How Does Residential Solar Work?

On the surface, solar is straight-forward. Sun is converted to electricity, and that electricity is made available to your house, sold back to the utility, or stored in batteries.

But when you peel back the surface layer, solar is more complicated, and many factors contribute to whether solar will make sense for you.

And more importantly, how do you make your power meter run backwards so that the power companies are paying you!?


To answer that question – Does Solar Make Sense For Me? – I recommend reading and learning as much as you can on the topic, starting with today’s article. I’ll explain solar system components, including photovoltaic cells and panels, inverters, grid-tied systems, off-grid systems, net metering and more.

This is just the start! Solar has enjoyed huge gains in popularity while making gains in efficiency and ease of use. Over the next 4 weeks I will cover the topic of residential solar in a series of four articles:

• How Does Residential Solar Work? Part I
• Quick Guide for Going Solar
• How Does Residential Solar Work? Part II
• Does Solar Make Sense for Me?

If you absolutely want the quickest way to evaluate solar companies and designs, read the upcoming 'Does Solar Make Sense for Me?' and 'Quick Guide for Going Solar' articles. 

What is a Residential Solar System?

In the most generic sense, a residential solar system is an electrical system that translates energy from the sun into electricity. This system most convert the electricity into a usable form for your house, and most include safety switches and other components to "tie" to the electrical power grid, and/or provide battery backup for off hours and grid outages.

The following drawing gives a high-level overview of a typical grid-tied solar system.

Note that details may vary (i.e. no batteries are used in this system; a central inverter is used whereas some designs may see benefit from several 'micro' inverters, etc).  But in all cases multiple solar panels are tied together and run through a series of components to tie into your house and the electric grid.

Photovoltaic Cells – The Basis of Solar Systems

Photovoltaic (PV) systems translate solar radiation into electricity, forming the basis of a solar electric system. PV cells are generally arranged on a rectangular or square panel (i.e. your traditional solar panel).

The cells consist of a material which allows solar radiation (sunlight) to create a voltage buildup. This voltage buildup can be translated into Direct Current (DC) power. Side note: houses typically run entirely on Alternating Current (AC) power, which means that an Inverter is required to convert the DC to AC.

We’ll talk about Inverters in great depth later, but for now remember that the inverter may be as important as the panels you select.

Polycrystalline vs Monocrystalline Cells

As the names suggest, polycrystalline cells are made up of many crystals, while monocrystalline cells are made up of one crystal. in fact, you can even see the 'mosaic' like structure of the polycrystalline cell.

But what does this mean in terms of home use? Polycrystalline cells are relatively easy to produce, and are thus cheaper. But the trade off is that monocrystalline cells can produce a little more power using the same amount of surface area as a polycrystalline cell.

When looking at Solar PV panels, it is generally best to just look at the cost/efficiency ratio of the panel and not worry too much about mono vs poly. The one exception might be if your space for panels is limited, in which case it may be worth asking for a monocrystalline panel.

How do Amorphous Panels Compare?

Amorphous panels are very thin sheets of silicon (sometimes called thin-film) that are cheaper and easier to produce, but produce less power per square inch. However, one major benefit is that the thin film technology allows for the sheets to be flexible, allowing it to be used in unique applications and architectures.

However, in most residential scenarios, monocrystalline or polycrystalline are necessary for a viable system.

How Do I Compare Solar Panels?

I've discussed the basics of monocrystalline, polycrystalline, and amorphous panels, but how can you really compare and determine which panel is right for you? First of all, unless you are highly knowledgeable in electricity and construction, you should be working with a minimum of three companies getting quotes and understanding and comparing their solutions.

But if you are like most people looking at a major investment, you will want to understand as much as you can on your own. You'll want to understand the proposals, double check the designs, and ask intelligent questions.

The basic specifications of solar panels to be aware of is size and watts of power produced.

Watts is the basic measure of power, which is obtained by multiplying volts and amps. Typical panels may produce from 160 to 240 watts of power.

