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Solar Home Grid-Tie System Sizing Part 3: Small System Economics

Solar Home Grid-Tie System Sizing Part 3: Small System Economics

In part three of this series we are now going to look at a grid tie system based on a small budget instead of roof space or total energy off-set. Even a solar array that won't off-set all of your usage will pay for itself fairly quickly while saving money on your electric bill each month, and can be expanded in the future as well. 

In this example, I'm using the parts list detailed below, which, aside from some site specific wire runs is a fairly complete package for a roof mounted grid-tie system. 

6 - Peimar 310W Black Solar Panels
1 - SMA SunnyBoy 3000W Grid Tie Inverter W/SPS
1 - 30 Ft MC4 Solar Panel Cable Extension Wire 
1 - Roof Top Junction Box / Combiner Box
1 - SnapNRack Roof Mounting System (4 - 122 Inch Black Rails / 2 - Rail Splice Bars / 10 - Black Mid Clamps / 4 - End Clamps / 4 - Black Rail End Caps / 8 - Black Mounting Feet w/Flashing for Comp Shingle Roof / 8 - Lag Bolts / 1 - Rail Grounding Lug) 

This package would come in at 1.86 kW (1,860 W) and cost $3,000 or less depending on some of the specific components. 

It's important to make sure that under different conditions an array this small will still work well with the inverter we are using, which is obviously over-sized in comparison to the array. Many inverter manufacturers offer "String Sizing" tools on their website, I also personally like to use the MidNite Classic string sizing tool just for the data it provides to double check with the inverter specs in all different kinds of systems, it's handy as I can get the following results for this particular set-up quickly. 

Array Specs:
Number of Panels In Series: 6
Number of Parallel Strings: 1
Total Modules: 6
Rated PV Array Power: 1860 Watts
Anticipated Array Power @ 104 F: 1748 Watts
Rated PV Array Current: 9.51 Amps
VMP (Maximum Power Point Voltage): 195.6 Volts
VOC (Open Circuit Voltage): 244.2 Volts
VMP @ -22 F: 238.8 Volts
VOC @ -22 F: 287.4 Volts

SunnyBoy 3.0 Inverter Specs:
Max. usable DC power: 3100 W 
Max. DC voltage: 600 V 
Rated MPP voltage range: 155 - 480 V 
MPPT operating voltage range: 100 – 550 V 
Min. DC voltage / start voltage: 100 V / 125 V 
Max. operating input current per MPPT: 10 A Max. 
Short circuit current per MPPT: 18 A  

For small systems like this, micro-inverters like Enphase Products are also a great choice, and you really don't need to worry about any of these little details as each solar panel uses it's own inverter, so a single panel system could even be taken into consideration.

I used the SMA SunnyBoy for this kit for a few reasons though. First, it's a great product and this system could be added to in the future without much additional cost per watt, which will keep the payback time nice and low. Another thing I really like about the SMA SunnyBoy line is a unique feature known as SPS, or Secure Power Supply. This feature allows a home owner to dedicate their array power to a dedicated outlet which provides up to 2000 W of power depending on the conditions, with this particular array you'd probably expect around 1800 Watts of available power given good solar panel orientation and sunlight - without requiring a battery bank, charge controllers or other hybrid system components. 

As a side note, this SPS feature could be used to charge a battery bank for emergency use using a lower cost standard battery charger and low cost inverter, because not everybody really needs the best of the best for a quick black out scenario, and these types of systems can also be fairly cost prohibitive if you're on a tight budget.

Going back to the simple payback methods described in the last blog post, I'll be using PV Watts to calculate the predicted system payback time. Here are the specs I'm using for PV Watts:

System Info:
DC System Size (kW): 1.860
Module Type: Premium
Array Type: Fixed Roof Mount
System Losses (%): 14.08 (Default Loss Calculations)
Tilt (deg): 26.57 (6:12 Roof Pitch, See last blog post for a roof pitch to degrees chart)
Azumuth (Deg): 180
NOTE: So far, my favorite online tool to use for determining azimuth angles is provided by Solmetric at the link below, it's a free tool but an account sign-up is required. Link: Roof Azimuth Tool

