The efficiency depends on various factors such as geographic location you are living in, shading, weather conditions, and panel placement angle. If all these factors perfectly fit into your region your 400-watt solar panel system will give you a good amount of energy to run your home appliances depending on their voltage needs.
Determine the amount of electricity you consume daily (in kilowatt-hours, kWh).
PSH is the number of sunlight hours per day that reaches about 1,000 watts of power per square meter. Consult a PSH map to find the average PSH for your area. Or you can just simply check it on the internet.
Formula for Number of Solar Panels needed= Daily Energy Consumption / Panel Wattage × PSH
This above formula will give you the number of solar panels to fulfil your daily energy usage. For example, if you live in California with a daily energy consumption of 12 kWh and 5.5 PSH, you would need to calculate the number of 400W panels as follows:
12000 W / 400 W × 5.5 = 6
This means you need approximately 6 panels to meet your energy needs in California.