Size is straight forward, but important to keep track of. First, you will want to understand how efficient a panel is for its size. You can divide the panels rating in watts by its size in square inches to perform a basic efficiency evaluation.

A Complicated Roof Makes Panel Size and Placement Important

The dimensions of the panel are also important since south-facing roof space is generally limited. You will want a panel that has dimensions that allow for maximal use of the roof space. Most panels range from square to rectangle in varying sizes.



Most manufacturers publish data sheets (spec sheets) that are freely availanle on the web. You may try looking at companies like Sunpower, Sharp, Kyocera, Canadian Solar, and Sanyo as a start.

Inverters

Inverters, as mentioned above, convert the direct current (DC) power generated by your solar panels, into alternating current (AC) that can be used in your home.

And again, on the surface this sounds like a very basic function, but as you investigate, you will see many different manufacturers all claiming to have the best inverter technology.

Grid-tied or Off-the-grid?

First, there are different types of inverters for different applications. Most residential solar systems are grid tied, meaning that you will still connect to the power grid and use your solar power to supplement power from your utility. In this setup, you may even be able to sell power back to the utility (called net metering).

Grid-tied inverters have protections to ensure that they do not send power back to the grid in the event of a grid failure. This is a safety mechanism to prevent additional problems and to prevent utility workers from encountering live power. An additional feature called anti-islanding is necessary for grid-tied systems to prevent the inverter from getting tricked into thinking the grid is active even if it has failed.

The other most common setup is an off-grid stand-alone setup, where banks of batteries are used to store power generated by solar, and the inverter converts that power into AC. Stand-alone means you are entirely off the grid. This may sound like the way to go, but these systems are very expensive, and you generally need much more space to have enough panels to account for your peak usage.

You'll need to know whether you want a grid-tied or off-the-grid system to select an inverter.

We'll talk more about grid-tied and off-grid system design in part 2 of our solar series.

Maximum Power Point

Power is calculated by multiplying voltage (V) times amperage (I). As such, there is only one point of operation where a PV cell produces maximum power - that point where the product of V and I is greatest - the Maximum Power Point.

Inverters can maximize the power output by adjusting the resistance applied to the circuit, which affects voltage and current.

Simple inverters will have one MPPT (maximum power point tracker), meaning that the inverter finds the MPPT for an array of PV panels. This makes it critical to have all of the panels in the array operating under similar conditions (i.e. facing the same direction, no shading, etc). If one panel is out of tolerance or shaded by a chimney, it could affect the efficiency of the entire array. Even very little shade (i.e. enough to cover 10% of one panel) may affect MPPT to the point where the entire array is producing 50% less power than optimal.

Many inverters offer two or even three MPPTs, meaning up to two or three arrays could be supported. This is especially useful if a home has complex roof lines or doesn't sit at a true North/South orientation. For example, a two MPPT inverter would allow you to have an array on a SE facing roof line and on a SW facing roof line.

Micro Inverters

Micro inverters are relatively new to the solar scene, and promise to alleviate some of the limitations of centralized inverters.

A micro inverter is tied to a single panel (usually attached to the back of a panel), resulting in each panel being able to lock on to its own independent MPP.  This means that systems should see an immediate theoretical gain of a couple of percent of efficiency as compared to a centralized inverter since individual panel variances will not affect the array as a whole.

Additionally, micro inverters will make system design easier.  Concerns of shading will be decreased since, with micro inverters, only the shaded panel will be affected, instead of the entire array as with central inverters.  And each micro-inverter means over voltage and power dealt with is lower, making for viable DIY "plug and play" solar options.


However, a failed micro inverter means time on the roof replacing components.   Some micro inverter manufacturers claim 20-25 year lifespans, which generally exceeds centralized inverters.  None-the-less, if you have a 24 panel system chances are good that you will have a few fail prematurely.

Next Week

Next week we'll continue our investigation into home solar systems with a "Quick Guide To Going Solar", and then the following week continue our look at residential solar systems, including how to make your meter spin backwards (net metering), and some design basics and considerations.