Off-Grid Solar Power

Matt Blank | KE7NOR

July 2024

Solar Power at Field Day

2024 was the first attempt at “crowd-sourced” solar power for Field Day.

The clubs came together and contributed panels, charge controllers, and batteries. We were a bit short of MPPT charge controllers and a local business, MidNite Solar, was able to help us out.

I set about 1300 W worth of panels out flat on the field, which was quite effective through the day since we’re in summer. Facing the sun is best for max output, but it was a good compromise not having to move them. Four 100W panels were dis-similar, so derated to the lowest current in this case.

Four charge controllers were set up in a weatherproof case to protect them from the elements, each one feeding into a common bus fuse panel. The bus was tied to a pack of 12V LFP batteries equivalent to 700 Ah, or 8.96 KWh.

The battery monitor wasn’t calibrated, so we didn’t have a good real-time picture, but when the system was disconnected Sunday morning, after running overnight Saturday it was sitting at around 65-70% SoC. That puts our overnight consumption at just under 4 KWh, for a period of around 12 hrs.

During full sun, the system was netting around 800 W (into batteries), which means we had a good margin for continuous operation. Of course, if this had been in winter, we might have needed 2-4X as much (rated) power to maintain that.

The end result was a very reliable, quiet, and clean solution that I think we’re looking forward to iterating on.

Quentin Caudron, K7DRQ

Why Solar Power?

Why Solar Power?

• Reliability in Disasters: Solar power provides a reliable energy source when the grid is down.
• Portability: Ideal for field operations and remote locations.
• Sustainability: Renewable and eco-friendly energy. Take advantage of the energy hitting the ground every day.
• Maintenance: PV equipment has no moving parts or consumables. The equipment can last for decades with little maintenance.

Key Equipment

Key Equipment

• Solar or PV (PhotoVoltaic) Panel: Converts light into electricity.
• Charge Controller: Converts the electricity from panels to optimize efficiency and protect your battery and electronics.
• Battery: Provides a buffer for high-power devices and night use.
• Inverter: Converts DC to AC for common appliances.
• Circuit Protection: Crucial at different points of the system to prevent arcing and short circuits.

Different Types of PV Panels

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• Rigid (Lowest cost and highest efficiency):
• Monocrystalline: Made of a single silicon crystal, higher efficiency, and higher cost. Have a consistent, dark color. The “single-malt Scotch” of cells.
• Polycrystalline: Made of a mix of silicon crystals fused together, these are less efficient and lower-cost. Usually a mix of light blues with a speckled look.
• Thin-Film: Flexible and lightweight, but lower efficiency. These may be used for mounting on curved surfaces where a sleek look is desired, but beware of UV damage. The plastic may turn cloudy and yellow leading to even less output
• Bifacial: Can generate power from both sides of the panel. Especially beneficial when mounted farther from a surface, where light can reflect onto the back side. Generally these have a clear “backing” allowing you to see around the cells.

Charge Controllers

Allows PV panels to operate at their “maximum power point”. Batteries are typically 12.8V, 25.6V, or 51.2V while solar panels produce their maximum power at 18-36V depending on the number of cells.

• PWM charge controllers are low-cost devices, which rapidly connect and disconnect the solar input to the battery to regulate the voltage and current in order to safely charge batteries. They work best when the battery and PV voltage are close. They should not be used with series strings and 12.8V batteries.
• MPPT charge controllers work like buck regulators, providing a constant power output, and adjusting their input automatically to match the maximum power point of the panel IV curve as lighting conditions change. They usually operate with higher input voltages, starting at a few volts above the battery voltage ranging up to several hundred volts. Most are programmable and work with various battery voltages and chemistries.

Batteries

Batteries are a crucial part of your system.

PV is by nature, variable. Even on a bright, sunny day, panels can be partially or completely shaded by various obstructions like vegetation, animals, aircraft, etc. If an entire cell or panel is shaded, the output could drop below the useful threshold immediately. In addition, transient demands might briefly exceed the maximum current of your panels.

