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6 Steps to Designing an Off-Grid Solar Power System

By Kris de Leon February 08, 2017

Solar Energy System for Off-Grid Living

There are many different kinds of solar power systems including grid-tied, hybrid and off-grid solar power. Out of the three main options for solar, off-grid solar power is by far the most independent of systems.

Off-grid solar power users enjoy producing and consuming their own electricity rather than relying on the voltage and price fluctuations from the local electric company while greatly reducing there carbon footprint and moving towards a more ecologically sustainable way of life.

Perhaps the most attractive part of living with off-grid solar power is when there is a storm or a blackout from your electric company you will continue to have power. With this power freedom comes the extra costs of off-grid living when compared to grid-tied solar power as well as power limitations by your inverters and the extreme importance to design your system perfectly in-line with your needs.

Detailed below are 6 steps to get you moving towards off-grid solar living. 

1. How much power do you actually need?

This is the most fundamentally important step towards designing your own off-grid solar power system. Your solar panels are only capable of producing power during the day and when there is sun. It is paramount in your system design to understand that your 24 hour use throughout the day has to be covered by the average amount of sun your solar panels collect during the day.

Although the task of figuring out how much power you need may seem a bit overwhelming, you can simply start with a pen and a pad. There are also online “calculators” available to enter the data you collect and do the math for you. Go around your home and make a list of all of the items you have that use electricity. If you are still in the construction phase walk your mind through each room and visualize what you will be using.

Write down the average watts of each device and for how many hours, on average, you will be using it. If you do not know the wattage of a particular item you can find it on the nameplate, packaging or by a Google search. Don’t overlook items such as water pumps, heaters, outside lighting and other things that may not be visible to you or an item you see every day.

For each item on your list take the watts per day and times them by the hours per day they will be operating. For example: if you have 10 LED light bulbs that are 5 watts each and they will run for 4 hours a day the total watts then will be 200 watts per day.

Remember – when going off-grid you only have what your panels produce and you batteries store so if in doubt, always overestimate the wattage and/or usage of the items on your list. It is always better to have more power than to be in the dark!

It will help in the design process to also make note of any items you have that are 240 volts rather than 120 volts (i.e. an electric water heater will most likely use 240 volts). This will help in the off-grid solar power system design when choosing the right inverter.

2. How many batteries do you need?

Now that you know how many watts you use per day we need to figure out how many batteries you need to provide the necessary power for your off-grid solar power system. Again, with off-grid living you are limited by the power generated through the day unless you have a back-up generator, wind turbine or other source of electricity.

Consider how many days you will want to power your home if there is no sun. The days of storage you desire, multiplied by the watts you need per day will give you the total amount of watts you will need to build your system provide for.

You will also need to consider what voltage your off-grid solar power system will be; 12, 24 or 48 volts. If you are running a small RV or boat a 12 volt system will be best for you. If you are running a cabin, home or business and using several thousand watts you will want to use a 24 or 48 volt system.

By increasing the battery voltage you reduce the amount of parallel connections between the batteries and at the same time reduce the current going to your inverter and the size of battery cable you need to connect them all (thick battery cable can be very expensive).

A brief lesson on batteries: Batteries are designed to operate at around 70 degrees Fahrenheit. Hotter and colder climates will affect the number of batteries you need. Deep-cycle solar batteries are rated in amp-hours. This is the amount of amps they can put out to your loads. Batteries are also rated, in terms of lifespan, by cycles. One full charge and one full discharge is one cycle. This discharge is defined by percentages called the “state of charge” and the “depth of discharge.”

To keep it simple – the more often you discharge (drain) your batteries and the deeper (lower voltage) they are drained, the shorter the lifespan of your batteries will have. A fully discharged (drained) battery will go from around 14.6 volts down to 12.2 volts. 12.2 volts would be a 100% depth-of-discharge. This is very bad for the battery and will reduce the life of your batteries from 5 or more years to just a few years. Although this may seem a bit intimidating there are many online calculators you can use to size your battery bank – it is just important to know the basics.

