What is kWp?

Watt-peak or Wp is the DC name plate rating of solar panels

It is the rated peak DC output under standard test conditions (commonly known as STC) of 1000 W/m2 irradiance, cell temperature of 25 deg C and air mass of 1.5. Solar irradiance of 1000W/m2 occurs at about noon time and hence most of the time, the irradiance will be lower than 1000 W/m2 and hence the net output of the solar panel will drop below the STC value.

Also the cell temperature may rise to as high as 65 deg C on a hot day and hence the output of the solar panel will also drop from the STC value.

Hence the name plate rating at STC is ideal condition and in actual operation, the net output will be reduced. Most established System Integrators should have taken these derating into consideration when computing the estimated energy yield of the PV system.


1 kWp = 1000 Wp. If one solar panel is rated 350Wp, a system of 100 solar panels = 100 x 350 = 35,000 Wp = 35 kWp.

Does cloud affect performance of solar panels?

Clouds reduce the intensity of irradiance falling on the solar panels. However, the average annual irradiation in Singapore of between 1580 to 1620 kWh/m2 has already factored the effects of clouds throughout the year. This worked out to an average irradiation of about 4.32 to 4.44 kWh/m2 per day (i.e 4.32 to 4.44 peak sun hours per day). However,  the daily irradiation may range from 1 to 6.5 kWh/m2 (i.e 1 to 6.5 peak sun hours) on a very wet and/or cloudy day to a very sunny day. The average figure is used to estimate the energy yield of the solar panels.

Why is it not recommended to install off-grid solar system with batteries in Singapore?

In rural countries where there are no grid supply, the PV system will be an off grid system which must be supported by batteries to overcome the intermittency of solar irradiation.

In Singapore where we have a stable and reliable grid supply, the PV system should be connected to the grid without batteries. In this way, whatever PV energy generated will be consumed by the loads (appliances) and the balance drawn from the grid. There is no concern of the intermittency of the PV supply as the system is tied to or connected in parallel to the grid and hence the loads will always receive a constant supply either from the PV or grid or combination of both. 

For any installation with a reliable grid supply, it is pointless to go for off grid system as the batteries will drive up the first cost and operating cost. The battery maintenance and replacement costs are much more than the savings in grid electricity charges.



What type of panel or cell technology is the most cost effective?

At this point in time (mid 2017), polycrystalline solar panels is still the most common cell technology and also the most cost effective.

Can I get paid for excess PV energy exported to the grid?

Yes, all customers whether contestable or non-contestable can enjoy net settlement (i.e rebate) for export of excess solar PV energy exported to the grid. The PV System Integrator will arrange for the application and installation of dual register meters and advise you on all the required PV generation meters, licences and applications.

What are the compliance requirements mandated by SCDF and how do they impact the PV system?

SCDF has issued a circular FSR-13 on 31 December 2015 that spells out fire safety requirements to enhance fire safety for rooftop PV installations.  You can view a copy of the circular here -  https://www.corenet.gov.#2B11DD2

Basically, the requirements include mandatory provision of staircase access,  2.5 m (1.5 m if there’s a 900mm railing or parapet wall) perimeter aisle, 1.5 m accessway between each 40 x 40 m array, product listing scheme, emergency shut down switches, signages, etc.

Building plans showing the PV Installation must also be submitted to SCDF for approval.

The SCDF requirements would reduce the net usable roof space that can deploy PV panels, apart from increase cost due to the provision of staircases, etc.


What is fill factor and what is its significance?

The short-circuit current and the open-circuit voltage are the maximum current and voltage respectively from a solar cell. However, at both of these operating points, the power from the solar cell is zero. The "fill factor (FF) is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is the area of the largest rectangle which will fit in the IV curve as illustrated below:

Examples of FF of various solar panels:

Solar Frontier CIS – 64.9%

Stion 150W CIGS – 68.2%

SolarWorld 325W Mono – 74.8%

Trina 315W Polt – 76.9%

Sunpower 327W – 76.5%

Panasonic 245W HTI – 79%

Crystalline modules usually have higher FF and the power curves drop steeply before and after the maximum power point. 

Thin film (CIGS) solar cells tend to have lower FF as shown on the graph on the right, but the power curves are less steep before and after the maximum power point. This explains why thin film modules experience less mismatch and hence less susceptible to severe power drop due to partial shading.


How are solar panels installed on rooftops protected by lightning?

Solar panels, including the mounting structures, should be bonded to the building’s lightning protection system. This is known as equipotential bonding. It has to be ensured that all metal parts of the entire solar array and structure are made electrically continuous and effectively bonded to the building’s lightning protection  system.

What is the straight pay back period for solar PV system?

The payback period depends largely on the size of the system and the prevailing and future grid tariff. For systems of 100 kWp and above, the straight payback period can range from 7 to 9 years, without considering interest on investment. Maintenance of PV system is very minimal and if this is taken into account, the payback period may increase to say 8 to 10 years.

What are the maintenance regimes for a solar PV system?

 PV systems require very little maintenance. Just basic spot cleaning of the panels once or twice a year

depending on the environmental conditions, inspection of electrical connections and mounting structures.

Most systems have online monitoring which will show the live performance of the system and give warnings

in the event of any malfunction or shortfall in performance.

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