These days, LiFePO4 is by far the best choice for most applications. It’s not quite as energy dense as other Li-Ion chemistries, but it’s lower nominal voltage and safety make it an excellent upgrade from lead-acid. It has excellent cycling characteristics and round-trip efficiency, and the capacity is much more predictable since it doesn’t suffer from the “Peukert effect”.

Also referred to as LFP, they’re commonly available in many sizes and getting more affordable all the time.

Inverter

Inverters are not necessary if your loads are designed for low-voltage DC like a lot of amateur radio and computer equipment.

Of course, AC appliances are ubiquitous and having one in your kit is not a bad idea, even if it’s a smaller one.

When it comes to inverters and electronics, especially radio, it’s best to stick with a pure-sine inverter. “Modified sine” inverters output more of a square wave, with jagged edges. This causes many devices to buzz and run hotter. Motors will struggle.

It’s also advisable to stick to UL and ETL compliant devices for the cleanest output.

Circuit Protection

No matter the size of your system, you should consider how to protect against faults. Batteries should always be fused as close to the battery as possible.

Solar panels may use an in-line fuse, which are available as a one-piece MC4 unit.

For higher voltage strings (above 20V) it’s a good idea to use a double-pole circuit breaker designed for PV disconnect. This allows you to manually disconnect without the risk of an electrical arc.

Fuses and circuit breakers should be sized about 125% of your expected load, and wiring should be sized to keep voltage drop below 3%. For example, with a 20A load you can run 14 AWG wire up to 6’, 12 AWG up to 10’ and stay within the 3% limit. With this in mind, you will get the most out of your equipment.

Connections

Understanding Solar Panel Ratings

• Power Rating (W): The maximum power output of the panel under standard test conditions (STC), typically measured in watts (W).
• Voltage at Maximum Power (Vmp): The voltage at which the panel produces its maximum power output.
• Current at Maximum Power (Imp): The current the panel produces at its maximum power output.
• Open-Circuit Voltage (Voc): The maximum voltage the panel produces with no load connected.
• Short-Circuit Current (Isc): The maximum current the panel can produce when the output terminals are shorted.
• Nominal Operating Cell Temperature (NOCT): The expected operating temperature of the panel under normal conditions.
• Temperature Coefficients: Indicates how the panel’s performance changes with temperature, typically given for power, voltage, and current.

Basic System Wiring

This is a simple overview of how components are connected. Note, it does not include circuit breakers or fuses, but these are important.

The power to run your devices is provided by the battery and energy to charge the battery is provided by the panel.

The inverter is representative of your load. They are often included because many loads are AC, but instead of an inverter you could also connect your low-voltage DC devices like a radio directly to the battery.

The charge controller takes the dynamic output from the solar panel and safely charges the battery.

From Newpowa

Series/Parallel Wiring

Multiple panels can be wired in series or parallel, depending on your specific situation.

I recommend keeping MC4 connectors when they are provided, because they make series connections very simple, and there are options for parallel connections also.

Series

Simplifies wiring by creating one continuous loop, perfect for taking advantage of the standard MC4 connectors and minimizing cable losses (no increase in current). Usually requires an MPPT controller with higher input voltage.

In Parallel

The sub current of all panels in a parallel string is added together, and you will need to be sure your wiring can handle this extra current.

Combining Different Panels

It’s possible to combine different models of panel together, even different brands or styles, but keep in mind:

When connected in series, the total voltage will equal the sum of the panel voltage, but current of the system will be limited by the lowest current panel.

When connected in parallel, the total current will equal the sum of the panel current, but the effective system voltage will be reduced to that of the lowest voltage panel.

This should be avoided, if possible. For permanent installations, each controller should have a balanced series/parallel set of identical panels.

