What Size Solar Panel to Charge a 35Ah Battery?

By Pence July 24, 2023 Solar Panels 0 Comments

Solar power is an efficient and sustainable way to generate electricity for your home, RV, boat, or any other off-grid application. A key component of any solar power system is the solar panel and battery setup.

Choosing the right size solar panel to meet your energy needs and properly charge your battery bank is crucial. In this comprehensive guide, we will examine how to select the proper solar panel wattage to effectively charge a 35Ah battery.

When building a solar system, one of the first steps is calculating your energy consumption needs. This will determine the battery bank size and capacity required. Once you know your battery bank specifications, you can then determine the appropriate solar panel size.

Some key factors to consider when selecting a solar panel for your 35Ah battery are:

Battery bank voltage – Most batteries operate at either 12V or 24V. The solar panel voltage must match the nominal operating voltage of your battery.
Battery capacity – The battery’s amp-hour (Ah) rating determines how much energy it can store. For this example, we will use a typical 35Ah battery size.
Charge controller – The controller regulates the solar panel power to safely charge the battery. It prevents overcharging and controls the charging stages.
Hours of sun – The solar radiation available in your geographical area impacts charging performance. More sunlight equals faster charging.
Future expansion – If expanding your system later, size the solar panel for the increased load.

Choosing the optimal solar panel wattage provides enough power to efficiently charge your 35Ah battery while avoiding significant oversizing which adds unnecessary cost. Continue reading as we examine sizing calculations and recommendations in detail.

Factors that Determine Solar Panel Size

Selecting the correct solar panel size for your battery involves more than simply matching the battery’s Ah capacity. To choose the proper solar panel setup, you need to consider these key factors:

Battery Bank Voltage

Solar panels must match the nominal operating voltage of your battery bank. Most lead-acid batteries provide a 12V nominal voltage. However, batteries can be configured into banks increasing the voltage to 24V, 48V or even higher.

The solar panels you select must have an output voltage corresponding to your battery bank. If your batttery bank is 24V, you would use 24V solar panels wired in parallel.

Battery Capacity

The battery’s amp-hour capacity determines how much energy it can store. Using our example battery size of 35Ah, this battery can deliver 35 amps for 1 hour before being fully discharged.

Higher capacity batteries allow more energy storage and longer runtimes, but require more solar power and time to charge.

Depth of Discharge

You should only discharge your lead-acid batteries around 50% to prevent damage and maximize their lifespan. The term depth of discharge refers to the percent of battery capacity removed during usage.

For our 35Ah battery example, this means we should only use around 17.5Ah (50% of 35Ah) before recharging. The solar panel system must be able to replenish this amount on a daily basis.

Charge Controller Specifications

The charge controller regulates power from the solar panels to the battery bank. It ensures the batteries are safely charged in three stages (bulk, absorption, float) and protects against overcharging.

Check the charge controller’s input voltage rating and maximum current/wattage. The solar panels must match these specifications.

Hours of Daily Sunlight

The amount of peak sun hours available in your location directly impacts solar charging rates and size requirements. More sunlight equals faster charging which allows you to use smaller panels.

As a baseline, 5 peak sun hours per day is average. If you live in an area with less sunlight, you may need to compensate with a larger solar panel system.

Future Expansion

If you plan to expand your battery bank in the future, size your solar panels to meet the increased charging demands. Buying slightly larger panels now leaves room for growth while minimizing costs over time.

Calculating Recommended Solar Panel Size

Sizing a solar system involves some key calculations. Let’s walk through an example using our 35Ah battery to determine the recommended solar panel wattage.

For this exercise, we will assume:

12V battery voltage
35Ah battery capacity
50% max depth of discharge
5 peak sun hours per day
20% safety margin

Step 1 – Determine Total Daily Energy Need

First, calculate your total daily energy requirement in watt-hours (Wh) using this formula:

Battery capacity (Ah) x Depth of discharge (%) x Voltage (V)

For our example:

35Ah x 50% x 12V = **210 Wh/day**

This tells us we need 210 watt-hours per day from the solar panels to sufficiently charge our 35Ah battery bank.

Step 2 – Factor in Sunlight Hours

Next, determine the minimum solar panel wattage needed by dividing the total daily Wh by the peak sun hours:

Total Wh / Peak sun hours

For our example with 5 peak sun hours available:

210 Wh / 5hrs = **42W**

This means in ideal conditions, a 42W solar panel would fully charge our 35Ah battery from 50% discharge in 5 hours.

