Top solar companies provide online calculators that make it easy to estimate how much solar power your home might need, what it could cost, and how much you could save.
While these tools can be a good starting point, many are designed with marketing in mind rather than engineering accuracy. If you don’t know how to interpret the data, you risk being swayed by inflated projections that don’t match real-world performance.
This guide will show you how to use solar calculators, step by step, so you can make informed decisions based on facts and achieve energy independence. And when you want results you can trust, MAG Solar team delivers precise, engineer-backed designs custom to your property.
Key Takeaways
- Pull monthly kWh usage from your hydro bill. Many calculators default to 100% offset, but most homes realistically offset only 30–70% based on roof size, budget, and net metering rules.
- Output depends heavily on local peak sun hours (PSH) and efficiency losses. Calculators that assume 5–6 PSH and 90% efficiency understate actual system size.
- Exact savings or payback claims are usually marketing. A reliable design needs engineering validation, accounting for roof area, panel wattage, shading, and battery needs if it is off-grid.
1. Start with Your Real Energy Data
Most calculators begin by asking: “How much electricity do you use?” Look at your hydro bill for monthly kWh usage. Example, if your bill shows 600 kWh/month, that equals:
600 kWh/month ÷ 30 days = 20 kWh/day
That 20 kWh/day is the target you want your solar system to partially or fully offset. Some calculators default to 100% offset. In reality, many homes only offset 30–70% depending on roof space, budget, and net metering policies.
Use calculators as a starting point, not the final design. Always cross-check with your actual numbers and only work with reliable solar energy companies like MAG Solar.
2. Input Average Peak Sun Hours (PSH)
The biggest driver of solar output is the amount of usable sunlight received per day, called peak sun hours (PSH).
- PSH = total solar irradiation (kWh/m²/day) ÷ 1,000 W/m².
- Vancouver, for example, averages about 3.5 PSH year-round.
Example: If you need 20 kWh/day, divide by PSH:
20 kWh/day ÷ 3.5 PSH = 5.7 kW array (before losses)
Watch out for hype: Many calculators assume 5–6 PSH for all regions, which inflates production estimates for cloudy climates.
3. Account for Solar Panel Installation Efficiency (Derate Factors)
No PV system runs at 100% efficiency. Real systems lose energy due to:
- Inverter losses (≈96%)
- Wiring losses (≈2%)
- Soiling/dust (≈5%)
- Shading & downtime (≈10% or more in poor sites)
Industry-standard derate factor: 0.72–0.80. Example, using 0.75 efficiency for our 5.7 kW array:
5.7 kW ÷ 0.75 = 7.6 kW required system size
Watch out: Some calculators quietly assume 90–95% efficiency. That makes the array look smaller and cheaper than it really needs to be.
4. Translate Array Size into Panel Count
Panels are sold by wattage. To get the count:
Number of Panels = System Size (W)
Panel Wattage
Example: For a 7.6 kW system using 400 W panels:
7,600 ÷ 400 = 19 panels
If roof space is limited, higher-efficiency panels are required. Be wary because some calculators understate roof area needs. Rule of thumb: 100 ft² per kW of panels.
5. Compare On-Grid vs Off-Grid Calculations
Most online calculators simplify to grid-tied systems, but some include off-grid options- Grid-Tied: Focus on kWh offset and utility bill reduction.
- Off-Grid: Requires sizing batteries for autonomy.
6. Validate Results with Manual Cross-Check
A reliable formula to double-check:
Array Size (kW) = Daily Load (kWh) .
PSH x System Efficiency
Example (Vancouver home, 20 kWh/day, 3.5 PSH, 0.75 efficiency):
20 ÷ (3.5 x 0.75) = 7.6 kW
If the calculator gives you 5 kW, it’s undersizing.
Residential Solar Installation Calculator: Worksheet for Homeowners
A step-by-step worksheet to use online solar calculators accurately and avoid marketing hype.
Example | Your Input | |
Step 1: Monthly Energy Use (from bill) | ||
Electricity Consumption (kWh/month) | 600 kWh | |
Average Daily Use (kWh/day) | 600 ÷ 30 =20 kWh/day | |
Step 2: Peak Sun Hours (PSH) | ||
Location PSH (hours/day) | 3.5 (Vancouver) | |
Daily Load ÷ PSH (kW) | 20 ÷ 3.5 =5.7 kW | |
Step 3: Apply System Efficiency | ||
Assumed Efficiency Factor | 0.75 (75%) | |
Adjusted System Size (kW) | 5.7 ÷ 0.75 =7.6 kW | |
Step 4: Number of Panels | ||
Panel Wattage (W) | 400 W | |
System Size ÷ Panel W | 7600 ÷ 400 =19 panels | |
Get Accurately Calculated Sustainable Energy Solutions with Solar Industry Experts
Solar calculators are valuable tools, but only if you know how to use them properly. A calculator that gives you an exact payback year or savings claim without showing its assumptions is leaning more toward marketing than engineering.
At MAG Solar, we specialize in turning these preliminary numbers into engineered designs tailored to your home or business. Whether you’re exploring power purchase agreements or looking to maximize available government incentives, we make sure your calculations reflect real-world performance and long-term value. Our experts handle residential, farm, and commercial solar installations.
