Solar energy offers a renewable and silent way to charge batteries for off-grid applications. Pairing a properly sized solar panel with a charge controller and deep cycle battery allows you to leverage the free power of the sun. This article provides complete details on selecting components, setting up the system, and maintaining a 120 watt solar panel charging a 12V battery.
Introduction
A small 120 watt solar panel system is well-suited for keeping batteries charged in sheds, RVs, boats, and remote cabins lacking grid power. By installing the panel, charge controller, battery, and wiring properly, you can reliably run lights, appliances, and electronics without generator or shore power.
Key Components
The main components needed are:
- 120W Solar Panel
- 12V Battery
- Charge Controller
- Wiring and Connectors
Properly selecting and sizing these components is crucial for optimal performance. The subsequent sections provide solar panel and battery sizing calculations, charge controller features, wiring gauges, as well as installation and maintenance best practices.
Solar Panel Sizing and Selection
When pairing a solar panel to a battery, you first want to consider the battery’s capacity and voltage to determine the appropriate solar panel specifications.
Our example uses a commonly available 100Ah 12V lead-acid battery. Lead-acid batteries contain liquid electrolyte and lead plates to store power. The “Ah” rating refers to how many amps it can provide for 1 hour until depleted. Our 100Ah battery can supply 100A for 1 hour.
To prolong lead-acid battery life, limit depth of discharge to 50-80%. So our 100Ah battery has 50-80Ah of usable capacity before requiring a recharge.
Next, size the solar panel system to meet the battery’s voltage and deliver enough current to fully recharge it within a reasonable timeframe.
Panel Wattage
A 120 watt solar panel matches the 12V battery voltage and provides ample power for charging our 100Ah battery bank.
Wattage = Voltage x Amperage
So a 120W panel produces roughly 10 amps (120 / 12 = 10) in full sunlight. 10 amps exceeds our 100Ah battery’s usable 50-80Ah capacity, allowing it to fully recharge in 5-8 hours under ideal conditions.
Sizing the solar panel to deliver more current than the battery’s capacity shortens the recharge time. It also compensates for low sunlight days when the panel underperforms.
Alternative Panel Configurations
While a single 120W 12V panel suits most small battery banks, you can alter the solar panel setup to fit your needs:
- Series wiring – Use a 24V panel and wire two 12V batteries in series to create a 24V bank.
- Parallel wiring – Connect two 12V 120W panels in parallel to double available current.
- Higher wattage – Upgrade to 200W+ panels to charge larger battery capacities quicker.
Ensure your charge controller can handle the voltage, amperage, and wattage limits of the chosen configuration.
Panel Voltage Considerations
Solar panels produce varying current depending on lighting conditions and load. But nominal voltage stays consistent.
12V solar panels ranging from 10-22V match well with 12V lead-acid or lithium batteries. The charge controller regulates the higher voltages down and prevents overcharging.
Be sure your panel’s voltage aligns with the battery bank voltage to enable proper charging.
Panel Efficiency and Placement
Today’s panels utilize higher efficiency monocrystalline silicon cells (22% efficient) vs older polycrystalline (15-17%). Higher efficiencies produce more power from the same sunlight.
Mount solar panels on a pole, rack, or roof with full southern exposure at the optimal tilt angle. This maximizes light exposure and power generation.
Charge Controller Requirements
Connecting a solar panel directly to a battery can lead to battery damage from overcharging. A charge controller regulates the charging process to:
- Prevent battery overcharge
- Limit reverse current flow at night
- Monitor battery state
MPPT (Maximum Power Point Tracking) charge controllers are up to 30% more efficient than PWM types. They’re recommended for solar panels paired with 12V batteries.
Sizing the Charge Controller
Match the charge controller amperage to the short circuit current (Isc) from the solar panel specification sheet. Go slightly higher to allow a buffer.
Our example 120W panel has an 8.4A short circuit rating. So a 10A or 20A MPPT controller works well.
The voltage should match the battery bank (12V). Some controllers allow 24V or 48V connections.
Added Features
Higher end MPPT controllers add helpful features like:
- Battery temperature sensing
- Adjustable charging settings
- Remote monitoring options
- Battery state of charge indicators
At a minimum, choose an MPPT controller with the proper voltage, amperage limits and at least an LED display showing system status.
12V Battery Selection
Two popular rechargeable 12V battery options used for solar energy storage are:
- Lead-acid batteries
- Lithium-ion batteries
While more expensive upfront, lithium-ion batteries offer significant advantages for solar applications including:
- High efficiency – 95%+ vs 85% in lead-acid
- Longer lifespan – 6 times longer than lead-acid
- Lighter weight
- Lower self-discharge
But tried and true flooded lead-acid batteries provide reliable power at the lowest upfront cost.
Sizing Battery Capacity
Determine the battery bank size needed using amp-hour (Ah) capacity:
- Calculate total Amp draw from all loads
- Estimate hours per day of required power
- Amp-hours = Amps x Hours
Then multiply by days of autonomy desired. This provides reserve power for multiple cloudy days if needed.
