Before planning a solar system, you need a basic idea of how much energy the home uses and how many panels may be needed.
A simple estimate starts with the home’s monthly energy use, average daily load, available sunlight, system loss, and panel wattage.
Quick Solar Panel Example
Example: a home uses 900 kWh per month, receives about 5 peak sun hours per day, uses 400-watt solar panels, and uses a 20% system-loss estimate.
Step 1: Find the Average Daily Load
Start with the monthly energy use.
900 kWh ÷ 30 days = 30 kWh per day
The home uses an average of 30 kWh per day.
Step 2: Add System Loss
Solar systems do not produce 100% of their rated output after real-world losses.
For this example, use a 20% system-loss estimate.
100% − 20% = 80% system output
To produce 30 kWh per day after losses, the array needs to produce:
30 kWh ÷ 0.80 = 37.5 kWh per day
Step 3: Estimate the System Size
Use the adjusted daily energy needed and the average peak sun hours.
37.5 kWh per day ÷ 5 sun hours = 7.5 kW
The estimated solar system size is 7.5 kW.
Step 4: Estimate the Number of Panels
Each panel is rated at 400 watts.
7,500 watts ÷ 400 watts per panel = 18.75 panels
Round up to the next whole panel.
Estimated number of panels = 19
Final Results for This Example
- Monthly energy use: 900 kWh
- Average daily load: 30 kWh per day
- System-loss estimate: 20%
- Daily solar production needed before losses: 37.5 kWh per day
- Average peak sun hours: 5 hours per day
- Estimated system size: 7.5 kW
- Panel wattage: 400 watts
- Estimated number of panels: 19
How Sparky Toolbox Helps
Instead of calculating average daily use, adding system loss, estimating system size, and calculating panel count by hand, enter the home’s energy use, sunlight information, system-loss percentage, and panel wattage into Sparky Toolbox.
The Solar Panel Calculator shows:
- Average daily load
- Estimated solar system size
- Recommended number of panels
That gives you a quick starting point for an early customer conversation or a basic solar-planning estimate.
Important Reminder
This is one simple estimate. Actual solar system design can change based on location, roof direction, shading, panel performance, system losses, utility requirements, available roof space, battery storage, equipment selection, and local permitting requirements.
Always confirm project details, equipment information, approved plans, manufacturer instructions, and local requirements before installation.