Short answer
The system size you need is roughly found by dividing your annual electricity consumption by the energy that one kW of panels can produce in a year where you live; then an allowance is added for shade, heat, dust and cable losses. The result is limited by roof area, connection rules and budget. The exact value is calculated for the project; a ready-made figure given from a single bill is often misleading.
First, two concepts: kW and kWh
The power of a solar system is expressed in kW (kilowatts), while the energy it produces and you consume is expressed in kWh (kilowatt-hours). kW is an instantaneous capacity: how much power the system can deliver under the best conditions. kWh is energy accumulated over time: the consumption you see on your bill is written in this unit.
This difference matters, because you buy the system in kW but see the return in kWh. Two systems of the same power, one on a shade-free south-facing roof and the other on a shaded east-facing roof, produce very different amounts of energy over a year. When you ask for quotes, instead of asking only for kW, ask for an estimate of how many kWh that system will produce per year on your roof.
Step 1: Work out your annual consumption from your bills
The basis of the calculation is the bills of the last twelve months. A single month misleads: in Şanlıurfa the July bill is swollen by air conditioners, while the April bill stays low. Add up the kWh values of twelve months to get the annual consumption. If you do not have your bills at hand, you can get your consumption history through the distribution company’s online service channels.
Look at the monthly distribution too. If consumption rises clearly in summer, that is good news for solar; the months when production is high coincide with the months when you need the most. If consumption rises in winter, for example because you use electric heating, the system’s contribution in winter months stays more limited.
For businesses, working hours are considered as well as the bill. A workshop that opens at eight in the morning and closes at six in the evening and a cold storage that also runs at night use solar differently, even if their annual consumption is the same.
Expert note: If you plan to add an air conditioner, heat pump, electric vehicle charging or a new machine in the coming period, include it in your consumption when you calculate. Adding panels later can create extra work inverter and extra permitting.
Step 2: Estimate what one kW of panels will produce at your location
The energy that one kW of panels will produce in a year changes with the solar exposure of your location, the direction and tilt of the panel, and shade. This value is calculated with solar map data and simulation software, by entering the coordinates and geometry of your roof. Although Şanlıurfa is a region with high solar exposure, there is a clear difference between an east and a south roof even within the same district.
A common mistake at this step is applying a general figure seen on the internet to every roof. An estimate made without taking shade, direction and tilt into account leads to disappointment after installation. During the site survey, the roof’s direction is measured and the surrounding sources of shade are noted with their positions through the day.
- 1Add up twelve months of consumption
- 2Make the local production estimate
- 3Subtract the losses
- 4Draw the roof area
- 5Check the connection rules
- 6Fine-tune the size to the need
Step 3: Take losses into account
The power on a panel’s label is measured under laboratory conditions. In the field, a system always produces less than that. The main losses taken into account are these:
- Temperature: efficiency drops as the panel heats up, and this effect is obvious in a Şanlıurfa summer
- Dust and dirt: production falls as the cleaning interval gets longer
- Shade: even brief shade can pull down the output of the whole string
- Cable and connection losses: a long, thin cable means more loss
- Inverter conversion loss: occurs when converting direct current to alternating current
- Panel aging: panels slowly lose power over the years
Step 4: Limit it by roof area and rules
The calculation gives you a system size, but your roof may not be able to carry that many panels. The area the panels will cover is drawn together with obstacles such as walking space left for maintenance, chimneys, water tanks and parapets. On a flat roof, when panels are mounted on a tilted frame, spacing is left between rows so they do not shade each other; this reduces the usable area.
Another limit is the grid connection rules. The system size that can be connected may change with the subscription type, the contract power and current regulations. For systems built to produce more than consumption, how the surplus energy is treated also depends on the same regulations. These limits are checked against current legislation at the quote stage.
Should it cover everything or only part?
It is not always necessary to build a system that covers all consumption. In a business with high daytime consumption, a system sized for the daytime load often gives the most efficient use; most of the energy produced is consumed immediately. At home, if nobody is there during the day, part of the production goes to the grid, and its value depends on current rules.
If the budget is limited, you can start with a smaller system and expand later. In that case, the inverter is chosen from the start to allow expansion, space is left on the roof for extra panels, and the cable route is planned accordingly.
What to ask when getting a quote
When you are told a kW figure, ask for the calculation behind it too. Which annual consumption was the starting point, how was the annual production per kW estimated, which losses were subtracted, how many panels fit on the roof? The answers to these questions should be in the quote in writing.
If the sizes in quotes from different companies differ, it does not mean one side is wrong; the assumptions may be different. When you put the assumptions side by side, you can see which quote looks at your roof and your consumption more realistically.
Consider a short example. Suppose one company makes its estimate without taking shade into account, and another takes into account the shade that the water tank on the roof casts in the afternoon. The first quote shows more production on paper, but in the field the second one turns out closer to reality. So read the assumptions the estimate is based on, and go with the one whose reasoning is clearest.
Finally, ask how the system will be monitored after installation. Most inverters come with an app that lets you follow production from your phone. Comparing actual production with the estimate is the easiest way to notice problems such as dust or a fault early.
Record the first year’s production month by month and compare it with the estimate in the quote. If there is a clear difference, the cause may be shade, dust, an inverter setting or an assumption in the estimate. This comparison gives you something concrete when you talk to the company that did the installation, and it shows the real yield of your roof if you later consider adding panels.
Frequently asked
How many kW of solar panels are enough for a home?
It varies from home to home. Your annual consumption, your roof's direction, shade and connection rules all go into the calculation. A calculation based on the bills of the last twelve months gives you a realistic value.
Can solar cover all the consumption on my bill?
If roof area and connection rules allow, it may be possible to cover it over a full year. However, night-time consumption comes from the grid or from a battery. How surplus production is treated depends on current regulations.
Are more panels always better?
No. Producing far more than you consume may not bring the value you expect under current rules, and it makes the investment larger than necessary. The right size is found by evaluating consumption and rules together.
Let’s calculate together with your bills
Get the bills of the last twelve months ready; Desa Bilişim will measure your roof and give you a reasoned system size and a written quote.