When combining the outputs of charge controllers in parallel, make sure they are all configured for the same battery/charging voltages before connecting. If they are auto-detecting, connect one at a time to the battery before connecting your PV.

Other Considerations

Ultra-Portable

• If your goal is to carry a system for POTA/SOTA use, you will want to carefully plan out your loads, and select the smallest components that will provide a good experience. You may not even need to bring a solar panel unless you’re planning to operate for an extended period.
• Bioenno batteries are pricey, but offer uniquely small form-factors. If you need 10Ah or more, the alternatives start to look a lot better. Their smaller packs have separate charge/discharge leads, so make sure you have the right plugs to connect everything.
• Small, packable solar panels exist, between 40-60W rating, but these often struggle to achieve their rated performance even in bright sunlight. Make sure to test your gear at home before you take it into the field.
• Some small panels also include USB ports for directly charging portable devices like phones and tablets.
• Genasun and Powerwerx offer some pocket-sized MPPT charge controllers that are great for packing in a backpack and charging with 100W or less panels.
• I don’t recommend purchasing any panels that are rated for less than 30W, or they will struggle to charge even a modern phone.
• Also avoid small power banks, flashlights, and “emergency” radios with built-in solar panels. They are too small to be effective, and charging devices in direct sunlight is bad for batteries.

All-In-One Solutions

There are a number of “power stations” that combine batteries, MPPT chargers (and high-power AC usually), USB, and pure-sine inverters. These get more advanced and cheaper all the time.

Commercial power stations are very difficult to beat with DIY components in terms of size, but the price does scale up greatly, and they are more difficult to expand. Repairs may be very difficult or expensive, though there isn’t a lot of data on this yet. They seem to be essentially consumable devices - but if you take care of them they work well.

For $1000 or less you can get >1KWh of batteries, multiple USB ports, regulated 12VDC out, 500W or more PV charging, and an inverter powerful enough to run any household appliance.

It may not be as fun as building your own, but like a Baofeng, it couldn’t hurt to have one either.

Weather

The amount of electricity you can generate will vary widely with lighting conditions. This would seem obvious, but what may not be obvious is that on an overcast day you might only get 10% of what you would on a sunny day. If it’s raining or close, more like 1%. This means a 100W panel would barely be able to charge a cell phone.

Plan for the weather and size up as necessary. It’s fine to run more wattage in PV panels than your MPPT controller is rated, because it will only draw up to it’s capacity. Adding a few extra in parallel can get you more usable power if the lighting conditions are bad.

Most panels, especially rigid glass and aluminum ones, are extremely durable and can withstand rain and hail. Generally it’s fine to leave them outside as long as the power connectors aren’t open. You should protect your charge controllers and batteries though.

Something else to consider is wind power, many MPPT chargers can also be connected to a small wind turbine. In stormy conditions this might provide a nice backup plan as long as you’ve got the right equipment in place.

Questions?

Links

https://diysolarforum.com/ - Discussion forum all about solar power

https://www.renogy.com/ - Good, low-cost batteries, solar panels, charge controllers, and inverters. Range of products from portable to RVs and buildings.

https://richsolar.com/ - Very similar product range to Renogy, another good option.

https://www.litime.com/ - Good, lower-cost LFP batteries. They also sell charge controllers, inverters, and accessories

https://powerurus.com/ - Similar to LiTime, mostly batteries, some chargers and accessories.

https://powerwerx.com/ - Many low voltage wiring supplies, small and cheap MPPT controllers, adapters, PowerPole connectors and tools.

https://signaturesolar.com/ - Higher-power supplies, good source for large panels, 48V batteries and off-grid/hybrid inverters

https://www.currentconnected.com/ - Similar to Signature Solar, carries some higher-end brands like Victron and MidNite Solar

https://batteryhookup.com/ - Surplus batteries and referral links. Excellent prices on a rotating selection of used and new batteries and cells. Amazon referral links to related products to help filter out scams.