How to Use the Inrush Current & Surge Sizer

1. Specify Inverter Capacity — input the continuous rating (Watts) and peak surge rating (Watts) of your inverter.
2. Add Inductive Motor Loads — enter a label for each load, specify its size in HP (Horsepower) or Watts, and choose the starting method.
3. Select Starting Method — choose Direct-On-Line (6x multiplier) for standard motors, Soft-Start/VFD (3x multiplier) for managed motors, or Resistive (1x multiplier) for heater/lighting loads.
4. Check Sizing Status — see if the system passes or fails. The model automatically evaluates both continuous running margins and peak starting surge limits.
5. Download Sizing Report — click "Download PDF Report" to save or print a branded engineering compliance report for your solar installation.

Why Inrush Current Math Matters for Solar Inverters

Designing an off-grid or hybrid solar storage system requires more than just summing up running wattages. Inductive loads—such as well pumps, refrigerator compressors, air conditioners, and power tools—contain electric motors that require a massive burst of energy to start from a standstill. This initial spike is called inrush current or surge demand, and failing to design for it is the most common cause of solar inverter shutdown trips.

When a motor starts Direct-On-Line (DOL) without any mitigation, the rotor acts as a short circuit for a fraction of a second. This locked rotor state draws 6 to 8 times the motor's running current (NEMA design codes detail these multipliers). While utility grids can easily buffer this spike, a standalone solar inverter must supply this current instantly. If the surge exceeds the inverter's peak millisecond rating, its internal overcurrent protection will trip immediately to protect its transistors.

To mitigate surge issues, solar engineers can oversize the inverter, add VFDs (Variable Frequency Drives), or install soft-starters. Soft-starters ramp up voltage gradually, lowering the startup surge to 2.5x or 3x running watts. This calculator models these startup methods side-by-side, helping designers size their inverter capacity accurately and determine if a soft-starter is required to avoid system overloads.

Worked Example: Well Pump Sizing on a Hybrid Inverter

A solar contractor is designing an off-grid solar system for a cabin. The cabin's primary load is a 2 HP submersible well pump. The contractor is sizing a 5,000 Watt hybrid inverter with a peak surge capability of 10,000 Watts (typical 2x continuous surge rating for 100ms).

Step 1: Calculate Running Watts. 1 HP is equal to 746 Watts of electrical power. Therefore, the 2 HP motor draws: 2 HP × 746 W/HP = 1,492 Watts running. This is well within the inverter's 5,000 W continuous limit (70.2% running margin).

Step 2: Calculate Starting Surge (DOL). If the pump starts Direct-On-Line (DOL), it draws a 6x starting surge: 1,492 W × 6 = 8,952 Watts. Although this is under the 10,000 W peak limit, it leaves a razor-thin 10.5% surge margin. If a refrigerator is running at the same time, the system will overload and trip.

Step 3: Analyze Soft-Starter. By installing a soft-starter on the well pump, the starting multiplier drops to 3x: 1,492 W × 3 = 4,476 Watts. The starting surge is cut in half, leaving a safe 55.2% surge margin on the 10 kW peak inverter. The sizing guidelines recommend a minimum continuous inverter capacity of 1,865W and a surge capacity of 10,742W for DOL starting, or a soft-start upgrade to guarantee stability.

Frequently Asked Questions