ISP Wireless - Guide to DC Power (Solar) for Remote Sites

Readers will learn how to set up a tower in an area that does not have a grid power supply readily accessible using solar as a good alternative.

The base of working out your requirements is: "Solar Power In" must be higher than "Power Out". You must capture more energy from the sun than you use. 

• Determining Power Requirements
• How to Store Power
• Selecting a Panel Size
• The Solar Controller
• Mounting the Panels
• Troubleshooting: The Battery Bank is Flat

Determining Power Requirements

How much power do you use? To work out your power requirements, you need to look at all the radios or devices you will have on your tower. Each device will have a wattage rating which can be obtained usually by googling the model and the words watt power. 

If you cannot find the watt rating of the device using Google, you can use a multimeter, a good option is a Kill A Watt. The Kill A Watt is a device with a similar shape to a surge protector with a small screen. It tells you how much power the device plugged into it, it will be using. Ubiquiti devices will have this information in their Data Sheets, which you can find in our Downloads page by navigating to the right product line using the top bar and then selecting the correct product from the left hand side menu. The Data Sheets will be under the Documentation subheader along with User Guides and Quick Start Guides.

For reference:

• NanoStation 5AC: 8.5 Watts
• NanoStation 5AC Loco: 8.5 Watts
• PowerBeam 5AC Gen2: 8.5 Watts
• NanoStation 5AC with UniFi Video Camera G3-Pro: 12.5 Watts + 8.5 Watts

Note: Most solar panels and batteries are 12 volts, but most devices you would install at a tower use 24v or 48v, so, you will need to wire your panels and batteries in series in order to up the voltage to what you need. We recommend you install an EdgeSwitch to power your devices because it can be wired directly to your 48v solar controller.

Once you have worked out the Watt ratings of all your devices, add them together. If we had a NanoStation 5AC and a UVC-G3-PRO, we would be using a total of 21 watts per hour. To work out how many watts are used per day, multiply the hourly usage by 24. For our example the ES-8-150, NS5AC, and UVC-G3-PRO, we are using 924 watts per day.

How to Store Power

A battery bank is used to store captured solar power for using at night or on rainy / overcast days. Your battery bank needs to be made of two or more deep cycle batteries that are able to power your equipment for a number of days without sunlight. A good goal in most areas would be to run for 4 or 5 days without sunlight.

What is a deep cycle battery?

A deep cycle battery is a special type of large capacity battery which can be discharged regularly to about 50% of its capacity. They are a similar size to a car battery but the difference is the chemicals inside.

A car battery is designed to be discharged only a small amount, then recharged instantly. Discharging a car battery will leave it with a very bad memory effect and this effect worsens as it goes without being recharged. Twenty large discharges may damage a car battery and stop it from ever being fully charged again, whereas a deep cycle battery can handle several hundred deep discharges, and go longer without being recharged.

Note: Even with the discharge capability of a deep cycle battery, it should never drop below 50% of its capacity, but can go several days or sometimes weeks without being recharged and still be as good as new.

A deep cycle battery will have a rating on the side. It will specify a voltage—usually 6 or 12, and an amp-hour / Ah rating. More expensive batteries will have a higher amp-hour rating and therefore have a larger capacity. It is recommended that you purchase 12 volt batteries because your solar panels will most likely be 12 volt. If you need to purchase 6 volt batteries, they can be wired in a special way to increase the voltage to 12. You can purchase more batteries to add capacity as we will see below.

Wiring Methods

There are 2 ways to wire batteries together. You can use both at the same time if you like.

• Parallel: This means that the amp-hour capacity of the batteries will be added together
• Series: This means that the voltage of the batteries will be added together

What size battery bank do you need?

You will need a battery bank that will run your tower for a set amount of days without sunlight, and still not be discharged more than 50%. To work this out, we take the daily power usage, multiply it by the number of days , and then multiply again by 2 so we factor in the 50% discharge limit.

In the example above of a NanoStation 5AC and a UVC-G3-PRO, we need 4 days of 504 watts, multiplied by 2. This means our battery bank capacity needs to be 4032 watts. But, we also need to consider the POE switch. For this example, I will use the EdgeSwitch 8 150W.

The EdgeSwitch has a maximum power consumption of 20W without POE, so we need to add that to the original 21W. That brings the total up to 41W. So, now you need 4 days of 984W multiplied by 2.

To convert the watts back to amp-hours, we would divide by the battery voltage rating (12) so 7872 / 48 = 162 Ah. Lets just round this total to 180 Ah and have some extra capacity.

To achieve this, we would want :

• Two sets of 4 12v 90Ah batteries wired in parallel, wired in series.
• One set of 4 12v 180Ah batteries wired in series.

Types of Batteries

• Flooded / Wet Cells: The older and more common type of deep cycle battery. Needs maintenance such as water top-ups. Water is lost in gas form when charging.
• Gel or AGM: Are sealed and maintenance free. There is no gas loss because the battery case is sealed and a special solar controller needs to be used so that the built-up pressure from the gas does not damage the battery. Gel or AGM are able to deliver more power and faster, so are ideal for starting engines or using on boats. They work well for our WISP radio towers too.

