4. How to buy a solar roof top from your installer.

Authors: Tetchi Capellan, President PSPA; Dipl.-Ing. (FH) Christian Brennig; Dipl.-Ing. (FH) Tatiana Abarzúa; Dipl.-Ing. (FH) Sebastian Stein (all BSW); Markus Dietrich, GIZ Consultant


Producing electricity partly for own consumption, and partly for sale to the distribution utility companies, is now available in the Philippines provided anyone has a roof suitable for solar energy. This guide provides an overview of important points to consider when planning and purchasing a small PV system intended for own consumption.

The guide is directed towards interested parties considering installing a grid-connected photovoltaic roof system up to 100 kW. It attempts to walk the reader through the different stages beginning from the day the idea to buy a PV system is conceived, up to the realization of the PV project. What this consumer manual intends is to assure quality and reliable installations. The system can be either a residential PV installation of a household or a larger commercial PV roof top system.

4.A Selecting components for solar roof top

4.A.1 What modules to buy

When buying a module it is important to check if the module passed the design qualification and type approval (See below the section "test specifications").  


  • Manufactured for maximum energy yield per m².
  • Mostly with aluminium profile frame which protects glass edges, facilitates mounting, improves statics of modules and can be used for fixation or mounting structures.
  • Most frames are produced with mounting holes and holes for water discharge.

Thin-film modules

  • Modules with cadmium (cdTe) and amorphous silicon solar cells (a-Si) are most widely used.
  • Other cell types cointain copper (as CIS/CIGS modules).
  • As less semi-conductor material is needed, the production of thin-film solar modules becomes cheaper and the selling price drops significantly.
  • Reminder: Thin-film modules have a lower efficiency than c-Si modules. Therefore larger area is needed for the PV installation.
  • Often used in facades or, as they weigh less and can be produced in flexible forms.

Test Specification 

  • Crystalline silicon modules: choose the modules with standard IEC 61215 which comprises the examination of all parameters linked to ageing of PV modules and describes various qualification tests based on the artificial load of the materials (radiation, thermal and mechanical testing).
  • Thin-film modules : choose the modules with standard IEC 61646, where additional test procedures are done to adapt to special properties of thin-film technologies.
  • Check the safety qualification  according to IEC 61730, as there are used in conjunction with above mentioned standards. Focus on fundamentals construction requirements for PV modules such us the prevention of electrical shocks, fire hazards and personal injury.

Module warranty

  • Many module manufacturers offer a product warranty of two or three years. some offer a product warranty of 10 years.
  • Most manufacturers guarantee a 25-year limited warranty on the power output, usually 90% of the minimum output power rating of the modules during the first 10 years of operation, and 80% during the following 15 years. Sometimes, manufacturer provides a warranty stating guaranteed power output of 97% of the minimum output power rating during the first year of operation and a guaranteed maximum linear degradation of 0.7% p.a. until the 25th year.
  • Reminder: Examination of European PV systems done by reasearch and testing institutes indicate that the actual degradation is far lower than the module power guarantee offers.


  • Check compliance of module type with IEC standards and type of  warranty.
  • Buy modules with measurement record and serial numbers as reference.
  • Secure technical data sheet and choose modules with low tolerance ranges of nominal output or positive sorting of power output categories.
  • If modules with high tolerance values (> 5%) are used by your installer, ask how they will be pre-sorted so connection of modules with a similar maximum power current within one string reduces losses caused by mismatch.
  • Modules with different power output categories (e.g. 215/220/225 Wp), should be string with modules from the same performance class.  

4.A.2Choosing Inverters

The inverter represents the connection between the photovoltaic system and the public distribution grid. It converts the direct current generated by the PV system into alternate current. Therefore, the inverters have to have to adapt to the grid frequency (60 Hz) and the voltage level, typically 230 V. inverters converts the direct current generated by the PV system into alternate current. Therefore, the inverters have to have to adapt to the grid frequency (60 Hz) and the voltage level, typically 230 V.

