The size of the inverter should indeed be based on the instantaneous power requirement (or peak power demand) rather than the total daily power consumption.

The inverter is responsible for providing power when the system is actively in use, which means it should be sized to handle the peak load (the highest wattage the system needs at any given moment) rather than the total energy required over a full day.

Let’s go over the correct approach for sizing the inverter based on the instantaneous power requirements:

Key Considerations for Inverter Sizing:

1. Peak Power Demand (Instantaneous Power): This is the maximum amount of power your household will need at any given moment. This includes all devices that might be running simultaneously, such as appliances, lights, etc.
2. Inverter Efficiency: The inverter will have an efficiency rating (usually between 85-95%). You should consider this factor while sizing the inverter.
3. Surge Capacity: Some appliances, especially motors (e.g., fridges, air conditioners), draw much more power when they start up. The inverter must handle this surge or start-up power.
4. Battery Voltage: This affects how much power the inverter needs to supply based on the DC voltage available from the battery.

Correct Approach to Inverter Sizing:

1. Calculate Peak Load: The inverter must be able to handle the total wattage of all appliances running at the same time. Add up the wattage of appliances that will be used simultaneously to get the maximum (peak) power demand.Example:
   • Fridge: 300W (running continuously)
   • TV: 150W
   • Laptop: 100W
   • Lighting: 200W
   • Other appliances: 50W
   • Total Peak Power Demand: 300W + 150W + 100W + 200W + 50W = 800W
2. Account for Surge Power: Inverters need to account for the starting surge of appliances like refrigerators or air conditioners, which could draw 2 to 3 times their normal running wattage when they turn on.
   • For example, a fridge may have a surge requirement of 1200W during start-up, even though its continuous running power is 300W.
   • The inverter needs to handle this surge, so the inverter size should be based on the peak running load plus any surge.
   Example: If your total peak load is 800W, but there’s a surge of 1200W for a fridge, you need to ensure the inverter can handle this. The inverter should be rated for at least 1200W (with surge capacity for higher startup loads).
3. Safety Factor: It’s good practice to add a safety margin (e.g., 20% to 25%) to ensure the inverter can handle unforeseen load spikes or temporary increases in power demand.So, the recommended inverter size will be:Inverter Size=(Total Peak Power Demand+Surge Power)×Safety Factor\text{Inverter Size} = (\text{Total Peak Power Demand} + \text{Surge Power}) \times \text{Safety Factor}Inverter Size=(Total Peak Power Demand+Surge Power)×Safety FactorIn our example:
   • Total Peak Power Demand = 800W
   • Surge Power = 1200W (for the fridge start-up)
   • Safety Factor = 1.25 (for safety)
   So, the inverter size should be:Inverter Size=(800+1200)×1.25=2000×1.25=2500W\text{Inverter Size} = (800 + 1200) \times 1.25 = 2000 \times 1.25 = 2500WInverter Size=(800+1200)×1.25=2000×1.25=2500WTherefore, in this case, you would need a 2500W inverter to handle both the peak power demand and the surge.

Steps for Correct Inverter Sizing:

1. Add the wattage of all appliances that will run simultaneously.
2. Account for surge requirements of appliances like refrigerators, pumps, and air conditioners.
3. Multiply the total wattage by a safety factor to ensure reliable operation.
4. Choose an inverter with a rating that covers the total continuous wattage demand plus surge capacity.

Summary:

• Instantaneous (peak) load determines the minimum size of the inverter.
• The inverter must be able to handle both the continuous power demand and any surge power from devices like motors.
• Safety margins (typically 25%) are added to ensure reliable operation during power fluctuations.

This ensures that the inverter will meet the demands of your system at any given moment, not just over the course of a day.