Step 3 – Add Safety Margin

It’s recommended to add a 20% buffer as a safety margin in your calculations to account for inefficiencies such as dust, high temperatures, aging panels, etc.

To add 20%:

42W x 1.2 = **50W**

Factoring in the safety margin, a 50W solar panel should sufficiently charge our 35Ah battery bank daily.

Solar Panel Size Recommendations

Based on the sizing calculations above, here are some solar panel recommendations for a 35Ah battery bank:

50W panel – This is the minimum size recommended using the 20% safety margin. A 50W panel should charge a 35Ah battery from 50% discharge each day with 5 sun hours.
100W panel – Doubling to a 100W panel leaves ample overhead but is still reasonably sized and affordable. It will recharge the battery faster with 2-3 hours of direct sunlight.
200W panel – Oversizing too far past the 100W range typically offers diminishing returns for the added cost. However, a 200W panel would provide maximum battery charging speed which is useful if expansion is planned.

Other factors like shading, panel angles, and temperature can impact real-world performance and may require further oversizing your solar panels.

Generally, a 100W panel offers a good balance of affordability, performance, and safety margin for a 35Ah battery bank. But your specific energy usage and sun availability may change the ideal size.

Charging a 35Ah Battery with a 100W Solar Panel

To demonstrate a real world scenario, let’s examine how a commonly used 100W solar panel would perform when charging our example 35Ah battery.

A 100W panel with a 12V nominal output voltage provides approximately 8.3A of charge current in optimal conditions. This can be calculated as:

 Panel wattage / battery voltage
100W / 12V = 8.3A

With 8.3A of charge current, here is an estimate of the 35Ah battery charging times from 50% discharge:

Sunlight Hours	Charge Current	Charging Time
2 hours	8.3A	4 hours
3 hours	8.3A	2.5 hours
4 hours	8.3A	2 hours
5 hours	8.3A	1.5 hours

As you can see, the 100W solar panel can fully recharge the 35Ah battery from 50% discharge (17.5Ah to replace) in just 2-4 hours depending on sun availability. This provides a nice safety buffer.

Factors like battery chemistry, temperature, and other losses will increase these charge times slightly. But a 100W panel provides ample capacity in real-world conditions.

Selecting a Solar Charge Controller

When pairing a solar panel to your battery bank, you need a charge controller to manage the charging process and protect your batteries. Here are some charge controller requirements for a 100W solar panel and 35Ah battery:

Voltage – Match charge controller to battery voltage (ex. 12V or 24V)
Max Input Power – Controller must handle the solar panel max watts (ex. 100W)
Max Charge Current – Controller capacity should exceed solar panel amps (ex. >8.3A for 100W panel)
Temp Compensation – Prevents overheating and boil over
Lightning Protection – Protects from surges and static discharge

Some suitable solar charge controller options for a 100W panel and 35Ah battery include:

Renogy Wanderer 30A – Rated for 12V systems and 400W max solar input. Has temperature compensation and lightning protection. Can parallel multiple controllers to handle larger systems.
Victron SmartSolar MPPT 100/30 – Supports 12V and 24V systems. 100V max input voltage and 30A charge current rating. Advanced MPPT technology for optimal performance.
EPEVER Tracer 3215AN – 15A controller good for up to 200W solar input. Affordable MPPT controller with temperature sensor and LCD display.

Always size your charge controller to adequately handle your specific solar panel and battery bank ratings for optimal performance and safety.

Factors that Impact Solar Charging Rates

While we used some generalized assumptions in the charging examples above, several factors impact your real-world solar panel charging performance:

Sunlight Intensity

The intensity of solar irradiation determines the power output from your panels. Cooler climates receive fewer peak sun hours per day, slowing charging. Positioning and angles are also important.

Battery Chemistry

Lead-acid batteries charge slower and less efficiently than lithium-ion designs. Refined battery technologies like LiFePO4 provide faster charging capabilities.

Temperature

Colder battery temperatures slow down chemical reactions and impede charging. Hot temperatures can cause overcharging if not controlled properly by the charge controller.

Dust and Shading

Dirty panels or obstructions like trees will cause partial shading and reduced power generation throughout the day. Proper panel cleaning and positioning is key.

