Choosing Solar Panels undergoes a few steps before understanding if you can be break-even (earning the money you put into them back) and making a profit on them.

1 – High level overview “in a nutshell” what Solar panels do and how it can benefit your home.
2 – Explanations about important information to understand Solar Panels
3 – How much can you save
4 – Future ambition – A custom calculator that can be used to calculate your businesscase

Solar Panels in a Nutshell:

When you are interested in Solar Panels, you need to understand the basic concepts in order to calculate the use case. This set of required information might be overwhelming at first, but the aim is to provide clear information what this means with sample data. They are packed in three categories:

1. Location: Where do you want to install the solar panels, what are the normal sun-hours, how is the shading and how intense is the sun.
2. Panels & usage: Choosing the right solar panels means you need to weight the features, initial (and installation) costs, the actual usage ‘when do you want to use it’ and connecting them to your home.
3. Run cost analysis: Put the location information, the panel information and run a cost analysis to determine if this investment is actually worth it.

With this information you should be able to determine if you can actually generate the solar energy that you expect. Suppliers often leave out essential information and portrait false pictures to the return on investments. So there is a lot of emphasis to calculate the real value rather then the sales pitch.

Solar Panel ROI Calculator – Calculate Savings & Payback | Cassini

Below you can try to calculate your own situation – No rights can be obtained from the outcome as it works pure to help you determine the use case –

 

 

 

 

What to look for when choosing Solar Panels:

While solar panels innovate rapidly and are progressing very well, this formula (created in 2021) still stands, since the basics are all the same.

Some remarks to be added to the list below.

Location: For the location there are specialized applications that show exactly how much sun a roof gets per year. This greatly increases the accuracy of the calculations – however if you do not have this information. There is information you can find online by going to your weather website.
The performance ratio corrects the total sun ratio. Think about the season (winter-sun is far weaker then summer-sun, dust on the panels, shadows caused by clouds etc). Clouded days can reduce the efficiency by 90% (so you keep 10% on that day instead of 100%).
The direction and angle differ per country, but for the Netherlands, Belgium and Germany South is 100% performance, Southeast/South West reduces the efficiency to 90%, East and West to 80% and North facing roofs reduce the efficiency to 50%
The angle of the roof reduces the efficiency too. 35° is 100%. anywhere between 35° to a flat roof reduces the efficiency (gradually) to 90%. The other direction from 35° to 75° reduces the efficiency to 90%, and vertical walls reduce the efficiency by about 20%.

Panels: The innovations go rapid and datasheets for the product offered are essential to include here. Without them you cannot properly estimate the real value of your solar panels.

Savings:

The next key question will be the generated savings. With the information above and the averages calculated it is straightforward to calculate the savings. Again, key information is required to make these calculations. This will continue in the same table – format.

% Energy directly consumed & governmental support: This is where the vast majority of suppliers fail to properly inform customers (unless they sell batteries, then all in a sudden they start to share this information). An average household will generate far too much during peak summer hours and not nearly enough in the dark winter months. This results in about 70% of the solar energy not being consumed directly. this is the key breaking point for non-government supported solar energy generation.

To illustrate the calculation below has 2 variables;
1. If the payback rate the same value as the purchase price.
2. If the payback rate is 10% of the purchase price.

Generated energy	Sample data	Calculations
Based on above calculations	2.880 kWh per year	2880
Purchase price for energy	€ 0.30 Euro per kWh	* 0,30
% of Energy directly consumed	30%	* 0,30
Annual energy saved by consumption		260,- Euro
Total Lifespan	20 Years (performance usually reduces significantly after	* 20
Option 1: Sell = purchase price	70% * € 0.30 Euro per kWh	(260 + 604) * 20 = € 17.280 Euro
Option 2: Sell = 10% purchase price	70% * € 0.30 * 10% Euro per kWh	(260 + 60) * 20 = € 6.400 Euro
Costs
Installation & Materials – This greatly varies per supplier	€ 6.500 Euro	– € 6.500 Euro
Option 1: Sell = purchase price	ROI over 20 years	€ 17.280 – 6.500 = € 10.780
Option 2: Sell = 10% purchase price	ROI over 20 Years	€ 6.400 – 6.500 = € – 100

In Conclusion

As often advertised, a solar panel use case greatly depends on factors unique to your home.
Innovations like increased capacity, better energy generation when shadows/clouds are present and ideal locations (facing south etc) determine if there is a potential.

But, even with a good potential use case like low installation costs, ideal locations etc. The market is not clear on what to do with the energy which is not directly consumed. This goes for daily demands, where in midday the energy generation peaks, but when the workday ends the energy demand rises. It also goes for summer and winter, where in the summer the energy demand is typically lower versus the winter. Appliances like airco’s heatpumps, electric cars etc greatly influence this.
In this example the directly consumed energy is ‘average’ with 30% directly consumed and 70% being sent back to the grid.
This is where the government and energy providers come in play. If they would nett your consumption (salderen in The Netherlands) you are likely benefiting from solar panels. The investment and return on investment is decent. If however the suppliers only pay back 10% of the price they charge for generating energy. (or have other penalties) the businesscase collapses completely.

Will home batteries be the answer? – Click here and see how you can add the home batteries to your calculation and see if they are interesting for you.

Frequently Asked Questions – FAQ

– Click on the question for the answer-

Question: Do these calculations always reflect the right situation?

Not entirely, unfortunately each situation is unique and frankly every year is unique too. In the last 5 years the amount of sun varied locally between 1750 and 2200 hours with different intensities. However, what is often done is taking very favorable numbers, leave out losses and project an ideal situation. This tool does incorporate that which is unique in the market. It can much more accurately work out scenario’s.

