The Things Solar Installers Notice

Why Two ACs Don’t Mean Double Surge (Most Installers Get This Wrong)

ByGLOBALNET ICT

Here’s a scenario I see all the time in the field:

  • Client has two AC units
  • Each AC is 1.5HP
  • Installer thinks: “Surge = 1.5HP x 2 = 3HP”

Next thing you know:

  • Inverter is oversized
  • System cost balloons
  • Battery and wiring are stressed unnecessarily

And the kicker?
Even with a “correct” surge calculation, the inverter may still trip — because installers misunderstand how surge actually works.

Let me explain what I’ve learned from real installations.

Step 1: Surge Happens Instantaneously, Not Cumulatively

When ACs start, the surge is very short-lived — usually 1–3 seconds.

  • If both ACs start at the exact same instant, yes, combined surge is higher.
  • But in reality, most ACs start a fraction of a second apart, even on the same panel.

This means:

  • The inverter only sees the largest single surge at any moment
  • Not the sum of both surges continuously

This is the #1 mistake I see: additive surge assumption.

Step 2: Inverter Protection Only Cares About the Highest Instantaneous Load

Inverters have internal protection:

  • They detect peak instantaneous current
  • Only trigger if current exceeds rating for more than a few milliseconds

This means:

  • Two ACs starting together rarely double the stress
  • The inverter handles the highest surge first
  • After the first AC stabilizes, the second surge is negligible

Step 3: Battery C-Rate Must Still Support the Highest Surge

Here’s where installers get caught again:

  • If your battery can handle 1.5HP AC surge, it will also handle two ACs, provided they don’t start exactly simultaneously
  • Oversizing for both ACs together wastes money and reduces system efficiency

Lesson: Size for highest single surge, not sum of identical surges.

Step 4: Thermal vs Instantaneous Stress

AC surge is instantaneous, not thermal:

  • A surge lasts seconds → small heating effect
  • Continuous high load causes thermal stress → long-term damage

Installers often oversize inverters thinking:

“Two ACs double the heat”

Reality: Thermal stress is negligible during surge if inverter is rated properly.

Step 5: Practical Example From the Field

  • Two 1.5HP ACs
  • 6kVA inverter rated for 1.5HP surge
  • 10kWh battery rated for 1C discharge

Installer oversizes thinking:

  • 1.5HP x 2 = 3HP surge
  • Upsizes inverter to 10kVA
  • Adds extra battery

Field result:

  • System cost increased 50–60%
  • DC cables and fuses oversized unnecessarily
  • System still works the same, but efficiency drops

Step 6: Installer Mistakes to Avoid

  1. Adding surges linearly
    • Only consider largest single surge at any instant
  2. Ignoring fractional start times
    • ACs almost never start exactly together
  3. Oversizing inverters unnecessarily
    • Leads to higher costs, battery mismatch, and energy waste
  4. Neglecting battery C-rate
    • Even the largest single surge must be supported
  5. Thinking “more is safer”
    • Oversized inverters are not more reliable — they stress battery & DC wiring

Step 7: The Real Installer Fix

Here’s how I now handle multiple identical appliances:

  • Identify highest single surge
  • Check battery C-rate and DC voltage stability
  • Stagger startup when possible
  • Avoid linear addition of identical appliances
  • Use tools like Globisun Solar App to calculate real surge scenarios

Result:

  • Correct inverter sizing
  • Reduced cost
  • Safe battery operation
  • Clients happy

Step 8: Installer Truth

Two ACs do NOT automatically double surge.
Most installers overestimate surge and oversize systems unnecessarily.
Proper understanding of surge timing and battery C-rate avoids trips, overspending, and early component wear.

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