How to Choose Portable Solar Panels for Off-Grid Living

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You have seen those ads, those campaigns, why you should switch to renewable energy. Over the decade, solar power and solar photovoltaic (PV) panels have stood out as the most promising form of renewable energy. The cost of acquiring solar panels has dramatically reduced, but deciding to choose a suitable solar panel for your use and capacity has become no easier. I wrote this article to reduce the burden and help you navigate those uncharted waters in choosing a solar panel and going off-grid.

 

Factors to consider in going off-grid

  • Energy Requirements:

Determining your daily energy consumption is a crucial first step. Energy consumption is measured in watt-hours or kilowatt-hours per day. You would have to determine the total energy consumption for all your equipment. For example, assuming your equipment are a 60W light bulb, your iPhone which consumes on average 15W, and your notebook consumes 35W to get fully charged. Your total consumption amounts to 110 watts. You can get this information by visiting the website of the manufacturer or the manual of the equipment. 

 

  • Evaluating your site location: 

Although The Sun releases an estimated 384.6 yotta watts (3.846×10^26 watts) of energy, much of this in the form of light and other types of radiation, the intensity of sunlight or available solar energy will vary from location to location. Insolation is used to describe the amount of incoming solar radiation that reaches the earth's surface. You must have an idea of the amount of sunlight you would be getting on any given month. On average, in the US, June is the month with most sunshine and December, the month with the least. Arizona and California receive the most sunlight while Wisconsin and Washington receive the least throughout the year. The National Renewable Energy Laboratory (NREL) provides an online resource for mapping available solar radiation https://www.nrel.gov/gis/solar.html. You want to size your system based on the month with the highest power consumption and/or lowest solar resource, usually December or January for most folks in the United States. We will assume 3 hours of sunlight during winter months as our worst-case scenario.

 

  • Determining the size of the battery bank:

 The battery bank in your inverter stores energy during peak-period for use off-peak periods. There are two main kinds of batteries: lead-acid and lithium. Lead-acid batteries have a lower upfront cost, while lithium batteries have the most extended lifespan. Lead-acid batteries are the most commonly used batteries. Determining how much solar power to keep it fully charged is critical. Accounting for the various losses:

  • The inefficiency of the inverter ranging from 5 – 20%. We'll assume a 15% loss from our inverter, i.e., 1.15factor.
  • Effects of temperature on battery capacity. Most batteries have lower efficiency at lower temperatures. We'll account for a 1.50factor for loss of efficiency of our batteries during lower temperatures of the winter months.
  • Loss of efficiency from charging and discharging of current from our batteries. We will assume a 15% loss for this. i.e., 1.15factor.

For a single day of reserved power for our inverter, we will need a total capacity of:

110W x 1.15 x 1.50 x 1.15 = 218 Wh

So, for a 110 Wh capacity for our equipment, we'll need batteries with a capacity of 218 Wh daily. Batteries are usually rated in Amp-hours (Ah). To convert watt-hours to Amp-hours, divide watt-hours by the system's battery voltage. For example:

For 12V, we will need battery a battery rated 18.2 Ah per day

For 18V, we will need battery a battery rated 12.1 Ah per day

Batteries are expected to last between 7 – 14 years if maintained properly. It is desirable to protect them from temperature extremes. Too high temperatures, you get higher efficiency but lower usage life. Too low temperatures, you get lower efficiency but longer usage life. The optimum temperature for your batteries and Solar PV Panel is 77F(25C). Lead-acid batteries need to be fully charged regularly. They require around 10 Amps of charge current per 100 Amp-hours of battery capacity for optimal life. If lead-acid batteries are not recharged regularly, they will likely fail, usually within the first couple of years. Not discharging your lead-acid batteries below 50% is a good standard practice to preserve its useful life. The maximum charge current for lead-acid batteries is typically around 20 Amps per 100 Ah (C/5 charge rate, or battery capacity in Amp-hours). Always refer to the battery specifications and user manual to confirm the charging guidelines. Not adhering to the guidelines will typically void your battery warranty and risk premature battery failure.

 

  • Figuring Out The Number of Solar PV Panels You Might Need:

Now that we have determined your battery capacity, we can size the charging system. The charging system needs to have enough capacity to fully replace the energy drawn out of the battery while also accounting for all efficiency losses. For our example, based on 3 hours of sunlight and 218 Wh per day energy requirement:

We will need a 73 W PV array size. Accounting for 10% loss of our Solar PV panels: 

73 W / 0.9 = 81 W. 

This means we will need 81 W minimum size for the Solar PV array. A broader PV array will make the system more reliable, especially if no other backup source of energy, such as a generator. Combining multiple portable systems and connecting them to a single solar controller is also an option to consider if you will be needing a higher capacity. The cost of a few extra solar panels is a good investment since your solar panels would require less maintenance over time if they never got overworked to capacity. Starting small and expanding later, depending on how your system performs is a good strategy. Your system size will ultimately be dependent on your total energy consumption, your site location, and how frequently you run batteries on off-peak periods.

 

  • Solar Charge Controllers:

Solar charge controllers regulate the amount of voltage that gets transferred from the RV solar panels to the house batteries. The charge controller will read what voltage the battery cells are at. Then they will change the voltage that is going into the batteries from the solar panels depending on the current state of charge. Charge controllers come in two 'flavors' - PWM (pulse width modulation) and MPPT (maximum power point tracking). PWM controllers are less efficient but are also cheaper than MPPT controllers. Decent solar charge controllers will have an LCD/digital display that will show you what is happening with the controller and batteries. It tells you voltages, amperage, power usage, and probably more information than you care about. They also have LEDs that indicate charge modes, faults, and other details.

 

Why choose our ROCKMAN 120W/18V Solar Panel

Not only does our ROCKMAN 120W/18V Solar Panel allows you to live in peace but it is also foldable, easy to install, and one of the most competitive solar PV on the market. It is suitable for backpacking for outdoor activities, including camping, road trips on your RV. 

 

Specifications of the ROCKMAN 120W/18V Solar Panel:

  • Peak Power: 120W 
  • Cell Efficiency: 23%
  • Power Voltage: 18V
  • Power Current: 6.6A Max
  • Open Circuit Voltage: 19V
  • Short Circuit Current: 6A
  • Lifecycle: 10+ Years 
  • Warranty: 24-Month
  • QC 3.0 USB Port: 9V/2.5A
  • Output Wattage 60W-72W (50%-60%)
  • Full Recharge Time: Rockman 200 - 3~4 Hours (with Full Sun)

Rockman 300 - 5~6 Hours (with Full Sun)

Rockman 500 - 8~9 Hours (with Full Sun)

Having a portable Solar PV Panel deployed on the ground that is tilt-able and can be pointed directly at the sun at the optimal angle is beneficial during months with less sunlight when the sun tilts at an angle. A ground-deployed portable solar panel also lets you camp in a shaded area in the afternoon, while the Solar PV batteries are still getting charged.

 

Caring for your Solar PV Panel:

  1. Dust and dirt on the surface should be cleaned using a soft brush. Use a damp cloth to wipe the surface of the solar module to remove any remaining dust or dirt. Any guano or other adhesive avoids performance reduction since a solar panel system needs to absorb energy from the sun
  2. To Maximize the lifespan of the module, avoid getting the package wet. Although your solar PV panel is water-proof, preventing corrosion or erosion for your system is vital for its longevity.
  3. Annual Inspection or preventative maintenance can go a long way towards the life of your solar system. 

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