Are you interested in designing an off-grid solar system? Here are the 6 steps to get you started.
Precursor: Trim Your Power Use
You will probably not be surprised to learn that designing a power system to run an off-grid home is not cheap. So it’s extremely worthwhile to consider improving your home’s energy efficiency before you design a power system for an inefficient home full of aging, power-hungry appliances and incandescent light bulbs. Making energy improvements first has a cascading effect on keeping the cost of an off-grid power system down.
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. The same is true with solar. You can’t just say “I’m going to buy two solar panels and a battery and hope it will be enough.” Use our kWh calculator to add up everything you’ll be powering with your solar power system. You’ve got to remember absolutely everything that will be powered by your system – remember, if you’re going off-grid, you won’t have grid power to fall back on!
2. Calculate the Amount of Battery Storage You Need
Now that you know how much power you need, you need to figure out how many batteries you need to store it.
- Think about the weather patterns in your area. How frequently do you get multiple sunless or low-sun days in a row? Your batteries need to be able to get you by during periods when your solar panels are producing well below their maximum. Do you need only enough storage for a day or two, or do you need to have enough batteries to store three or four days (or more) worth of power?
- Do you have another power source, like a generator or turbine, that can kick in in a pinch?
- Will you be storing the batteries in an insulated room or will they be in a cold location?
Batteries are rated for storage at around 77°F (25°C). The colder the room they live in, the less efficient they are and the bigger the battery bank you’ll need – over 50% bigger for below freezing! Each of these answers affects the size – and therefore the cost – of your battery bank.
What voltage battery bank will you use – 12V, 24V, or 48V? Generally, the larger the system, the higher voltage battery bank; this keeps the number of parallel strings to a minimum and reduces the amount of current between the battery bank and the inverter. If you are planning a small system and simply want to be able to charge your cell phone and power 12V DC appliances in your RV, then a 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. Most full-time off-grid homes are best off with a 48V system.
So how many batteries do you need for your off-grid solar system? Use our off-grid calculator to calculate what size battery bank you need based on your answers to the questions above.
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 discovering how much energy you need to make per day from our 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 given location is referred to as that location’s “sun-hours.”
The number of sun-hours at a location is not the number of hours of daylight at the location. It is how many hours 1000 Watts worth of solar radiation strikes a square meter of surface area at the location over the course of a day. Obviously, 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, whereas 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.
When designing an off-grid solar system – especially one that will be counted on to provide your electricity day in and day out, be conservative. Take the worst case scenario of sun-hours for your area by using the season with the least amount of sunshine during the period in which you’ll be using the system (i.e. winter if your system will be powering a full-time residence). This way, you won’t end up short on solar energy for part of the year. If your system will be used for a summer camp or seasonal vacation cabin, you don’t need to plan for winter, but if it is a year-round home or a hunting cabin, you need to use the number of sun-hours that corresponds 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.
But there’s a bit more to consider with selecting a 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’re using a 12V panel and a 12V battery bank, you can use a PWM. If your solar panel voltage is different form 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. If you’re doing a whole-home system, chances are very high that your best bet is a 48V battery bank and an MPPT charge controller (but we can help you confirm this if you want to give us a call at 877-878-4060 and talk through your system plans).
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 (which you almost certainly are if your system is going to be powering a residence), 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, but many are fixed. So check the output voltage specs of the inverter you are interested in carefully 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. The most common appliances that require 240V power are electric clothes dryers and electric ranges. 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 (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 – how much instantaneous power it needs to be able to output based on which loads it will need to supply simultaneously. 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, (12V, 24V, or 48V), 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 overcurrent protection
- What breaker boxes will be used
- Your solar panel mounting solution
- 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. Or if you decide you want some help, fill out our free off-grid system quote request form.