Okay, let's break down your calculations and address your questions about the charge controller and breaker. You're on the right track!Understanding the Issues Battery Capacity vs. Daily Needs

You're correctly calculating the total capacity of your batteries in Wh. The critical question is: what's the daily energy consumption of your gate opener? This determines how much energy you need to replenish each day.

Panel Output

Your calculation of 500Wh/day per panel is based on 5 hours of "peak sun". This is a good starting point, but remember that real-world sunlight is rarely at its peak for the full 5 hours. Cloud cover, panel angle, and seasonal variations will all affect output.

Charge Controller Selection

The charge controller's job is to efficiently regulate the voltage and current from the solar panel(s) to the battery, preventing overcharging and maximizing battery lifespan.

Safety Factor

You're right to include a safety factor.

Let's Review and Refine Your Calculations1. Battery Bank Sizing

You're moving from 3 x 35Ah batteries to 2 x 35Ah batteries. Let's calculate the total capacity: 2 batteries 35Ah/battery = 70Ah total capacity 70Ah 12V = 840Wh total battery capacity2.

Panel Output

Two 100W panels in parallel provide 200W. You've assumed 5 hours of peak sun, resulting in 1000Wh. That's the ideal scenario. You need to be more realistic. It will also affect how long it takes to charge your batteries from a discharged state.

Peak Sun Hours

A more conservative estimate is often 3-4 hours of effective peak sun hours, depending on your location and time of year. Let's use 3.5 hours for now: 200W 3.5 hours = 700Wh3.

Energy Balance

Now, compare your battery capacity to the expected daily solar energy input: Battery Capacity: 840Wh Daily Solar Input (Estimated): 700WhThis suggests that on an average sunny day with 3.5 hours of effective peak sun, you should be able to replenish most of the energy consumed by the gate. However, this depends entirely on how much energy your gate uses.4.

Charge Controller Amperage

This is where your question is focused.

Parallel Panels

You're correct; connecting panels in parallel maintains the voltage (around 12V-18V for charging a 12V battery) and adds the current.

Panel Current

Each 100W panel at 12V produces approximately 8.33 Amps (100W / 12V = 8.33A).

Combined Current

Two panels in parallel will ideally produce 16.66 Amps (8.33A + 8.33A).

Safety Factor

Apply a safety factor of at least 25% (multiply by 1.25): 16.66A 1.25 = 20.83A

Therefore, you need a charge controller rated for at least 21 Amps, but I would round up to a 30 Amp Charge Controller. I would suggest a MPPT (Maximum Power Point Tracking) type Charge Controller due to the increase in efficiency.5. Breaker/Fuse

This is for protection in case of a short circuit or overcurrent situation.

Consider the Panel Wire Gauge

Select the wire gauge able to handle the maximum current.

Sizing

A breaker or fuse should be sized slightly higher than the maximum expected current. In your case, 20.83A is your expected maximum. So a 25A or 30A breaker or fuse would be appropriate. Place it between the panels and the charge controller (closer to the panels).

Important Considerations and Recommendations Gate Power Consumption

Measure the power consumption of your gate opener. Use a multimeter to measure the current draw during operation (opening and closing). Then, multiply the current (in Amps) by the voltage (12V) to get Watts. Multiply Watts by the number of operating cycles per day, the duty cycle, and the total time the gate is actively opening/closing per day.

Battery Chemistry

You haven't specified the battery chemistry. Lead-acid batteries should not be discharged below 50% of their capacity for optimal lifespan. Lithium batteries can handle deeper discharges, but require specific charging profiles. Ensure your charge controller is compatible with your battery type.

Monitor Performance

After installation, monitor the battery voltage regularly. If the batteries are consistently running low, you'll need to either reduce the gate's usage or increase the solar panel capacity. A simple voltmeter is sufficient.

PWM vs MPPT Charge Controllers



PWM (Pulse Width Modulation)

Simpler and less expensive, but less efficient, especially when the panel voltage is significantly higher than the battery voltage.

MPPT (Maximum Power Point Tracking)

More efficient at extracting the maximum power from the panels, especially in varying sunlight conditions. More expensive, but often worth the investment. I recommend MPPT.

Location, Location, Location

Panel placement is critical. Ensure they are facing south (in the northern hemisphere) at the optimal angle for your latitude to maximize sunlight capture.

Revised Plan (Example - Based on Assumptions)1. Panels

Two 100W panels connected in parallel.2.

Batteries

Two 35Ah 12V batteries (70Ah total) - Choose a chemistry suitable for solar applications (e.g., deep-cycle lead-acid or lithium).3.

Charge Controller

30A MPPT charge controller, compatible with your battery chemistry.4.

Breaker/Fuse

25A or 30A breaker or fuse between the panels and the charge controller.5.

Wiring

Use appropriately sized wire (gauge) for the current and distance.6.

Monitoring

Voltmeter to check battery voltage regularly.

Key Takeaway:

The most important factor is knowing the power consumption of your gate opener. Without that, you're just guessing. Once you have that number, you can accurately size the solar panel and battery system. And remember, err on the side of slightly more* solar capacity than you think you need, as it's better to have excess energy than to run out. Flag for review