There are Inverters for grid-connected systems and for stand-alone systems. As the name suggests, grid- network.

Most inverters have a 10-year limited warranty.

Place of Installation 

  • As much as possible, the inverter should be installed near the house meter. Shorter DC cabling cuts system loss of solar power.
  • Inverters get Slightly warm during their operations. to achieve higher performances ,there should be adequate ventilation.
  • Always comply with the external conditions demanded by the manufacturer, particularly permissible humidity and ambient temperature. 


  • Ensure adequate ventilation.
  • Consider range of permissible ambient temperature for inverter (avoid power limitation due to high temperatures.
  • if an inverter is mounted outdoors it has to be protected against rain and isolation.
  • Pay attention to local high tide levels.

4.A.3 Selecting mounting systems and warranty

Unlike in the past, manufacturers now offer easy to install mounting systems for flat roofs. For large roofs on industrial and commercial buildings, frames are now generally lightweight, aerodynamic, self-supporting, and without roof penetration challenges. These features are ideally suited for roofs with membrane or bitumen surfaces as they represent a slight additional load to the roof.


When installing the modules, no holes should be drilled into the frame. Call your module manufacturer before you drill further holes, as warranty commitment may be invalidated.

Fixing of modules 

  • When fixing or clamping the solar panels on the rails of the chosen mounting system the appropriate points have to be considered.
  • The weight of the modules is approx. 20kg per module. Always check the kind of roof surface.
  • In case no information is provided by the manufacturer, the module attachment should be along the longer side of the module, at around 1/4 of the module length. the pre-drilled module holes are mostly in this part of the frame. 


  • Choose a corrosion-resistant and statically tested mounting system.
  • Check if chosen mounting system complies with the requirements of module manufacturer.
  • Consider maximum local wind loads.

4.B Sizing of the PV system

4.B.1 Do I need a small PV systems

The electrical power of a PV system is measured in kilowatt peak (kWp) and refers to the maximum power under standard test conditions (STC). For the installation of a 1 kWp power plant, you will only need an area of about 8 m². If you have a small roof area, you may want to choose solar modules with higher efficiency.

Remember that only a portion of your household electricity demand will be covered by the PV system. It is therefore important to get a rough estimate of the overall power of the PV system obtained - based on the space allowed on your roof – and match the PV energy output with your consumption.

Solar allows you to reduce your consumption during the daytime. It is essential to improve the demand profile of your home by utilizing washing machines, dryers and other loads whenever the PV system delivers a lot of electricity and reaches peak capacity. Managing your load this way will reduce the peak demand for electricity. Under the net-metering regime, consuming self-generated solar power makes more economic sense than selling excess power to the public grid. Of course, storing energy is an option. As the prices for battery decreases in the near future, a purchase can be considered in order to use all electricity provided by the PV system.

4.B.2 Can I install large PV systems up to 100 kWp

The installation of a PV system is a good investment, which can provide higher returns than a savings deposit with only an average interest rate of .25% - 1.25% per annum.

Larger PV systems for commercial use will therefore make sense if you can arrange loads to coincide with high solar generated electricity. Sometimes this is realized applying remote controlled relays or sockets to switch on additional selected loads. More recently load management components are offered that also consider weather forecasts and thereby set time frames corresponding to the projected electricity generation.

In the case that the owner of the building is the operator of the PV system, it can be considered to include the solar roof top in the property insurance. The insurance coverage should already be provided during the construction phase.

