The Brains of the Solar System
Solar panels and battery banks generate and store power as Direct Current (DC). However, almost every appliance in your house—from your refrigerator to your television—runs on Alternating Current (AC).
You cannot simply plug a television into a solar panel. You must install a Solar Inverter.
The inverter acts as the translator between the two systems. It rapidly switches the DC power back and forth to simulate the 60 Hz AC sine wave required by standard household wall outlets. If the inverter is undersized, it will become violently hot, automatically shut down (tripping an internal breaker), and plunge your entire house into darkness.
The Two Types of Wattage
To properly size an inverter, you must calculate the total "Load" (the amount of power) that the inverter will be forced to supply at any given exact second. Appliances have two different power ratings you must account for:
1. Running Watts (Continuous Load)
This is the amount of power an appliance requires to run normally. A standard LED light bulb requires 10 Watts. A television might require 150 Watts. A microwave requires 1,000 Watts.
2. Surge Watts (Starting Load)
This is the hidden danger in off-grid solar. Any appliance with an electric motor or a compressor (like a refrigerator, well pump, or air conditioner) requires a massive, instantaneous surge of electricity just to start the motor spinning.
A refrigerator might only require 400 Watts to run, but the exact second the compressor kicks on, it requires 1,200 Surge Watts for a fraction of a second. If your inverter cannot handle that momentary 1,200-Watt spike, the entire system will crash.
How to Calculate Inverter Size
You do not size an inverter based on your daily energy usage (kWh). You size it based on the absolute maximum number of appliances you intend to run at the exact same time.
The Formula
- Walk through your house and list every single appliance you might feasibly turn on at the exact same moment (e.g., the fridge is running, the TV is on, 5 lights are on, and someone turns on the microwave).
- Add up the Running Watts of all those appliances.
- Identify the single appliance in that list with the largest motor (usually the fridge or a well pump), and add its Surge Watts to the total. This gives you the absolute Maximum Peak Load.
- Add a Safety Factor (usually 20% to 25%). Inverters operate much more efficiently and run significantly cooler when they are not constantly maxed out at 100% capacity.
Inverter Size = Total Maximum Peak Load × Safety Factor
Example Calculation
You are living in an off-grid cabin. At 7:00 PM, you expect the following to happen simultaneously:
- Refrigerator is running (400W continuous + 800W surge)
- Television is on (150W)
- Lights are on (50W)
- You turn on the Microwave to heat dinner (1,000W)
- Total Peak Load:
400 + 800 + 150 + 50 + 1000 = 2,400 Watts - Add 25% Safety Factor:
2,400 × 1.25 = 3,000 Watts
You should purchase an inverter rated for 3,000 Watts continuous power. (Do not rely on the inverter's marketed "Peak/Surge" rating for your continuous calculations, as cheap inverters can only sustain peak ratings for milliseconds before burning out).