Selecting a Panel Size

This depends upon the average daily sunlight hours in your area during winter time. Insolation is the correct measurement to use. 

Insolation is a measurement of the sun's energy that reaches a specific area of the earth's surface. It is more accurate than sunlight hours as it takes into account the angle of the sun and various other environmental factors. Sunlight hours can easily be used where you are using less than 20 watts of load.

In the example scenario, we have 4 hours each day on average during the winter months. You can find out your area's sunlight hours from your local weather agency. The solar panel will need to be able to capture enough solar power to run your equipment for the day, as well as recharge your battery bank following a rainy or overcast period. It also needs to be able to do this during winter.

So if we have 4 hours of sunlight to capture 24 hours worth of power (924 watts), that's a basic requirement of a 231-watt solar panel array. (924 / 4 = 231 watts per hour). 

Now we need to factor in the recharging after a rainy or overcast period. If it rained for 4 days, and on the 5th day it was sunny, that's 5 days worth of power that needs to be captured. In the example that's 4620 watts worth of power. We also need to capture this power as fast as possible before it rains again. So to capture 4620 watts of power during 4 hours of sunlight, that means we need an 1155-watt solar array. If we wanted to, we could set a goal of recharging the batteries over 2 days. That means we have 8 hours to capture 6 days worth of power, which a 693 watt array would be able to do. If you decide to set a longer recharge period, you can save some money on solar panels, but will need to spend more on adding more capacity to your battery bank incase you only get one sunny day and it returns to rain.

It is also probable that if you have an average of 4 hours of sunlight per day in winter, that also means half the days will be rainy or overcast, and the other half would be sunny. This means that on the sunny day, there could be up to 8 hours to capture the required power instead of the 4 assumed above. It is best to oversize your panels just in case, so you should not rely too much on extra average sunlight hours from rainy days.

Types of Solar Panels

• Polycrystalline: Are cheaper to produce, but are not as efficient as mono. Panels are usually larger for the same watt rating.
• Monocrystalline: Are more expensive to produce but are more efficient than poly and can capture the more watts per square foot of solar cells.

Overcast Days: Some panels will still capture solar energy when there is a light overcast of cloud (bright white clouds) and they can sometimes work at up to 40% of the panel's rating. You may be interested in checking how your panel performs by using a multimeter and testing the panel's output on an overcast day. 

Next we will look at the bit that joins everything together.

The Solar Controller

A solar controller can perform 6 main tasks:

• Prevents power from the battery traveling back up and getting lost through the solar panel at night.
• Prevents the panels from overcharging the battery by disconnecting them when the battery is full.
• Gives an indication on the battery bank's State Of Charge.
• Stops the batteries from being discharged too much by disconnecting your devices when the battery SOC gets too low.
• Counts how much power you have generated and how much power your devices have used.
• Helps lower the amount of maintenance work required on your batteries by charging in certain ways.

Old solar controllers used to be called solar regulators. This is simply because they stopped the battery getting overcharged and would simply use a relay and volt meter to check when to disconnect or reconnect the panel.

You will want to make sure you get a solar controller that will display the state of charge for your battery bank. This helps diagnose problems if your tower stops working and your customers need it fixed urgently.

Sizing Solar Controllers

Your panels will have a maximum amp output current. If you have wired your panels in parallel, you will need to add together the maximum amp output current of each of them. You will want a controller that can handle at least this amount with plenty of capacity to spare. It may be wise to purchase a controller that handles double the current than what you need in case you wish to add more radios or panels in the future.

Also check if you have used gel or liquid flooded batteries and that the controller or regulator will work with your batteries. This is especially true if you use AGM or gel batteries. The controller needs to be set to charge in a specific way so that it doesn't cause high gas pressure inside the sealed battery. Flooded batteries need ventilation because they are not completely sealed and gas escapes when charging.

The Load Output

One important feature of a solar controller is the load output. This is where you connect your devices. You must be careful with this feature though. Some controllers assume it will be to automatically control lighting, therefore will switch on the load output in the evening and switch it off during the day. For our 24-hour radios, you need to make sure the load output can be switched on all the time and will not turn off by any automatic feature before purchasing the controller. Some controllers like the Steca brand pictured above-right, allow you to press a button to switch the load on or off, or use the menu to enable automatic functions such as turning it on x hours after darkness and off in the morning. The important thing is that it does allow the manual-only switching mode.  

Mounting the Panels

You will want to mount your panel so its surface is perpendicular to the sun's rays during winter. During winter, the sun will be on a lower angle than in summer. Because there is more sunlight hours in summer, we are not too concerned if the panel is less efficient because of its angle. But in winter, it makes more of a difference. Therefore, you should mount your solar panel so it is most efficient in the winter.

Connecting it all Together

Troubleshooting: The Battery Bank is Flat

So what do you do when you find its been raining for too long and your battery bank has gone flat? Ideally, you should have a few backup pairs of batteries to swap into to your bank to run your tower for 2 days.

The first step would be to check the solar controller. Some with LCD screens will show an error code to help you diagnose the problem. If it's just been raining too long then you will need to add another solar array and expand the size of your battery bank to prevent it from happening again.