Solar Panel Calculator
Calculate how many solar panels you need from monthly kWh usage, peak sun hours, and panel wattage.
Loading tool...
What is Solar Panel Calculator?
The Solar Panel Calculator is a free tool that estimates the size of a rooftop photovoltaic system you need. Enter your monthly electricity usage, your location's peak sun hours, and the wattage of the panels you are considering, and it computes the required array size in kWp, the number of panels, expected energy production, the roof area required, a suggested string configuration for your inverter, and simple bill savings.
Key Benefits
- Know your ideal system size before installer visits
- Compare quotes with realistic production numbers
- Check if your roof physically fits the array
- Understand string configuration for inverter matching
- Estimate payback from monthly savings
Common Use Cases
- •Homeowners planning a rooftop solar investment
- •Comparing 400W vs 550W panel quotes
- •Sizing a partial-offset system for rental properties
- •Pre-checking roof feasibility before a site survey
- •Estimating production for a holiday home or small office
How to Use the Solar Panel Calculator
- Enter your energy usage: Type your average monthly electricity usage in kWh from a recent bill, and your local peak sun hours per day (4-6 is typical for sunny regions).
- Pick your panel: Enter the wattage of the panel you are considering (300-600W is common today) plus its Voc and dimensions if you want string and roof checks.
- Set losses and offset target: Keep the default system losses or adjust for shading, then choose what percentage of your bill you want the system to offset (100% for full offset).
- Review the results: Read the required array size in kWp, panel count, expected production, roof area needed, string configuration, and estimated savings. Copy the summary for your installer.
Key Features
- Array sizing from monthly kWh, peak sun hours, and performance ratio
- Panel count and total kWp for any panel wattage
- Daily, monthly, and annual production estimates
- Roof area check with 20% spacing allowance
- String configuration from inverter max DC voltage and panel Voc
- Monthly and annual bill savings estimates
Frequently Asked Questions
How many solar panels do I need for my home?
It depends on your usage and sunlight. As a rule of thumb, divide your monthly kWh by 30 to get daily usage, divide by peak sun hours, then divide by panel wattage. For example, 450 kWh per month with 5 sun hours and 550W panels needs about 6-7 panels. This calculator does the math for you including system losses.
What are peak sun hours?
A peak sun hour is one hour of sunlight at 1,000 W/m² intensity — the standard test condition for panels. Most locations get 3-6 peak sun hours per day on average. Thailand averages around 4.5-5.5, while Germany gets about 3 and Arizona about 6.5.
Why is the calculated system larger than my daily usage suggests?
The calculator applies a performance ratio (typically 65-80%) that accounts for inverter losses, wiring, temperature derating, soiling, and shading. Panels rarely produce their nameplate rating in real conditions, so oversizing the array compensates for these losses.
What is string sizing and why does it matter?
Panels wired in series form a string, and the total voltage adds up. Your inverter has a maximum DC input voltage (often 600V or 1000V) that must not be exceeded, especially in cold weather when Voc rises. The string configuration shown tells you how many panels per string and how many parallel strings your array needs.
Does this work for off-grid systems?
The sizing logic is the same, but off-grid systems also need battery storage sized for night-time usage and cloudy days, plus a charge controller. Use this calculator for the PV array portion, then size batteries separately based on your nightly kWh consumption.