Fans + Fridge + AC Overnight: The Correct Way to Size Battery & Inverter

ByGLOBALNET ICT

One of the questions I get most often from installers and clients is:

“I have 3 fans, a fridge, and an AC. How big should my battery and inverter be to last overnight?”

It sounds simple — just add the wattages, right?
Wrong. If you size based on total energy alone, your inverter will trip, batteries will drain too fast, and your system will fail.

Let me walk you through the correct approach, based on real-world installations.

Step 1: Calculate Continuous Load vs Surge

Continuous load

  • Fans = 3 x 70W = 210W
  • Fridge = 150W running
  • AC = 1.5HP ≈ 1100W

Total continuous load = 1460W (~1.46kW)

Surge load

  • Fridge compressor startup = ~450–500W extra
  • AC compressor startup = 3–4x rated power ≈ 3300–4400W
  • Fans have minimal surge (~100W collectively)

Key lesson: You cannot size the inverter just by running load — you must consider the highest instantaneous surge.

Step 2: Choose the Correct Inverter Size

The inverter must:

  1. Handle continuous load comfortably
  2. Support largest single surge without tripping

Example:

  • Continuous load = 1.46kW
  • Largest surge = AC startup = 4.4kW

So you should select an inverter rated at least:

  • 5–6kVA (6kVA is common)

Important: Do not sum all surges linearly. Two appliances rarely start at exactly the same moment, and the inverter only reacts to instantaneous maximum.

Step 3: Battery Sizing — Energy + C-Rate

Battery sizing is not just about kWh, it’s about current delivery (C-rate).

Step 3a: Calculate energy required

  • Overnight duration = 10 hours
  • Fans + fridge + AC (average AC duty cycle ~50%)
  • Average load ≈ 1.46kW – 1.1kW (AC runs intermittently) → let’s estimate 1.2kW

Energy needed = 1.2kW × 10h = 12kWh

Step 3b: Apply depth of discharge (DoD)

  • For lithium battery, max DoD ≈ 80%
  • Effective battery size = 12kWh ÷ 0.8 ≈ 15kWh

Step 3c: Check C-rate

  • Largest surge = 4.4kW
  • Battery voltage = 48V
  • Required current = 4400W ÷ 48V ≈ 92A

Your battery bank must deliver ≥92A without exceeding its C-rate.

Many installers ignore this and pick a battery with enough kWh but too low a C-rate. Result: voltage sag, inverter trips.

Step 4: Wiring & DC Considerations

Even if inverter and battery are sized perfectly:

  • DC cables must handle surge current
  • Loose lugs or long runs increase voltage drop
  • Low voltage triggers inverter protection

Rule: Oversize DC cabling for surge capacity, not just continuous load.

Step 5: Account for Appliance Duty Cycles

  • Fans run continuously
  • Fridge cycles on/off → 50% duty
  • AC cycles depending on thermostat → 50–60% duty

You don’t need to size for full AC wattage 100% of the night.
Use average load for energy calculation, max instantaneous load for inverter sizing.

Step 6: Use Real Tools to Avoid Guesswork

This is where I stopped guessing:

  • The Globisun Solar App calculates:
    • Continuous and surge load
    • Battery kWh + C-rate
    • Inverter size
    • Wiring voltage drop

Using a tool ensures your overnight system works without trips or early battery degradation.

Step 7: Installer Truth

Sizing for overnight loads isn’t just about kWh. It’s about surge, C-rate, duty cycle, and wiring.

A 15kWh battery paired with a 6kVA inverter:

  • Works if surge is calculated
  • Voltage drop is minimized
  • Battery C-rate supports largest spike
  • Appliances run overnight reliably

Ignore any of these — and the system fails silently or loudly overnight.

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