DC-DC Charging Losses

Inefficiencies in wiring and power conversion from DC-DC reduce total energy delivered to your batteries. Oversize your system to compensate for these losses.

Battery Age

As batteries degrade over time, their effective capacity and charging rates will drop. Older batteries require more time to fully charge.

All these factors demonstrate why it’s important to provide a safety buffer in your solar panel sizing assumptions during system design.

Best Solar Panel Angle for Your Location

Optimizing the tilt angle of your solar panels can maximize their power output throughout the year. The best angle depends on your latitude and seasonal sun path at your location.

Some tips for solar panel tilt angle:

At latitudes below 25° N/S, use a tilt = latitude
At higher latitudes above 25° N/S, use a tilt = latitude – 10°
Face panels towards equator (south in northern hemisphere, north in southern hemisphere)
Adjust angle 2-4 times per year if feasible
Flat roofs: Tilt at 10-25° works year-round

Also consider shading obstacles and height of the sun above horizon during peak seasons. Position panels to receive maximal sunlight for most of the day.

Advantages of Larger Solar Panels

Installing a solar panel that’s larger than the minimum size needed comes with several benefits:

Faster battery charging – More wattage and current results in reduced charging times.
Handles weather variability – Clouds and rain impacts are minimized with buffer capacity.
Increased energy harvesting – Bigger panels offset losses and convert more sunlight overall.
Future expansion capability – Additional batteries can be added without resizing the panels.
Battery longevity – Full recharges and avoiding deep discharges extends battery life.
Emergency power reserves – Excess solar power during the day creates a backup supply buffer.
Supports larger loads – Runs more devices and appliances with ample solar input.

The marginal cost increase of a larger panel is usually worthwhile for the performance advantages. Oversizing the solar array acts as insurance against unpredictable weather.

Maximum Power Point Tracking (MPPT) Solar Chargers

A maximum power point tracking (MPPT) solar charge controller can further optimize energy harvest from your solar panels.

MPPT technology constantly adjusts the electrical load on the panels to force operation at the peak voltage and current point. This maximizes the wattage extracted from the panels to charge the battery.

Benefits of MPPT vs regular PWM solar chargers:

10-30% more energy conversion efficiency from panels.
Faster charging and better regulation of battery.
Allows higher voltage panels to charge low voltage batteries.
Works with panels of different wattages and types together.
Handles environmental fluctuations better.
More intelligent battery maintenance and protections.

MPPT controllers cost more but usually pay back through increased solar power utilization. They’re a smart investment for larger solar systems.

Expanding Solar System Capacity in Future

As your power needs grow over time, there are couple straightforward ways to expand the solar system capability:

Add more solar panels – Increasing the number of panels adds more wattage and current. Wire additional panels in parallel to existing array. Requires a charge controller sized for higher input current.
Install larger panels – Swap out existing panels for higher wattage models. This is simplest if space limits additional panels.
Add more batteries – Expand total energy storage capacity. Requires increased solar input to keep additional batteries charged.
Upgrade to 48V system – Shift to a 48V battery bank instead of 12V. This allows handling more power with smaller cables. But requires different panels, charges and inverter.

A properly designed system makes incremental expansions easier in the future. With thoughtful planning, the solar array can grow to meet increasing electrical demands down the road.

Creating a Solar Sizing Calculator Spreadsheet

Determining the right solar panel size for your application requires some simple calculations using key system parameters.

You can streamline the process by creating a customized solar sizing calculator spreadsheet. Here are the steps to set one up:

In a blank spreadsheet, create columns for battery voltage, capacity (Ah), peak sun hours, system loss %, etc.
Input the specifications of your battery in the respective columns. For example, 12V and 35Ah.
In another column, use formulas to calculate wattage, amperage, wire sizing, etc. based on the inputs.
Create a results summary section showing recommended solar panel wattage, voltage, current, etc.
Allow input cells to accept different values. Add conditional formatting to highlight results.
Save this as a solar sizing calculator template that you can reuse for different systems.

With a custom spreadsheet, you can quickly run through various scenarios to identify the optimal solar panel sizing to charge your batteries. It’s a handy tool for both design and future expansion planning.

Adjusting key assumptions like peak sun hours or system losses automatically updates the solar panel requirements. This allows you to run different configurations to find the best fit.

