Off Grid Solar System Design Tool

6 Steps to Designing an Off-Grid Solar Power System

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September 2, 2016

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6 steps to Off-grid Solar

Are you interested in designing an off-grid solar system? Here are the 6 steps to get you started.

#1) Figure out how much power you need

This is the most important step, and many people try to skip over it. Don't!

Planning a solar system without knowing how much power you need is like planning a car trip and not knowing how far you are going, and in what vehicle. Ok, now go buy gas for the trip. How much? Well, that depends on your distance and gas mileage. Same with solar. You can't just say I'm going to by 2 solar panels and a battery and hope it will be enough for my needs. Use our load calculator and enter what you will be powering with your solar power system. You've got to remember absolutely everything that will be powered by your system – seemingly little changes can make a big difference.

#2 Calculate the amount of batteries you need

Now that you know how much power you need, you need to figure out how many batteries you need to store it.

Do you need only enough storage for a day or two or do you need to have enough batteries to store 3 or 4 days, or more, worth of power?
Do you have another power source, like a generator or turbine, that will kick in if the sun doesn't shine?
Will you be storing the batteries in a warm room or will they be in a cold location?

Batteries are rated for storage at around 80 degrees Fahrenheit. The colder the room, the bigger the battery bank you need – by over 50% more for below freezing. Each of these answers affects the size, and cost, of your battery bank.

What voltage battery bank do you need – 12V, 24V, or 48V? Generally, the larger the system, the higher voltage battery banks are used to keep the number of parallel strings to a minimum and reduce the amount of current between the battery bank and the inverter. If you are just having a small system, and want to be able to charge your cell phone and power 12V DC appliances in your RV, then a basic 12V battery bank makes sense. But if you need to power much over 2000 watts at a time, you'll want to consider 24 volt and 48 volt systems. Besides reducing how many parallel strings of batteries you'll have, it'll allow you to use thinner and less expensive copper cabling between the batteries and the inverter.

Use our off-grid calculator to calculate what size battery bank you need based on these answers.

#3 Calculate the number of solar panels needed for your location and time of year

The second half of our off-grid calculator can help you figure out how many solar panels you'll need for your solar system. After knowing how much energy you need to make per day from the load calculator, you'll need to tell it how much sunshine you'll have to harvest from. This available energy from the sun for a location is referred to as "sun hours."

The number of "sun hours" is how many hours the available sun shining on your panels at an angle throughout the day equals sunlight, as if it were shining directly on your solar panels when they get the most power. As you know, the sun isn't as bright at 8AM as it is at noon, so an hour of morning sun may be counted as half an hour, where the hour from noon to 1PM would be a full hour. And unless you live near the equator, you do not have the same number of hours of sunlight in the winter as you do in the summer.

You want to take the worst case scenario for your area, the season with the least amount of sunshine that you will be using the system. That way, you do not end up short on solar energy for part of the year. If it's a summer camp, you don't need to plan for winter, but if it is a year-round home, or a hunting cabin, you need to tell it the number of sun hours that correspond to winter.

#4 Select a solar charge controller

Alright, so we have batteries and we have solar, now we need a way to manage putting the power from the solar into the batteries. An extremely rough calculation to figure out what size solar charge controller you need is to take the watts from the solar, and divide it by the battery bank voltage. Add another 25% for a safety factor.

Now there's a bit more to consider with selecting the charge controller. Charge controllers are available with two major types of technologies, PWM and MPPT. In short, if the voltage of the solar panel array matches the voltage of the battery bank, you can use a PWM charge controller. So, if you have a 12V panel and a 12V battery bank, you can use PWM. If your solar panel voltage is different than the battery bank, and can't be wired in series to make it match, you need to use an MPPT charge controller. If you have a 20V solar panel and you have a 12V battery bank, you need to use MPPT charge controller.

#5 Select an inverter

Now that we have efficiently charged batteries, we need to make the power usable. If you are only running DC loads straight off your battery bank, you can skip this step. But, if you are powering any AC loads, you need to convert the direct current from the batteries into alternating current for your appliances. It is very important to know what type of AC power you need. If you are in North America, the standard is 120/240V split phase, 60Hz. In Europe and much of Africa and some countries in South America, it is 230V single 50Hz. In some islands, it is an interesting mixture of both. Some inverters are configurable between voltages and/or frequencies, many are fixed. So check the specs carefully of the inverter you are interested in to make sure it matches your needs.

If you do have the North American standard, you must figure out if you have any appliances that use 240V, or if they are all just 120V. Some inverters are able to put out 240V, and you can wire the output to use either 120V or 240V. Other inverters are stackable, each one outputting 120V, but when wired together, or stacked, can create 240V. And others are only capable of outputting 120V, and cannot be stacked. Again, read the specs to determine which inverter is right for you.

You also need to know how many watts total your inverter will need to power. Luckily, way back in step one, you created a loads list that figured out both the constant watts and surge requirements of your loads. Please note that an inverter is designed for a specific voltage battery bank, like 12, 24 or 48 volt, so you need to know what voltage battery bank you are going to have before you settle on the inverter. Keep this in mind if you think you may be growing your system in the future. If you plan on having a higher voltage battery bank later, be aware that the lower voltage inverter won't work in the new bigger system. So either plan ahead and go with the higher voltage to begin with, or plan on changing out your inverter in the future.

#6 Balance of system

OK, we're kind of cheating by lumping everything else into one final step for balance of system, but there are a lot of other little components needed, including:

the fuses and breakers for over current protection
what breaker boxes will be used
how you are going to mount the solar panels
what size wire you will need

Once you've gone through these 6 steps, you'll be off and running to designing your own DIY off-grid solar system.

About Author

Amy Beaudet
Amy Beaudet has been in the solar industry at the altE Store since 2007. She's been a sales rep, an instructor, and an all around solar evangelist, sharing her passion for solar around the world. When not at work, she's either sailing or skiing, depending on the season, but odds are good she's still talking about solar on the boat or on the slopes. See more of Amy Beaudet's blog posts.