than doing the design once.

Panel choice matters too. For larger irrigation systems, 600W+ modules often reduce the total number of panels, rails, clamps, and wiring points. Older 330W panels may look cheaper, but they can double the installation work on an 18.5kW project.

That means more crew time under the sun, more connectors to inspect, and more places for faults to appear during the next dry season.

FAQ: solar water pump inverter sizing

Should the solar pump inverter be the same kW as the pump?

In many irrigation projects, yes. The inverter is usually selected at the same kW level as the pump motor, then the PV array and DC input voltage are designed with enough margin. Heavy-load or high-head projects may need one practical step of margin after checking motor current and site conditions.

Can I fix a weak irrigation system by choosing a larger inverter?

Not usually. A larger inverter will not fix low PV voltage, weak PV power, wrong panel strings, poor cable sizing, or a mismatched pump. Check the solar array and voltage window first.

What DC voltage should I target for a 220V solar pump inverter?

Many 220V single-phase solar pump inverter systems target around 310V DC Vmp and around 380V DC Voc. Always check the inverter datasheet and panel datasheet before final string design.

What DC voltage should I target for a 380V three-phase pump?

Many 380V three-phase solar pump inverter systems target around 540V DC Vmp and around 650V DC Voc. With panels around 41V Vmp, that often means 13 to 15 panels in series.

What information do I need before sizing the inverter?

Send the pump nameplate, rated voltage, phase, rated current, pump power, head and flow requirement, panel datasheet, cable distance, and site temperature range. Those details decide whether the system will run well in the field.

Product Path After Sizing

Once the pump kW, phase, current, PV string, head, and flow are confirmed, connect the sizing result to the product family. Many small 220V pump systems should be checked against the SP1 inverter path. Many 380V three-phase irrigation systems should be checked against the SP4 inverter path. For a complete package, compare the result with the solar pump system design guide.

Final field check before ordering

Before you order the inverter, check these five items:

• Pump kW, voltage, phase, and rated current
• PV array power at 1.3 to 1.5 times pump power
• PV string Vmp and Voc against the inverter input range
• Site heat, dust, cable distance, and irrigation schedule
• Protection settings for dry-run, overload, and over-voltage

This related guide on essential protection features in solar water pump inverters explains the protection side after sizing.