Residential Rooftop Solar Power

Posted on Sat 08/11/2012 by


Large Rooftop Solar Panel Array


Is rooftop solar power in a residential household application an economically viable way to supply residential power?

The short answer is no.

The long answer is still no, and that needs careful explanation. In recent years there have been ploys to make it seem that there could be a way to make it viable, both as a source of residential power, and also on an economic basis. That economic basis is important here because to make it seem viable, then that includes subsidies, and these subsidies are not minor amounts. With the inclusion of these subsidies, it gives the impression that on a long term basis, rooftop solar power might actually look like it could be economically viable.

That economic aspect is not a simple thing to explain, and rather than just give some basic points as to why it is not viable, both as a method for residential power supply, and also for that important economic aspect, it needs that in depth explanation, and that can only be achieved in a long and involved Post, so rather than do that, I will split this into a short Series of three further Posts after this Introduction.


So, how does rooftop solar power work?

Monocrystalline Silicon Wafer Solar Cell. (Image in the Public Domain.)

The image at left is of a Solar Cell. The process works on light shining onto the cell, and that light is then converted to electricity, in this case a small direct current (DC) flow. A large number of these cells are connected to the larger solar panel, (shown below at right) and you can see the array of solar panels in the image at the top of the Post. One cell produces a small amount of power, so connecting many of them in the form of the panel, then a larger power can be generated, and the same applies for using more than one panel. The more panels, the more power generated.

As the power produced is DC, it needs to be changed to the normal residential power that is supplied to all houses, and that is AC power. This is achieved with an Inverter, which converts the DC generated by the panels to the usual AC power supplied to the home.

The main source of the power is the solar panels, which work with light shining on them, so this form of power can only generate its power while light is shining on the individual cells on the panels, hence power can only be generated during daylight hours. The more power is generated during Summer, so some people might think it has to do with heat, but this is incorrect, and the reason more power is generated in the Summer months is because the daylight hours are of longer duration. In fact, excessive heat on really hot days actually lowers the amount of electricity being generated.

Also, because they are working on the light, then that means they generate most of their power under direct Sunlight. The second a cloud passes across the face of the Sun, then these cells produce a lot less power, and in some cases that power generation may drop to only 20 to 30% of their capability. What also happens is that once the cloud has passed that power takes time to build back up to its maximum level.

Prolonged periods of overcast can lead to panels generating barely 10% of their power. The same applies for seasonal changes, as in Winter, the overall daily total power generated can be a lot less that for Summer, because there are less hours of daylight, and the Sun is shining from a lower angle and not as directly onto the cells as it is with the Sun closer to directly overhead as it is in Summer.

The same also applies for where you live. The closer to the Equator, then the more power is generated, because the light is shining almost directly down onto the cells. The further away from the Equator, then the Sun is shining at an angle onto the panels, hence less power is being generated. In cases like this, the panels can be mounted at angles greater than the angle of the roof itself, as shown in the image at the top of the page. Panels can also be attached to moving ‘tables’ so that their angle to the Sun is optimised so the greater power can be generated. Again, all of these extras for optimal generation will add to the initial cost of the system itself, sometimes by a considerable amount.

With respect to the further North or South these systems are installed, then those areas may be subject to Winter snowfall, and for areas where there is a regular Winter snowfall, then having rooftop solar power would not be a feasible idea.

The panels themselves will only generate their maximum power while ever the unimpeded Sunlight is shining on them, and over the years that power will gradually diminish. Most of the commercially available solar panels are capable of producing an effective amount of electricity for around twenty years. A typical warranty from manufacturers of solar panels is that they will generate 90% of their rated output for the first 10 years, and 80% for the second 10 years, and their generating capability drops off substantially after that 20 years. Solar Panels could be expected to function for a period of 30, and maybe even up to 35 years, but as you can see from the warranty percentages, the power drop off after 20 years is substantial, so while they will still be generating some power, that level of power will be drastically diminished, and would not even approach a fraction of the power produced when the panels are new.

Something else that has not really been explained all that well, is that for the panels to be generating at their maximum, then those panels must be kept pristinely clean, as any film of dust on the panels seriously diminishes their generating capacity, in a similar manner to how cloud cover also affects that capacity for generation.

Another disadvantage is that once the installation is in place the residence owner is virtually locked into staying at that house for up to 25 years if they expect to receive the monetary value these installations are perceived to return.

The exercise in the following Posts is to show if an installation is viable, both for the supply of power for the residence, and also to show the economics for a typical installation that may be able to supply the power that an average residence might use. To do that exercise we need to establish a baseline. That baseline is the amount of power that the average residence will use on a daily basis for the best case 25 year lifespan of the system itself. In the second Post, we will use the example of a system that is grid connected, and for the third Post we will use the example of a stand alone solar system that supplies all the power requirements for an average residence that is not connected to the power grid.

The following are the links to each of those Posts. Click on the link and you will be taken to that Post.

PART ONE   Baseline. Power consumption from the grid only for 25 years.

PART TWO   Grid connected solar system for 25 years.

PART THREE    Stand alone solar system for 25 years.