So let’s say you need 4,000 watts per day and want to prepare for 3 days of storage with no sun. That means you need a system that will produce and store 12,000 watts. For a system that uses 4,000 watts per day a 12 volt battery set-up will be perfect. 12,000 watts divided by 120 volts equals 100 amps (Watts divided by Volts equals Amps). You will need 100 amp-hours of storage to provide power to all of your devices for 3 days. The average 12 volt deep-cycle battery has around 100 amp-hours of storage potential. You can discharge that battery to around 15% which leaves you roughly 15 amp-hours usable per battery. Your battery bank should be then 7 or more 12 volt 100 amp-hour batteries connected in parallel.

This information is just to help you understand the basics – don’t forget about the online calculators that will do the math for you! You can shop for a variety of solar batteries at SolarTech Direct.

3. How many solar panels do I need for my off-grid solar power system?

To calculate the number of panels you need you will need to determine the solar irradiance or “sun hours” of your location throughout the year. There are several online tools that help with this. Simply enter your location and the calculator will tell you how many “sun hours” you enjoy throughout the year. Take the worst case scenario from these numbers and divide it by the wattage of your solar panels.

Back to the numbers; if you need to produce 12,000 watts and enjoy 4 “sun hours,” on average, throughout the year and you have a 300 watt solar panel – you will need 10 solar panels. 12,000 watts divided by the hours of sun your solar panels can collect that sun divided by the amount that can be collected by your panels. As with the rest of the information – online calculators are available to guide you in this process.

Shop for a variety of solar panels at SolarTech Direct to meet your needs. 

4. Selecting your electrical components

There are two more main components you will need for your off-grid solar power system – a charge controller and an inverter. Your charge controller takes the power from the panels and charges the batteries with it. It also makes sure your batteries are not over or under-charged.

There are two kinds of charge controllers – PWM and MPPT. PWM is the more basic and economical choice while MPPT will give you the most power possible from your panels and typically includes extra features such as maintaining your batteries and storing data.

To size your charge controller simply take the wattage of your solar panel (in this case 300 watts), divide it by your battery bank voltage (in this case 12 volts) and add 25%. 300 watts divided by 12 volts equals 25 amps. Add an extra 25% and you now know you will need at least a 30 amp charge controller. It is important to always design your systems with room to grow. In this case, a larger charge controller would allow that. Getting a much larger charge controller than you actually need can only help you and your system.

The second and perhaps the most important components of your system is your inverter. Off-grid solar inverters are available in two forms of electrical output; modified and pure sine wave. In nearly every case it is suggested to get a pure sine wave inverter over a modified. Pure sine wave electricity is what you would find in a home outlet and is better for motors, condensers, flat-screen TV’s and other sensitive electronics.

Next is to determine the output voltage that you need. Off-grid inverters are available in 120 volts and 240 volts (as well as 230 for European countries). If you have a device that uses 240 volts then you will need a 240 volt inverter. If not, the 120 volt is best for you. Simply match your inverter voltage to the battery voltage you calculated to determine what voltage your inverter need to be. Consider the list you made with all of your devices and ask yourself – what is the maximum amount of watts I will be using at one time? Use this number to decide the wattage output rating of your inverter.

5. Protective devices, mounting and balance of systems

It is always important to install the necessary fuses, overcurrent protection devices, disconnects, etc to protect your components and create a safe and reliable system. Skipping these components will surely be more costly in the future.

You will also need to consider how you plan to mount your solar panels, at what angle and where. There are scores of options available for both roof and ground-mounted systems – just make sure to consult with your supplier to make sure the mounting system is compatible with your panels.

View the mounting and racking systems available. 

6. Quality, Quality, Quality!

There are hundreds of websites that offer pretty good economical solar materials at unbelievable prices. As a professional solar installer I cannot stress enough the importance of quality materials. Make sure to consider how many years the manufacturer has been in the industry, product warranties and reviews. As a DIY off-grid solar power installer you will surely want the online and telephone technical support provide by top-tier solar companies!

Now that you have all of the information you need get out there and start shopping at SolarTech Direct! Building your own off-grid solar power system will surely be one of the most rewarding projects you have yet to encounter as well as one that will bring light to you for decades to come.

Kyle Johnston
This post was written by: Kyle Johnston 

Originally from Easton PA, Kyle Johnston is a solar installer and owns and operates 
Lux Aeterna Solar Energy in Antigua, Guatemala. He provides system analytics, product comparisons, finance studies, energy audits and everything else to make the switch to renewable energy easy for his customers.

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