Question to ask for Home Installation 

  • Is the quality of the roofing suitable for the mounting of a solar system?
  • Is the chosen area for the PV installation free of any shading caused by neighbouring buildings, antennas or trees?
  • Is an alternative roof more adequate for PV installation, like the roof of a garage ?
  • Are expansion gaps between modules considered (approx. 10 mm)? 
  • will fire compartments not be overbuilt and are sufficient distances to fireproof  walls taken into account (1.25mm)?
  • Are switches for demand response considered (control of loads)? 
Roof Space Requirements 
Solar cell materials  Module efficiency
Required surface
area for 1 kWp
High Performance silicon
(rear contacts, HIT)
17-20% 5-6 m²
Monocrystalline silicon 11-16% 6-9 m²
Polycrystalline silicon 10-15% 7-10 m²
Thin-film :
6-11% 9-17 m²
Cadmium-Telluride 6-11% 9-17 m²
Micromorphous silicon  7-12% 8.5-15 m²
Amorphous silicon  4-7% 15-26 m²


  • Choose roof area free of shading, as shaded modules significantly reduce the PV output.
  • If your roof is oriented towards south you will have the highest yield with a module inclination of 8°
  • Inclination between 0° and 20° is appropriate too.
  • Inclining the modules provides a washing effect by rain.
  • If it is possible for you to realize a building integrated PV system you could save money for roof tiles and other roofing material. 
  • Flat roofs: Consider sufficient distance between module rows to avoid shading.
  • Consider what portion of your present electricity demand you would like the PV system to meet. The ideal is to plan for system lower than peak demand.

Question to ask in Large Scale Installation 

  • Is the quality of roofing of your building suitable for the mounting os a solar system 
  • Is the chosen area for the PV installation free of any shading?
  • How is the realation between nominal power of the PV generator to nominal power of the inverter (Describe as DC/AC ratio)?
  • Can loads be easily switched on and off (demand response)?
  • Large on-roof PV systems: Is the PV plant divided in different arrays with intermediary spaces (for maintenance and safety reasons)?
  • Has an insurance policy been concluded? 


  • Check your electricity demand and consider which degree of self consumption you would like to achieved with the PV system.
  • Consider if you can re-arrange some loads to peak capacity time in order to maximize the use of solar generated electricity
  • Check with your LGU for Electrical and Building permit.
  • Large roofs: consider modules with lower efficiency at a more competitive price.
  • A relation between nominal power of PV generator and inverter of 1:1 is preferable (DC/AC ratio 100%) because a lower ratio would lead to losses.
  • Consider an intermediary space of 1 m and a maximum width of 40m as protective measure for the event of a fire in the building (thus fire fighters can maintain safety distance for fire fighting purposes at electrical installations).
  • Check with your property insurance firm about insuring the PV Installation.

4.C How do I recognize a good offer?

The first question to ask is: Where will I find a good PV installation company? System installation can be found by advertisements in printed media and internet search. There is a solar association in the country and you can search the website for companies that can provide installation services.

Sometimes, component providers and its local distributors frequently offer lists of authorised installers of their components. Always ask for references of already realized PV systems.

When choosing an installer, the most obvious way is to get their track record. Don’t forget to ask the host of the PV system if they are satisfied with the recorded PV yields and the overall installation process.

Did the installer comply with agreed deadlines? What are the impression about the installer’s know-how and diligence? How is the visual aspect of the PV installation?

Payments for the provision of services are made through instalments (partial payments). For instance a payment is made after signing of contract. Then, another payment is remitted to the installer as soon as the delivery of material to construction site is done. Then, the final payment is completed when the installation of components and connection of modules to the inverter, or after grid connection.

Choice of Installer 

  • In the installer able to explain the working of the PV system in simple words and a comprehensible way?
  • Does the installer provide electrical plans and other documents required for LGU  Electrical and building permits?
  • Is the processing of permits included in the services?
  • Does the offer include the delivery of all required components and services to produce electric power and feed it to the grid?
  • Are grid-connection, and commissioning of the PV plant included? In other words: is it a turn-key installation?
  • Is the client stipulated to provide any item(e.g. scaffolding or labour)?
  • Are optional and mandatory items distinguished and terms and conditions of the contract included ?
  • Does the installer observe current applicable standards and guidelines and know compliance standards set by the electrical and distribution codes calid in the Philippines ?
  • Payment conditions: are instalments included?

