How To Size Your Off-Grid Solar Power System

Having your solar array connected to the power grid definitely has its benefits. You can take advantage of net metering, and in case of a cloudy day, you have the grid to back you up. Still, many are opting to disconnect and build their photovoltaic (PV) systems completely off the grid. Off-grid solar is great for those with RVs, boats, or a backyard shed or guest house. For those who live in isolated areas that lack the infrastructure, off-grid solar might be a necessity. Going off the grid means you keep all the power you generate, and there’s no interruption in service when the power grid fails.

However, you’ll need to consider some important factors if you plan on building an off-grid PV system. Adequate energy storage is a necessity. You’re going to need plenty of backup power stored for those days when the sun isn’t shining. You’ll also need to do some in-depth calculations to assess what size PV array you’ll require. In this article, we’ll guide you through the steps.

Read also: The Best RV Solar Panel Kits

Step 1: Assess your Energy Needs

Before you even start looking into solar panels, you need to know what devices you’re powering and how much energy they use. Make a list of all the electrical appliances you’ll be powering, from lights and refrigerators to phone chargers and power tools.

Next, split your list into AC and DC devices. Anything with a normal household plug is an AC device. DC devices run on 12 volts (V), 24V, or sometimes 48V.

Once you know what devices you’ll be powering, you can determine each device’s wattage. You can find the wattage by looking at the manufacturer sticker, searching online for information about the device, or using a wattage meter to measure the wattage directly. Then, write down how many hours you use each device daily. It’s best to overestimate each time, just in case.

Create tables that look like this:

You now need to decide if you want to use a 12V or 24V system. This will decide everything about your PV setup, from the inverter down to the solar panels you buy. Small systems, such as those on an RV or boat, should use 12V systems, while larger solar arrays do best with 24V.

A good rule of thumb is that if your energy needs are less than 1,000 watts, go for a 12V system. If you use between 1,000 and 3,000 watts, then a 24V system is best. If you require more than 3,000 watts, then you might even need a 48V system.

The reason you want to raise the voltage for higher wattages is that it decreases the current that will flow through your system. Higher amperage systems cost more because you need to find heavy-duty parts that can handle the high current. They also generate more heat and can be dangerous to work with.

Base the voltage of your system on your estimated daily usage. In our example, the total daily consumption of our appliances, both AC and DC, is 900 watt-hours, but the potential peak power usage of our system is 1,325 watts. While we only expect to use 900 watt-hours on a daily basis, we want to accommodate our peak load, so we’re going to build a 24V system.

Now that we know what we will be powering and how much power we need, we can move on to our next step.

Step 2: Choosing a Battery

Now it’s time to find yourself a battery. What you want is a battery that can output the wattage you need to power all of your devices. Batteries are differentiated by voltage (V), representing power output, and amp-hours (Ah), which represents capacity. Wattage equals amps (A) times volts. For example, let’s say we are using a 12V battery with a 125Ah capacity, that battery will store 1,500 watt-hours of energy. On the other hand, a 24V 62.5Ah battery will also have the same 1,500-watt-hour capacity.

Since we’re installing a 24V system, we’re going to need a 24V battery. We also need a battery that can give us over 1,325 watts on a single charge. A 24V battery that can give us 1,325 watts will have a 55Ah capacity. To give us some headroom, we’re going to go up a few sizes and use a 70Ah battery. A 24V 70Ah battery will have a capacity of 1,680 watts.

You should also consider a battery’s depth of discharge, or DoD. This represents how much of the battery’s rated capacity you can actually use. Lead-acid batteries have a DoD of 50%, while lithium and Ni-Cd batteries have a DOD of about 80%, and flow batteries have a DoD of 100%.

For our example, we’re going to keep it simple and go with lead-acid batteries. Since they have a DoD of 50%, our 70Ah battery will only give us 840 watts, meaning we’re going to need two lead-acid batteries to fulfill our needs.

We also have to remember that the sun may not shine every day. We’re not always guaranteed to have ideal weather conditions for charging our battery bank. So we want to have several days of autonomy to keep us going when it’s cloudy. Let’s say we want three days of autonomy. That means we need to triple our battery capacity. Six 70Ah batteries would give us 5,040 watts, which is more than enough power to carry us through three days of bad weather.

Step 3: Choosing an Inverter

It’s time to start looking for a power inverter. Power inverters convert DC electricity to AC, and since solar panels generate DC power, we only need to worry about having enough capacity for our AC appliances.

According to the chart above, the total wattage of our AC appliances is 1,115 watts. You should always leave some headroom for safety, so an inverter with a capacity of 1,500 watts would suffice.

Unless you plan on powering very simple devices, you should buy a pure sine wave inverter to help your electronics run smoothly. Most of our everyday electronics can’t handle square wave inverters—they’ll make a humming or droning sound when turned on, and they won’t have much power.

Modified sine wave inverters are a cheaper option that serve as a halfway point between square wave and pure sine wave inverters. Still, the modified sine wave can cause sensitive electronics to malfunction and overheat, and motors will run less efficiently. Anything with a microprocessor, which encompasses just about all modern electronics, requires a pure sine wave to operate correctly.

The output your inverter should have depends on your needs. Most homes and businesses use 120V single-phase power. Larger appliances like stoves, washers, and dryers use a 240 V split phase. You should also keep in mind that most off-grid inverters can’t connect to grid power.

