This week we'll look at design considerations of home solar systems, including panel placement, shading, and lifespan of equipment. We'll also look at how to get the most efficiency from a system.
And more importantly, we'll answer the question posed in part I - How do you make your power meter run backwards so that the power companies are paying you!?
Grid-Tied vs Off-Grid – Revisited
As mentioned in the first installment of this series, you have two main choices for solar installations. You may choose to install a system that supplements your utility power (grid-tied), or you may choose to have a system which allows you to sever your ties and go ‘off-grid’.
Going off-grid is costlier and more impacting to a typical lifestyle. To go off-grid, you must generally install larger systems (i.e. most costly up-front costs). Costs are further increased by the installation of large battery banks to supply power off hours or for cloudy periods.
Off-grid generally requires that you alter your behaviors to account for the system design. For example, you may have to let your house get a touch warmer in the afternoons, or reduce your nighttime power usage to ensure you do not fully drain your batteries. Generally other power saving measures must be taken to prevent wasteful use of electricity.
As such, many people going off grid are installing systems in remote locations, or as part of a ground-up house design where other power saving features are integrated into the building.
For most people’s suburban lifestyles, grid-tied is the way to go. Roof space often doesn’t allow for the system size needed to fully power a typical house in an off-grid setup, and utility power is generally already in place. In these cases, solar PV systems ‘take the edge’ off utility rates, and provide some insurance against rates going up in the future.
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| Basic Grid-Tied Solar System |
Some grid-tied systems also permit battery backup, but this can vary depending on local regulations.
Net Metering – How to Make Your Meter Run Backwards
If you have a grid tied system, you may be wondering how to make your meter run backwards – i.e. sell power back to the utility provider. In theory, this can occur when you generate more power than you are using, and is referred to as “net metering”.
As a preface, some utilities do not allow net metering, so this is something you’ll want to know about your local utility. Net metering can help a solar PV system pay for itself much sooner, so it is an important consideration.
When you purchase power from a utility, you pay a retail rate. This rate varies across the USA from anywhere around $0.08 per KwH to as much as $0.25 per KwH. Most utilities charge based on a tier – i.e. one rate for your first 1000 KwH, another rate from 1001 – 2500 KwH, and a third higher rate for 2501+ KwH, for example. The key here is that this is a retail rate.
The utility purchases power or resells generated power at a different rate, called the wholesale rate. The wholesale rate is generally much lower than the retail rate.
With net-metering, you sell power back to the utility when your meters spins backwards. However, this is at the wholesale electricity rate.With net metering, power you use is billed to you at the retail rate. Power you sell back to the utility is sold at the wholesale rate. With this in mind, it is important to realize that all power you generate is not equal. This leads to a key design detail – the most cost effective systems are designed to offset your use, but not generate more power than you use.
If your goal is to have the quickest payback possible, your system would not be designed to support peak power usage on the peak day of the year.
In fact, it would likely be designed to support 40-60% power usage on a typical summer day. This allows for enough of an economy of scale to spread out the costs of your inverter, ensures that the majority of your power generated offsets your retail usage, resulting in minimal power sold back to the utility. Remember, a KwH that offsets your own usage is worth more than a KwH that is sold back to the utility.
With net metering, the utility will install a special meter that can track power in both directions, giving them a means to see how much power was sold back. Many utilities implement complicated net metering schemes, which can result in some of your power that was sold back not counting. Be sure to read the fine print of how your utility implements net metering.
Residential Solar Design Considerations
As you’ve been reading Part I and today’s article, you’ve probably noticed a few design considerations sprinkled throughout. I’ve consolidated the most basic design considerations here.
Roof Layout
Do you have a nice broad south facing roof? Your roof needs to have a broad face with some southward component (in the Northern Hemisphere). It is OK if it faces ESE or WSW or anywhere in between, as long as it has a south component.
Complex roof layouts may require two or three strings of panels (i.e. if you can't fit enough panels on one exposure), you may opt to have two smaller strings on different exposures - but you'll either need micro inverters or a two MPPT inverter.
Shading
Your roof should be free of shading, especially during peak times of the day. Chimneys and flues can present challenges. If you have a lot of chimneys and flues, a micro-inverter system may make sense for you (see Part I). Additionally, consider your trees and how tall they will grow. Also, you may be surprised about the amount of shading a neighbor’s house may cause.
Heat
Solar PV panels operate less efficiently at high temperatures. This is a sad fact for those in very hot locations, resulting in slightly less performance during the hottest times of the year. However, this can be somewhat of a blessing for people that do not have a due south facing roof, since their panels will operate cooler and will offset some of the lost efficiency.
Longevity
Inverters are the single costliest component of most systems, and typically the first component to fail. Most inverters have a 5 to 10 year warranty. Be sure you have a good warranty and that the inverter manufacturer is reputable in the industry.
PV panels can last 25 years, 30 years, or longer. However, they do lose a bit of efficiency with age. Solar PV warranties typically ensure a percentage performance by a specific year. For example, a warranty may guarantee a panel operates at 80% of new at year 20.
Net Metering
As mentioned above, net metering provides a safety net for excess power generated (if you are not storing it in batteries). However, remember that net metering is not as cost effective as offsetting your own use. Do not over build your system simply to sell power back to the utility at the measly wholesale rate.
Rebates and Tax Credits
The federal government offers a 30% tax credit with no upper limit until the end of 2016. This substantial rebate is applied AFTER all other local and utility rebates (don’t let the solar install companies fool you). Many states and some cities offer additional rebates, as do many utilities.
Most utility rebates are capped (for example, $2 per watt of capacity, up to a 4 kilowatt max). Thus, if you installed a 5 KW system, you would be eligible for 4000 x $2 = $8000 utility rebate. The KW above 4 KW would not be eligible.
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