A full list of PV installer can be found in Chapter 7 of the Guide.


  • Ask your installer to conduct on-site inspection to consider existing electrical installations at the building.
  • A calculation of the estimated yield should be included in the technical proposal so you can calculate the potential savings.
  • Include  in the contract, the installer's declaration stating compliance with current applicable  standards ,guidelines and rules.
  • Check on insurance for the PV installation. It should provide security against storms, hail , lightning,overvoltage,theft , vandalism and other circumstances.
  • Always include regular inspection of the PV system and a prompt service in case of a defect.

4.C.1 Comparing offers

In order to compare different offers, it is necessary that all basic items are listed. Some components can be a useful add-ons, but not critical to the operations of the PV system - like a data logger, an operation and maintenance contract, warranty extensions (usually offered for the inverter) and an insurance policy for the PV system.

In case of comparable offers, a local company is preferable. Another useful aspect to consider is the possible date of installation of the PV system.

Items to be Included 

  • PV modules : Number, manufacturer, cell technology , type(s) certificates
  • PV generator capacity (STC)
  • Inverters : Number, Manufacturer, type(s), rated output power
  • Mounting system : Manufacturer, type, certified statics 
  • DC-cabling (modules to inverter): type, length and diameter 
  • Protection devices 
  • Lightning and surge protection (if applicable)
  • Installation of mounting system, PV Modules, inverter and DC Cabling 
  • Grid Connection 
  • Commissioning (incl. tests)
  • Compilation and delivery of documentation, instruction for maintenance and operation, warranty terms including on workmanship


  • Demand a module layout plan with proposed location of modules and wiring
  • Demand a block diagram of whole PV system
  • Check if all necessary items are included (list shown above).
  • Demand information about statically testing of offered mounting system. 
  • Check if offer and terms and conditions clearly state the obligations of PV system provider and client. demand more details if this information was not provided.
  • you can also ask for advice in the internet , e.g. at www.photovoltaicboard.com

4.D Return of investment

In contrast to conventional power plants PV systems have no fuel costs. Moreover, there are no mechanically or thermally highly stressed parts; therefore the maintenance is limited primarily to few activities.

Nevertheless it is prudent to allocate a small financial reserve for the eventual replacement of defective parts. For instance the inverter may have to be replaced once within 20 years. Besides you have to know (1) your yearly demand for electricity, and (2) the exact price you are currently paying for this electricity. The more money you spend on electricity, the higher the return of investment (ROI) for the installed PV system will be.

The quote for the PV installation should also include an estimate of cost-effectiveness. This calculation has to be based on the expected yield, the electricity demand and the achievable degree of self-consumption. Besides, the energy produced which is not consumed onsite and is fed into the grid will be credited to the next monthly electricity bill.


  • Ask your installer to provide an estimate of cost-effectiveness
  • Provide the installer with your current electricity purchase price and previous rate of increase (1-5% p.a.).
  • Make sure that expected yield, self consumption, and excess feed-in are according to current pricing for purchased electricity and feed-in tariffs .

4.D.1 How to calculate the return of investment on your solar installation

The example below shows a sample ROI calculation of a solar roof top in the Philippines. Please check with your solar installer and your DU the numbers applicable to your solar roof top.

Disclaimer: This example is a hypothetical calculation which should illustrate the methodology of calculating the return of investment of a solar roof top under net-metering rules in the Philippines.