Retail Electricity Rate:
Rate Type: Residential
Rate ($/kWh): 0.11

PV Watts Results:
Annual Output: 3,005 kWh/Year* (System output may range from 2,881 to 3,080 kWh per year near this location.)
Annual Average Daily Solar Radiation: 5.72 Hours
Annual Energy Value: $330.00

If we look at the most simple payback method of the original package cost divided by the annual energy value, we come up with 9.09 years to payback. Taking the Federal Residential Renewable Energy Tax Credit of 30% into consideration, that number drops down to 6.36 years. There could and usually are state, local, and utility rebates that Solar Grid-Tie systems would qualify for as well that would drop this even further! Check out the Database of State Incentives for Renewables & Efficiency for more information on rebates and incentives specific to your location and utility provider.

Thanks for reading, hopefully this inspires you to check into a system for your home, the sooner it's on the roof the sooner it's going to pay for itself! Our team is always here to help from designs to sale and into the future as a new friend of the solar crew! 
Solar Home Grid-Tie System Sizing Part 1: Using a Utility Bill

Solar Home Grid-Tie System Sizing Part 1: Using a Utility Bill

Grid-Tied (a/k/a “grid interconnected”) solar systems are the most common and simple types of solar electric systems homeowners install. These complete solar power systems are connected to the utility grid and generate electricity while the sun is shining and the grid is running. If the system produces more energy than your home uses, then the system sends (sells) the excess energy back to the grid, which would offset any electricity you buy from the utility at night (check with your utility provider for details, as these policies vary). 
If you consume more energy than your system is able to generate, then the utility would supply your additional energy demand as usual. Grid-tied solar panel power systems are great for reducing your energy bill.

Sizing a system can seem confusing at first, mostly due to the flexibility of sizing a grid-tie system. We can base a system on a certain amount of production to off-set current usage, to fit into a budget, or to just take advantage of the roof-space available to install the solar panels on. 

Figuring out the size of system to offset current usage is pretty straight forward and simple though, so we will start the series off with this. Here is an easy method to help you figure out the correct solar panel array size for your home.

Step 1: Find your monthly average electricity usage from your electric bill.This will be in kilowatt-hours (kWh). Due to air conditioning, heating and other seasonal usage, it is a good idea to look at several bills. You can add the typical summer, fall, winter and spring bills and divide by four to find the average monthly usage.

Step 2: Find your daily average electricity use. Divide the monthly average number of kWh use by 30 (days).

Step 3: Find your location's average peak sun hours per day. Find what is known as an insolation map, or average sun hour list. For example, Colorado is around 5 peak sun hours. Alternatively, a PV Watts is a great online tool from NREL that will provide great insight for month by month and average solar radiation for your specific location. (https://pvwatts.nrel.gov/)

Step 4: Calculate the system size to provide 100% of your electricity. Divide your daily average electricity use by average sun hours per day. For example, if the daily average electricity usage is 30 kWh, and the site is in Colorado, system size would be: 30 kWh / 5 h = 6 kWh. It's a good idea to multiply this number by 1.15 in order to account for factors such as inverter efficiency, dirt or pollutants that may accumulate on panels, etc., if you want to cover all of your use. (One of our solar home packages in the 7 kW range would be great for this!)

Take a look at some of our system options for grid-tie packages here, or contact our sales team for more information, we are eager to help out and provide you with everything you need to go solar! 
Electrical Components 101

Electrical Components 101

Want a brief summary of some of the common electrical components used in a solar system?  Whether you're interest is grid-tied solar or off-grid solar, you'll get a taste of fuses, disconnects, wire and more.
Solar Batteries 101

Solar Batteries 101

Do I need solar batteries for my system?  

 Everyday we talk to customers looking to use solar energy to power all kinds of different things. For instance, if they want to simply reduce their electric bill from the local utility, then we're clearly looking at a "grid-tied" solar system. But often people want to power some type of device directly from a solar panel. Or they need mobile power, or power for something remote and away from electric utility lines, or back-up power. More often than not, this means batteries! And not just any batteries, but solar batteries (batteries suitable for solar systems).

 Can't I just plug it directly into a solar panel? 