If you’re looking to power your boat, you’re going to want an off-grid inverter-charger that has the capability to plug into the grid for when you’re onshore and need to charge your battery on a cloudy day. The same goes for RVs because you want to be able to conveniently use grid power to charge your batteries when you travel through the city.

Step 3: Solar Panel Type and Wattage

Now that we know how much energy we’re going to use and the size of our battery and inverter, we can start to calculate how much wattage we need from our PV system.

Our battery bank can hold up to 5,040 watts. Let’s say we want our solar array to charge our battery bank within one day. If we assume that we get five hours of full sunlight daily, then we divide 5,040 watts by five hours, which gives us 1,008 watts. If we use 250-watt solar panels, then we take 1,008 watts and divide that by 250, which gives us 4.03 panels.

So, about four 250-watt solar panels should be able to fully charge our battery bank over the course of the day. Of course, we want to leave room for inefficiencies and changes in the weather, so we’re going to install five solar panels just to be safe.

Since we have 24V batteries, we also want 24V solar panels. The amp output of a 24V 250-watt solar panel will be 10.4A. This is under ideal conditions, as variation in sunlight will affect the power output, and the amp output, of our solar panels.

When wiring solar panels, you can choose to wire either in series or parallel. In series, you add up the volts while amps stay the same. In parallel, you add up the amps while the volts stay the same. You can also rig a combined series-parallel setup if you have a larger array with at least eight panels. All methods have their pros and cons, but because a PV system wired in series can completely shut down if there is a failure anywhere in the system, we’re going to wire in parallel. That means we will be adding the amps.

Each one of our five panels puts out 10.4A. So our total amperage will be 52A. We’re going to need to know this for our next step.

Remember that five hours of sunlight a day isn’t the norm. If you live in a sunny place like Southern California, then you might get a full five hours of sunlight all year round. But if you live in a place such as Seattle, which is known for overcast skies and rainy days, then you should plan to have a larger solar array to compensate for the lack of sunlight.

If the amount of sunlight drops significantly during the winter months, then you should size your solar panels based on the least amount of sunlight available during the year. If the available sunlight drops by half to 2.5 hours a day during the winter, then we would double the size of our PV array to 10 panels.

You should also ensure that you have the right tilt and orientation for your latitude, as this affects your power output. Make adjustments to your array if you expect shading. You should add extra panels if you expect a large portion of your array to be shaded at any time.

Of course, there are other factors to consider when choosing solar panels. Higher-quality panels will give you more wattage and efficiency, meaning you’ll need fewer panels. This is something to keep in mind if you’re short on space. If you’re on a budget, lower-wattage modules cost less but require more solar panels. The best idea is to take a look at the range of panels produced by top-rated solar manufacturers.

Read also: How Much Power Does a Solar Panel Produce?

Step 4: Charge Controllers

Charge controllers protect your battery bank as well as the electrical circuits in your PV system. They prevent the battery from overcharging and keep electricity from flowing from the battery to the solar panels at night. In short, you need a charge controller.

Charge controllers come in two types: PWM (pulse width modulation) and MPPT (maximum power point tracker). MPPT controllers are the better option, as the technology is newer and more efficient. MPPT controllers can regulate the voltage when the voltage from your solar array is different from that of your battery bank. PWM controllers are cheaper and ideal for smaller systems.

Like your battery bank and inverter, controllers should be rated to the capacity of your system in both amps and voltage. Mismatched charge controllers can result in energy losses of up to 50%.

Remember that the total amperage of our solar panels will be 52A, so we need a charge controller that can handle the amperage of our panels. Charge controllers come in a variety of voltage and amp ratings. Though our panels output 52A, we want to give ourselves some headroom for safety. After searching online, we found a PWM charge controller that is rated for both 12V and 24V systems up to 100 A. PWM controllers are best for PV systems wired in parallel.

Newer charge controllers have a feature called low-voltage disconnect, or LVD, which powers down the load when a battery is close to being discharged. This prevents the battery from fully using its capacity, as the chemistry in some batteries is such that it can be damaged from being excessively discharged. Though this feature may cost a bit extra, it’s especially useful if you want to prolong your battery life.

Explore our article to learn how to operate solar charge controllers.

Things to Consider

Remember that you’re building an off-grid solar array, so you should strive to get the highest quality equipment and do the best possible job with your installation. You’re not going to have the power grid to back you up, and you don’t want to take any chances with any failures that could leave you without power for days. We recommend you explore our off-grid solar setup wiring guide.

You should also think about your energy needs when deciding to wire in series or in parallel. Wiring in series can help your system stay efficient if your panels are far from the inverter or battery bank, but a failure at any point in the system will cause the entire PV system to shut down. Wiring in parallel is less efficient but more reliable.

Remember that deciding to take on higher voltages for the same wattage will drastically reduce system costs. Choosing a 48V system over a 24V system for a 3,000-watt power requirement lowers the amperage of your system. This means you can buy thinner cables and cheaper fuses, saving you hundreds, if not thousands, of dollars. High amp systems also generate more heat and carry a higher risk of electrocution.

Lastly, remember to maintain your PV system. Keeping your panels clean and inspecting your wires and fuses will ensure that you can continuously generate electricity for as long as possible.