No Assumptions    
1 Life time of PV  Years 20
2 Rated Capacity of PV System (RC) kWp 6
  Module Efficiency (ME) % 16%
  Hours/Year Hours 8,760
3 Yield (RC *ME* hours/year) kwh/year  8,410
4 Degradation Factors  % 0.5%
% Own Consumption % 70%
6 DU Total Customer Charge PHP/kWh 12.5718
7 Annual Increase in DU / Generation Charge % 3%
8 % Net Export  % 30%
9 DU Generation charge PHP/kwh 5.4951
10 Operations and Maintenace /year/kWp PHP 900
  Cost of Installed PV system PHP/kWp 120,000
11 Cost of installed  PV system total PHP 720,000
  Key Performance Indicators     
  Total Solar Energy Produced  kwh 160,438
  Total Cost  PHP 828,000
  Total Savings  PHP 2,133,227
  Break Even  Year 8
  Internal Rate of Return  % 12%


4.E Important aspects considering the installation

4.E.1 Quality aspects of installation and mounting

On flat roofs, protection mats should be provided as an installation foundation, as these protect the roof membrane. Enough space should be provided in between PV arrays for accessibility in times of servicing and repair purposes. The commissioning of the PV system hast to be carried out by a registered electrician. During the commissioning a protocol has to be written by the PV plant installer. This document points out important data about the PV plant and records measured electrical values.

Issues to consider during installation

  • The running cables has to be short circuit proof, if possible laying the positive and negative lines seperately, with double isolation.
  • The cables have to be UV resistant and properly fixed on the substructure or be conducted in adequate fixings to prevent a lying on the roofing or on sharp edges.
  • Some modules require the earthing of a pole (e.g. thin-film modules with string-ribbon cells:negative pole;sun flower cells: positive pole)
  • For self consumption: appropriate meters have to be installed. Under the net-metering rules, either a single or bi-directional meter, or two undirectional meters are installed to measure delivered and received electrical energy seperately. A third meter may also be installed to measure total RE generation 

Protocol of Plant Commissioning 

  •  Before switching on the PV Plant several values are logged: earthing resistance of the grounding system,insulating resistance of the PV plan, Insulating resistance of DC main line , short circuit current for each string , open circuit voltage of the generator, open circuit voltage for each string.
  • After switching on the PV installation : check the operation current of each string as well as voltage drop of diodes and fuses. Important is that the inverter feeds in and that the generator is not running in open circuit but in the operation point, generally the maximum power point (MPP)


  • Check if cables are properly fixed.No lying on the roofing or on sharp edges.
  • Include protection class for outdoor installation 
  • Ask installer to use appropriate cables and material for DC installation  
  • Preferential use of solar cables (Characterization : PVI-F).
  • Connectors have to fit together and should not be permanently under water 
  • Avoid looped circiuits 
  • Observe distance of at least 0.5 m between cabling and lightning protective systems. 

4.E.2 Quality assurance during operation time

The installer should offer a warranty on construction works for a certain period of time (at least one year). This has to be reviewed with the installer and should form part of the documentation of the PV plant. The inverter should be checked on a regular basis (on vision panel or data logger).


  • Check monitoring data( on inverter or online via data logger)
  • Monthly : Check yield values on meter

4.E.3 Maintenance of the PV system

For maintenance measures by the PV plant owner, regularly monitor performance to avoid disruptions. Maintenance work consists mainly on control of operational readiness of the system in order to rapidly resolve any failures. An operational manual for the inverter and a documentation of the installation (like the “Anlagenpass/PV Passport)” provided by the installer are necessary. For illustration purposes and to show how such documentation could look like, the German PV Passport has been included in the annex of this guide. It could also contain a maintenance contract and also maintenance recommendations.


  • Maintain the system at least twice a year. This includes visual inspection and removing of leaves lying on the modules , as shading would reduce PV yield
  • Consider cleaning of the modules twice a year, particularly near to sources of high emission like motorways, railroad tracks, or pig farms, Use lime-poor water . Do not use cleaning supplies.
  • Twice a year check if the roof OK. Check if all modules are attached and fixed. Have Cables been bitten by animals? Are the connectors OK ? 
  • After a lightning: Did the lectric-surge arresters remainintact? (Check vision panel).


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