Question: How much shadow do I have?

Determining how much shade your trees will throw on your solar array is the single greatest hurdle to estimating system performance. Shading depends on several interrelated factors:
Click here for more information.
Tree Height & Canopy Density

• Height dictates the length of the shadow at any given time of day. A 5 m tree will cast a much shorter shadow than a 15 m tree at the same solar elevation.
• Canopy density (leaf thickness) controls how much light filters through. A sparse canopy may only reduce irradiance by 10-20 %, whereas a dense evergreen could block 80-100 % of direct sunlight beneath.

Sun Angle & Time of Day

• Morning vs. afternoon shading: Shadows in the early morning (when the sun is low in the east) may fall across panels oriented south or southeast; afternoon shadows (sun low in the west) may fall across panels facing southwest or west. Blocking the peak-production hours (roughly 10 AM-2 PM) has a disproportionate impact on total energy yield.

Seasonal Variation

• Summer: High solar angles mean shorter shadows at midday-but longer shadows in the early morning and late afternoon.
• Winter: Low solar angles all day long translate into much longer shadows, potentially covering panels for hours on end.

Tree-to-Panel Distance

• The farther a tree is from your array, the less shading it causes. A rule of thumb: shadows extend roughly the tree’s height multiplied by the cotangent of the sun’s elevation angle. In summer at noon, this factor is small; in winter at 9 AM or 3 PM, it can be 2. or more.

Practical Measurement Tips

Online Shade-Analysis Tools

• Sites like PVGIS, SolarPathfinder’s digital app, HelioScope (Commercial Solar Software) or Google’s Project Sunroof (US only) can simulate shading patterns across the year based on your geographic coordinates.

On-Site Observations

• On a clear day, mark shadow tips on the ground hourly between 9 AM and 4 PM.
• Record the length and direction of each shadow, noting date and time. Repeat in midsummer (around June 21) and midwinter (around December 21) for a full picture.

Quick “Ballpark” Walk-Through

• If detailed measurement is too time-consuming, pick a sunny day and walk the perimeter of your planned array at key times (sunrise + two hours, solar noon ± 1 hour, sunset – two hours). Estimate the percentage of your array footprint that falls in shadow at each point.

Rough “Ballpark” Shading Estimates
These figures are only for very early planning-actual shading losses can be higher or lower depending on your specific conditions:

• Shading Category Approx. Annual Energy Loss Typical Scenario
• Minimal (0-5 %) < 5 % loss Small deciduous trees > 10 m away; low density canopy.
• Light (5-15 %) 5-15 % loss Medium-sized trees (5-10 m) 5-8 m from panels.
• Moderate (15-30 %) 15-30 % loss Large trees (10-15 m) within 5 m; occasional midday shading.
• Heavy (> 30 %) > 30 % loss Tall, dense canopy trees immediately adjacent; winter shading all day.

Note: Even short periods of shading during peak-sun hours can cost you more energy than shading of equal duration in early morning or late afternoon.

For a truly accurate yield estimate, combine your shading measurements with a solar-performance model (e.g., PVSyst, Helioscope) that accounts for row-to-row masking and irradiance transposition.

Question: How does a solar array generate energy in the winter vs the summer?

While less complex then the shadow question and of equal relevance, the amount of sun per month varies greatly. But it depends a lot on your location. Here lies one of the key issues why “only” 30% of solar energy usage is actually a good number. Since many solar arrays are calculated on the average energy usage per month (or the total energy consumption over a full year), it means you will likely generate too much in the summer and not enough in the winter. Below is a typical example how much sun is cast per year.
Do know that the angle of the sun is not consistent and thus you will generate less energy in the winter because while the sun is shining the angle and the intensity are sub optimal.

Question: How do I know what tilt my roof has, and why does this matter?

Why Roof Tilt Matters for Solar Panels

1. Face the sun
   o Solar panels work best when they point straight at the sun.
   o If your roof is tilted too flat or too steep, panels lose some of the sun’s energy.
2. Year-round performance
   o The sun sits higher in summer and lower in winter.
   o A tilt close to your latitude (in the Netherlands that’s about 52°) gives the best average output over the year.
3. Keeps panels clean
   o A bit of slope helps rain wash off dust, leaves or snow automatically.

How to Measure Your Roof Tilt

1. Rise-and-run method
   o Pick a rafter in your attic (or safely climb onto the roof).
   o Measure 30 cm along the rafter (that’s your “run”).
   o From that point, drop a plumb line straight down and measure how many centimetres up to the rafter (that’s your “rise”).
   o If your rise is 22 cm (8.7 in), for example, your roof pitch is “22 cm in 30 cm” (about 36 degrees).
2. Smartphone app
   o Download any free inclinometer or “slope meter” app.
   o Lay your phone flat on the roof (or on a board against the roof).
   o The app displays the angle in degrees.
3. Hand-held inclinometer
   o A small tool you rest on the roof surface.
   o It shows the tilt in degrees on a simple dial.

Putting It All Together

Optional seasonal tweak
o Tilt 5 to 10 degrees higher in winter for more low-sun output.
o Tilt 5 to 10 degrees lower in summer for higher-sun days.
By matching panel angle to the sun’s path, you’ll get the most energy, keep panels cleaner, and make your solar investment pay off sooner.

Measure your roof’s tilt using one of the methods above.

Compare that angle to your local latitude (about 52° N in most of the Netherlands).

Adjust your solar mounting if your roof is much flatter or steeper.