Finally, adjust capacity down slightly to account for 80% depth of discharge limit in lead-acid batteries. This extends lifespan.
Matching Voltage
Match battery bank voltage to the solar charge controller and panel specs. 12V is typical for small solar installations.
Wiring Batteries in Parallel
To increase capacity, batteries can be wired in parallel. This combines the amp-hour ratings while retaining the same voltage:
- Connect positive (+) terminals together
- Connect negative (-) terminals together
Two 100Ah 12V batteries create a 200Ah 12V bank.
Electrical Wiring and Connections
Choose connecting wires and cables properly sized for the solar panel amperage ratings to avoid heat buildup and voltage drops.
Wire Gauge Rating
Select wire gauge based on the 3% voltage drop rule. This states total voltage loss across the wire should equal 3% or less.
Refer to voltage drop tables to cross reference the cable length and expected electrical current. This provides the ideal wire gauge rating.
For a 10A 120W solar panel, #12 AWG wiring works for runs up to 30 feet. Go up to #10 AWG for longer distances.
Weatherproof Connections
Use exterior rated wiring and waterproof connectors suitable for outdoor installation:
- MC4 connectors
- Weatherproof junction boxes
- Conduit for wire runs
Proper connections prevent shorts, fires, and system failures.
Step-by-Step Installation Guide
Once you’ve selected components, it’s time for installation. Follow these steps:
1. Mount Solar Panel
Securely mount the panel on a pole, rack, or roof with maximum sun exposure:
- Tilt panel 20-40 degrees to catch more winter sunlight if in northern hemisphere.
- Face panel true south for evenly distributed sunlight (northern hemisphere).
2. Install Charge Controller
Mount the charge controller near batteries using screws:
- Indoor/ventilated area protected from weather
- Clean, dry, climate controlled space optimal
3. Connect Batteries to Charge Controller
- Observe proper polarity – connect positive lead to charge controller positive terminal.
- Connect negative to negative.
4. Connect Solar Panel to Charge Controller
- Run wiring from solar panel into building.
- Connect positive solar panel wire to charge controller solar positive terminal
- Connect negative to solar negative terminal
- Ensure secure, weatherproof connections
Double check wiring polarity throughout. Reverse connections can severely damage components.
Operation and Preventative Maintenance
While solar panels and batteries require less routine maintenance than generators, following best practices improves safety and longevity:
Check Charge Controller Readings
Monitor display readings on the charge controller periodically:
- Battery state of charge – Should reach 100% on sunny days
- Battery voltage – Ranges from 11.5V to 14.5V typically
- Alerts – Cleared/addressed. May indicate faults.
Take corrective action if measures consistently low.
Prevent Battery Over-Discharge
Don’t allow batteries to sit in a low state of charge for extended periods. Recharge from solar or other source soon as possible.
Over-discharging shortens lead-acid battery life dramatically.
Inspect Electrical Connections
Check wiring for loose, corroded or damaged connections annually:
- Tighten loose screw terminals
- Clean oxidation from connectors
- Replace damaged wiring
Faulty connections cause system failures or electrical shorts.
Clean Solar Panels
Dirt, dust, snow, and debris block sunlight from reaching solar cells reducing power output.
Brush or wipe panels clean once per month or after snowstorms for maximum production.
Expanding the System Over Time
While a single 120W solar panel provides ample 12V battery charging for smaller applications, the modular nature of solar lends itself to system growth when needs increase.
Add More Panels
Connect additional 120W panels in parallel to produce more current and charge capacity. This setup features two panels:
The charge controller combines current from both panels to charge the battery bank faster.
Upgrade Batteries
Install larger capacity or additional parallel batteries over time as electricity demands increase. The charge controller regulates the higher loads.
Use Higher Wattage Panels
Replace panels with 200W, 300W or higher output models. The system easily scales up by the panel’s wattage rating.
Higher solar input combined with added batteries increases the solar system size to meet expanding electrical needs down the road.
Pros and Cons of Solar Battery Charging
Advantages of keeping batteries charged with solar:
- Renewable – Won’t run out like gas/propane
- Silent – Quiet compared to noisy generators
- Low maintenance – Batteries last 5-10 years
- Free sunlight – Saves money over utility power
Disadvantages include:
- Higher upfront cost than generators
- Space required – Batteries, panels take up room
- Power fluctuates – Based on changing weather
- Batteries wear out over years of use
On balance, solar energy provides a reliable means of meeting electrical needs in remote areas by leveraging renewable sunlight.
Conclusion
A 120 watt solar panel has ample capacity for effectively charging a small to medium-sized 12V battery bank. By properly sizing components, installing the system correctly, and conducting preventative maintenance, the battery will stay charged from renewable solar energy for years of service. This eliminates the need for shore power or generators for off-grid applications.