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The Truth of a Solar Promise -kWp vs. kWh

What’s in it for me? You will know that kilowatt-hours (kWh) is more important than kilowatt-peak (kWp) when you receive a solar proposal from a reputable solar installer.

Kilowatt-hours (abbreviated as kWh) is the product of power generated or consumed over a specified time. It is the amount of energy generated by your solar when the sun rays hit your solar panels. It is also the amount of energy required to operate your lightings, television, cellphone charger, refrigerator, air conditioning, etc.


The insolation (incoming solar radiation) is most effective when the sun is hitting the cells of the solar panels at a perpendicular angle (90 degrees). Considering that our planet revolves around the Sun, the Earth’s equator slightly tilted by around 23.5 degrees (the reason why we have longer nights and days), but even though, the sunlight strikes the equator directly most fo the year. Since the Earth is roughly spherical in shape, we need to consider the latitudinal location and compensate the angles with respect to the equator so that the solar panels are receiving the highest amount of sun rays most of the time. In our country, the Philippines, we are on the Northern Hemisphere, therefore facing the panels on the south direction is the best option. If you are in Manila, the exact latitude is 14.5995° N. Therefore, you should face your solar panels with 14 to 15 degrees to the south direction. In Tawi -Tawi, a 5 degree tilt is sufficient, while in Batanes a 20 degree tilt is recommended. Of course, these conditions are not really met onsite, especially for arooftop with an existing roof pitch and orientation. You don’t have to remove your roof and revise the pitch angle. Instead, you can compensate by adding more panels to get the target solar harvest. Therefore, when installing solar panels, using the south facing roof is always the first option.


What about the East and West facing panels?

This part of the roof is the second option to place your solar panels. If you are using more electricity in the morning, then you can put more panels on the east-facing part and less on the west-facing part. If you are using more electricity in the afternoon, then putting much portion of panels in the west part is the best move. If you are using a string technology inverter, be mindful of the panel connection and the minimum number of panels in a string. All panels connected in a string should be on the same roof planes (optimized system and microinverters are an exemption).


Look for the kWh and not the kWp in the solar proposal.

You will realize that most solar proposals are based on the kWp. Kilowatt-peak (abbreviated as kWp) is the product of the solar panel size in watts and the total quantity. It becomes the norm since government and distribution utility should regulate the generation size capacity at a certain point of connection and to take account the impact by too much distributed generation.


For example, your solar installer provides a quotation for 3.645kWp Grid-Tie Rooftop Solar Photovoltaic System. Take note that on a grid-tie/on-grid/embedded solar photovoltaic system, it does not matter what are the size of your loads. The grid-tie inverter will continue to provide the maximum power available it can harvest from the sun. The grid-tie inverter has always a big brother we call the distribution utility, whenever its capacity is not enough to supply the load, the big brother will help. Export limiting and net-metering is a different scenario.


In this example, the below table should be the projected energy harvest of the 3.645kWp solar.

You will notice that solar energy harvest is not consistent throughout the year. Solar harvest is maximum in the summertime in the months of March, April, and May. These data can be easily projected using the inverter OEM analysis tool or other software available online.


The truth of a solar promise is how much kWh of energy your solar can provide you all throughout the year monthly. Your solar installer should provide you this data on the contract as this is the real deal of projecting the energy savings thru solar energy.


Points to remember:

  • Do not expect a zero-bill on a grid tie-inverter without net-metering. Regardless of your solar system size, you cannot attain zero bill since you still have night-time consumption.

  • Zero-bill is attainable in three ways:

  1. For Grid-Tie Inverter + Net-Metering, make your system size larger as to compensate the nigh time consumption with net-metering. Determine your daytime consumption and nigh time consumption first. Say your daytime energy use is 60% and nigh time use is 40%. Take note that we are buying electricity for example 10 pesos per kWh, but we can only sell them at less than half the price of, say, 4 pesos per kWh.

  2. Install a Hybrid-Grid Tie Inverter + Net-Metering. Excess solar will be first stored in the battery to be use at night. When the battery is full, excess solar will be sold the grid. This comes at a higher cost since there is battery. Its advantage is you have the back-up power during blackout.

  3. Install an Off-Grid inverter system. All your loads will be supplied by the solar day and night.


Jayson Francisco, PEE

11 April 2021

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