Usually no. Most devices that need electricity require steady, reliable current (amps) and voltage to operate as designed. The voltage produced by a solar panel when it captures the sun's light varies constantly. Clouds, weather, and the changing position of the sun throughout the day are to blame. Also, solar panels only produce DC (direct current) electricity, which means it must be inverted to AC electricity by way of an "inverter". But an inverter requires steady, consistent energy as well. However, there are a few types of "loads" that can be powered directly from a solar panel. Certain types of attic fans or water pumps can operate effectively with the varying voltage throughout the day. But for most other loads, the DC energy from the solar panel must be first inverted to the proper form or stored in a solar battery system.

What what is a battery, really? 

Putting solar aside, for a moment, we are all familiar with batteries. We use all kinds to power all kinds of devices. But at the end of the day, a battery is simply a device to store energy produced somewhere else at some other time. Like a water storage tank filled by a pump, a battery holds or stores a certain amount of energy.

How much depends on the battery's size, construction, type, chemistry, environment, etc. Unlike the throw-away AAAs in your TV remote, batteries used for solar energy systems big and small are rechargeable. A rechargeable battery can be discharged and recharged over and over again (one discharge and recharge is called a "cycle"). The battery's chemistry and type determine how deeply the battery can be discharged and the number of cycles it can perform during Its lifetime.

Batteries for Solar? 

There are a number of factors to consider when buying solar batteries. Because the battery bank is one of the most expensive components of such a system, it is important to carefully design the battery bank so that it performs as expected for as many years as possible. When designing a battery bank we first analyze how the batteries will be used. How much energy must the battery bank supply to the “loads” each day? A load is any device that uses energy. People are often surprised when we ask them to list all of the loads they will use each day, the watts of each load and how many hours a day they expect to use them. But

that detailed info is needed to determine the daily watt-hours of storage which determines the battery bank size. From there we consider other factors to recommend the best battery bank for the specific situation.

Other factors 

Because we're trying to keep this overview fairly basic, we will discuss factors like battery chemistry, sizes, capacity, and more in later blog posts. But we want to comment on just a couple more important points to keep in mind as you look for solar batteries.


There are quality long-lasting batteries and there are cheap short-lived batteries. We stick with quality brands. You can find cheap batteries all over the place. But if you want your batteries to last and give your solar system the most bang for the buck, it's usually worth it to get quality. We often supply quality brands that are not even on our site today, so please ask!

Deep Cycle 

 For solar systems, you want to stick to deep cycle batteries only. You don't want car or “starting” batteries for a solar system. If the battery doesn't list Amp Hours and instead only lists CCR (Cold Cranking Amps) then avoid it, even if the battery is labeled Deep Cycle. Deep cycle batteries are meant to slowly discharge down to a certain point and then recharge fully. And repeat that cycle over and over for many years. Car batteries are designed to a very different thing--quickly start a car engine in a short burst of current. Just remember deep cycle.


Because solar battery systems store energy for gradual discharge (usually throughout the night), a good deep cycle battery will list its storage capacity in Amp Hours as measured over a 20 hour discharge rate (designated as "C/20"). Sometimes batteries will show different discharge rates (C/100, etc). Just make sure to use the C/20 rate to compare capacities among batteries. The more amp hours the better. Deep cycle batteries generally should not be discharged beyond 50% of its rated capacity. If they go beyond that level, the battery can be damaged and its ability to fully recharge will be reduced. For that reason, it's really important to size the battery bank correctly or else your battery life will be cut way short.


 If you are running an RV solar system, boat system, outdoor yard lights, water pump, remote instrumentation, etc., then chances are very good that you are operating equipment designed to run on 12 volts of DC electricity (sometimes designated VDC). So you will want to keep your battery bank at 12 volts. If you are powering a large off-grid home, then you'll want to be at 48 volts. Smaller cabins at 24 volts. We design battery banks to match these system voltages by using individual batteries that may be at 12 volts, 6 volts or 2 volts. Again, we'll get into the specifics in a future post, but for now that is why you'll see batteries with these different voltages listed on our site.


While there is a lot of hype today about lithium ion batteries, lead acid batteries still lead the way as the most economical, long-lasting battery for solar energy systems. For most people, lead acid batteries still make the most sense. But if the batteries need to be light, fully cycle daily, and at moderate temps, then lithium may be a great choice. If price isn't a big deal. But today, lithium prices are pretty high compared to lead acid.

Click here to see our lines of batteries for your application.  Questions, just ask!