Common Solar Panel Sizing Mistakes to Avoid

When calculating the required solar panel size, there are some key mistakes that can lead to problems:

Undersizing the wattage – This can lead to chronic undercharging of the battery.
Overestimating sun-hours – Results in undersized panels for the location.
Ignoring losses – Undercompensates for real-world efficiency losses.
Mismatching battery voltage – Leads to ineffective charging and equipment damage.
Using old solar radiation data – Outdated average sun-hour values give incorrect sizing.
Not considering shading – Fails to account for shadows that will reduce energy yield.
Assuming “more is better” – Excessive solar input can overload the battery.
Not verifying wire ratings – Undersized cables risk overheating and fire hazard.
Not confirming charge controller limits – Exceeding current/power rating damages the controller.

Double checking your design assumptions and having safety margins in the calculations will avoid these missteps and their consequences.

Expanding Your Solar System

If you need to expand your solar panel system later to handle increased loads, it’s relatively straightforward. Here are some tips:

Use a charge controller that allows paralleling multiple units. This enables system expansion over time.
When first installing, leave room on your battery bank racking to add more batteries in the future.
Plan wiring runs during initial construction to support additional panels. Use conduit where possible.
If roof or ground mounting, ensure the structure/rack can handle extra panel weight when expanding.
Oversize components like wire gauges and fuses so they don’t need replacement when adding panels.

With smart planning and strategic component selection, you can design a modular solar power system that can grow over time as energy demands increase.

Frequently Asked Questions

How long does it take to charge a 35Ah battery with 100 watt solar panel?

With a 100W solar panel and optimal conditions, it will take approximately 2-4 hours to recharge a 35Ah battery from 50% discharge, depending on the amount of sunlight. More peak sun hours per day will result in faster charging.

How many watts is a 35Ah battery?

A 35Ah 12V battery has 420 watt-hours of capacity. This is calculated by amp-hours x voltage, or 35Ah x 12V = 420Wh. The watt-hours indicate total energy storage.

How many solar panels do I need to charge a 50Ah battery?

Using the same sizing formulas discussed above, a 50Ah battery would require roughly a 70 watt solar panel minimum, or 140 watts with a 100% safety margin. The exact number of panels depends on the wattage of the models selected.

How do I calculate what size solar panel I need to charge my battery?

Determine your total battery capacity in amp-hours
Estimate your average daily depth of discharge
Multiply capacity x discharge % x voltage to get daily watt-hours
Divide by average peak sun hours to get minimum solar wattage needed
Add a 20-40% buffer for your safety margin

How long does it take to charge a 35Ah battery?

The time to charge a 35Ah battery depends on many factors like charge current, battery type, temperature, and more. As an estimate, charging a 35Ah battery from 50% discharge would take:

5A charge rate – 7 hours
10A charge rate – 3.5 hours
20A charge rate – 1.75 hours

How many amps is a 12V 35Ah battery?

Amps is a measure of current rather than capacity. The maximum current a 12V 35Ah battery can deliver is 35A. However, the recommended continuous discharge current should be no more than C/5 or 7A.

What is the voltage of 35Ah battery?

The voltage of a 35Ah battery depends on how many cells are wired in series. Common configurations are 12V or 24V. You would need multiple 12V 35Ah batteries together to achieve higher voltages like 48V.

How many volts is a 35 amp battery?

Again the voltage depends on the number of cells and battery configuration, not the amp rating. Typical voltages for 35 amp batteries are:

Single 12V battery
Two 12V batteries in series = 24V
Four 12V batteries in series = 48V

How many amps is a 35Ah battery?

The amp-hour rating refers to capacity, not maximum current. A 35Ah battery can deliver 35 amps for 1 hour before being discharged. The recommended maximum continual current draw is C/5, or 7A for a 35Ah battery.

Conclusion

Determining the optimal solar panel setup for your battery bank involves careful sizing calculations based on your energy usage and location. For a typical 35Ah battery, a 50W panel is the absolute minimum, while a 100W panel provides a good balance of charge performance and cost.

There are many factors that impact charging efficiency and speed in real-world conditions. Allowing a 20-40% buffer when sizing your solar system helps account for losses and maintains the ability to fully charge your batteries each day.

With smart solar panel selection and proper battery management, you can build an efficient and reliable off-grid power system to meet your energy needs.