7 practical tips before buying a solar panel

When we are looking for where to buy solar panels, we mainly look at the price, but is price the most important criterion when choosing a photovoltaic solar module?

There are a multitude of manufacturers that bring new models of very different qualities to the market every year. How can we compare all these solar panels and find out which one is the most suitable for our installation?

There are many factors to take into account: solvency of the manufacturer, warranty conditions, performance references in other installations, quality in the manufacturing process, certification tests and a long etcetera. These are undoubtedly important factors but often difficult to find out. When we want to buy a photovoltaic solar panel we normally look at the price and the technical characteristics. And it is here where we must pay special attention to the details to “translate” all this information and know what is relevant to make the decision of which panel to buy. Selecting a quality solar panels and solar inverters brand from the hundreds of different models and manufacturers could be a difficult task. These are the main parameters to take into account before buying a photovoltaic solar panel:

Monocrystalline or polycrystalline

The basic difference lies in the manufacturing process. Monocrystalline silicon cells are obtained from a single crystalline structure, whereas in polycrystalline cells the silicon paste is left to solidify slowly in a mould, resulting in a solid made up of small silicon crystals. The theoretical efficiency of monocrystalline cells is slightly higher, as is their price. The difference is not critical, and for me it is not a determining factor unless we have space problems. To distinguish between them we can look at the colour of the panel: monocrystalline modules have a darker shade, while polycrystalline modules are bluer.

Working voltage and number of cells

For isolated solar installations with batteries, the solar panels are 36 cells (12V) or 72 cells (24V).  If we want to use the module in a small isolated installation that feeds a battery box (monoblock for example) we will need 12V panels, while if our installation is a little bigger and has a larger accumulation system (two monoblock batteries or 12 stationary glasses) we will need 24V panels.

Grid-connected installations and self-consumption solar kits usually work with 60-cell panels that are not valid for stand-alone installations unless they are connected to a charge regulator with MPPT maximum power point tracker.

Power output

It is the theoretical power output of the solar module, the term we usually use to refer to the size or capacity of the panel; however, it is a power that we almost never get in normal operating conditions because this value has been measured in standard measurement conditions (STC): 1000 W/m2 of irradiation, at 25ºC of cell temperature (we would have a cell temperature like this when the ambient temperature was approximately -7.5ºC) and a spectral distribution of 1.5 AM, conditions that are very rarely reproduced in our day to day life. This value is used to compare two modules “of the same size”.


Due to the manufacturing process and the different components that make up a solar panel, the output power may vary significantly with respect to that indicated in the technical characteristics sheet. This variation is called tolerance and it can be indicated in W or in %. Let’s imagine that we have a 100W panel with a tolerance of +/- 5%. This means that the panel can deliver a power between 95W and 105W. If the panel has cost us 100€, in the first case it will cost us 1.05€/Wp and in the second case 0.95€/Wp.

Ten euro cents per Watt is a difference to take into consideration, especially in installations with a large installed power. Most of the major manufacturers offer panels with a positive tolerance of 0/+5%: this ensures that we will get at least the power we have paid for.


The efficiency of a photovoltaic solar panel is the power that 1 square metre of photovoltaic solar panel is capable of producing when it receives an irradiation of 1000 W/m2. If in the technical characteristics sheet of a 245W photovoltaic solar panel we see that it has an efficiency of 15%, this means that for every 1000W/m2 of irradiation that 1 square metre of panel receives, it will produce 150 W of power. Nowadays it is quite common to praise the quality of higher efficiency solar panels, and often, higher efficiency solar panels are considerably more expensive. In my opinion, if we do not have space problems, it does not make much sense to pay a higher price for higher efficiency solar modules. On the other hand, if we have a very “tight” roof, we will have to install higher efficiency solar panels to optimise the installation to the maximum.

Electrical characteristics at nominal operating temperature

The power of the panels is given in standard measurement conditions (see point 3), but these conditions do not normally occur when the solar panel is in operation. Therefore, the power value at nominal conditions is more relevant (as these conditions are usually more common): 20ºC ambient temperature, 800 W/m2 irradiation, air speed of 1 m/s and a spectral distribution of AM 1.5. A typical power value for a 245W panel at operating conditions is 180W. A good solar panel or solar generators should reflect this data in its data sheet.

Thermal characteristics

In my opinion, one of the most significant technical parameters for predicting the future performance of a solar panel. There are two important parameters:

Nominal Operating Cell Temperature (NOCT): this is the temperature reached by the cells of the module under normal operating conditions, mainly at 20ºC ambient temperature and irradiance of 800W/m2. It is important to know that the NOCT is directly related to the temperature that the cells reach at a given ambient temperature, and that the lower the temperature of the module, the better it will work and the more power it will deliver. Therefore, the lower the NOCT the better.

Power Temperature Coefficient: indicates the percentage loss of power output of the panel for each degree above 25ºC that the temperature of the module’s cells increases. The lower the better.

Where to buy solar panels?

If you still have any doubts before shop solar kits, you can contact Renogy at any time. The Renogy team will be happy to answer any questions you may have. Renogy supplies a large option of solar panel for sale, so you can constantly get photovoltaic solar panels that will always fit your power needs. With years of service and experience, at renogy official online store you can buy solar panels that are monocrystalline, polycrystalline, off-grid, or rv batteries, agm batteries, gel batteries. Our deep expertise ensure you get quality solar panel efficiency, reasonable pricing, and knowledgeable customer service.