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    OGM
    Power Systems

    Inverter Sizing for Off-Grid Homes: The Complete Guide

    Undersize your inverter and you'll trip breakers on startup. Oversize it and you waste money and efficiency. Here's how to get it right.

    30-45 Minutes
    Critical Calculation
    Pure sine wave inverter installed in an off-grid power system
    A properly sized inverter is the heart of any off-grid electrical system.

    The #1 Mistake Off-Gridders Make

    Choosing an inverter based on total battery capacity. Your inverter size should be based on your loads, not your batteries.

    Rule of thumb: Sum the continuous wattage of everything that runs simultaneously. Add surge demands. Size up 20-25% for headroom. That's your minimum inverter rating.

    Inverter Types at a Glance

    Pure Sine Wave — required for sensitive electronics
    Modified Sine Wave — cheap but causes appliance noise
    Grid-tie only — NOT suitable for off-grid use
    Hybrid inverters — can work with or without grid

    Recommended Inverters

    Pure Sine Wave Inverter

    3000W Pure Sine Wave

    ideal for medium off-grid homes with standard appliances.

    Shop Now
    5000W Inverter

    5000W Large System

    For whole-home off-grid with well pumps and HVAC.

    Shop Now
    01

    Sizing Fundamentals

    10 Mins

    Goal: Understand the difference between continuous wattage, peak/surge wattage, and your actual power demand.

    • Continuous wattage — what the inverter can run 24/7. This is the rated wattage (e.g., 3000W inverter = 3000W continuous).
    • Peak/surge wattage — extra capacity for motor startup. Typically 2-3x continuous rating for a few seconds.
    • Running watts — actual watts used while an appliance is running (lower than surge).
    • Overload tolerance — most inverters can handle 150% load for a few minutes. Don't rely on this.

    Quick Formula

    Inverter Size ≥ (Highest single surge load) × 1.5

    OR

    Inverter Size ≥ (Sum of all continuous loads) × 1.25

    Use the larger of these two calculations.

    02

    Load Calculation Worksheet

    15 Mins

    Goal: List every load, identify which run simultaneously, and calculate your true peak demand.

    List common off-grid loads and their typical wattages:

    ApplianceRunning WattsSurge Watts
    Refrigerator (energy star)150-200W500-800W
    Well pump (1HP)750-1000W2000-3000W
    Microwave800-1200W800-1200W
    Chest freezer100-200W400-600W
    LED lighting (whole house)50-150W50-150W
    Laptop/electronics20-100W20-100W
    Instant hot water3000W+3000W+

    ⚠️ Avoid running these simultaneously unless you have a 5000W+ inverter: Instant hot water + stove + air conditioning + well pump.

    03

    Surge Capacity: The Critical Factor

    5 Mins

    Goal: Ensure your inverter can handle the startup surge of your largest motor without tripping.

    • Fridge/freezer compressors — need 3-5x running watts for 1-3 seconds
    • Well pumps — the biggest surge load in most off-grid homes; can need 3000W+ surge from a 1000W running load
    • Air conditioning — 3-5x surge, compressor plus condenser fan
    • Pool pumps — similar surge profile to well pumps

    Example

    Well pump: 1000W running → 3000W surge needed. If this is your highest surge load, you need an inverter that can handle at least 3000W surge. A 2000W continuous inverter with 4000W surge capacity would work.

    04

    Voltage Matching: 12V, 24V, or 48V?

    Goal: Match your inverter input voltage to your battery bank voltage.

    12V

    Small systems only. Max ~2000W recommended. High current = more cable loss.

    24V

    Mid-size systems. Good for 2000-5000W. Balance of efficiency and simplicity.

    48V

    Best for 5000W+ systems. Half the current of 24V. Much more efficient.

    • 48V is the standard for serious off-grid homes — lower current means thinner cables, less heat, better efficiency.
    • Match your battery bank voltage — a 48V inverter MUST be paired with a 48V battery bank.
    • Hybrid systems: Some inverters auto-detect voltage, but always check specs.
    05

    Installation Best Practices

    Goal: Install your inverter safely with proper wiring, grounding, and ventilation.

    • Mount near battery bank — keep DC cable runs short. Every metre of 12V cable at high current = heat and voltage drop.
    • Proper cable sizing — use a voltage drop calculator. For 3000W at 24V (125A), you need at least 00 AWG / 70mm² cable.
    • Dedicated fuse/breaker — on the positive DC input, as close to the battery as possible.
    • Grounding — frame ground the inverter chassis. AC ground to your system ground rod.
    • Ventilation — inverters are 85-95% efficient. The 5-15% loss is heat. Never enclose an inverter in a sealed cupboard.
    • No reverse polarity — connecting battery backwards will destroy the inverter instantly and void your warranty.

    Conclusion

    Inverter sizing is the most consequential decision in your off-grid power system. Get it right and your system runs reliably for years. Undersize it and you'll be replacing inverters every few years. Use the load calculation worksheet, account for surge demands, and always size up 20-25%. For most Australian off-grid homes, a 5000W pure sine wave 48V inverter is the sweet spot.