What size solar panel to charge 100ah battery

What Size Solar Panel To Charge 100ah Battery?

What Size Solar Panel To Charge 100ah Battery? Panel efficiency, battery type and starting charge level are all relevant factors when working out the size of solar panel needed to charge a 100ah battery. You would need at least 240 watts of solar panel in ideal conditions, assuming 5 hours per day of sun and a 12v battery.

240-watt solar panel combinations could be formed from two 120w panels, three 100w panels, five 50w panels or others. In real-life conditions, you might need 300 watts of solar panelling due to cloud and shadow.  

Are you wondering which solar panel you need or what size solar panel to charge a 100 amp hour battery?

Are you curious about the sun’s energy and how it can be converted by modern technology into clean, green electricity?

Our overview guide could help determine the best solar panel options for you. However, if volts, amps and watts are new or forgotten words, skip to the bottom first for a quick explanation of key terms.

Let’s get started…

100ah battery 1

Why use solar panels to charge batteries?

Sun energy is easy to maintain, affordable and clean source of electricity. Tapping into solar energy supports more self-reliant and sustainable living, whether on the road in an RV or camping or setting up a full system of solar batteries in your home or office.

Solar energy kits are designed to provide independence from the grid and give simple access to solar energy, opening doors to off-road opportunities. Installing solar panels in an RV or carrying a portable charging kit will keep you powered up deep in the countryside and far off the beaten track.

100ah battery 1

What size are different solar panels?

The ‘size’ of solar panels could refer to physical dimensions or power output in watts. When installing in a vehicle, boat or cabin, consider both power and total space are equally important.

Panels come in many different wattages/sizes of power. For example, you can buy panels from 1 watt suitable for powering LEDs and charging small power stores to 1000 watts suitable for powering a house.

Re physical dimensions, an average 100-watt solar panel measures around 47 x 21.3 x 1.4 inches. Higher wattage solar panels will generally be larger and lower wattage solar panels smaller, but this also depends on the design and efficiency of the solar panel itself.

100ah battery 2

What size solar panel to charge a 100Ah battery?

A 12-volt battery can be charged by any solar panel but will charge more quickly with a higher wattage panel and more slowly with lower wattage. Timing depends on the type of battery and depth of discharge.

Do note that the open-circuit voltage of an average ’12v’ solar panel is actually 21 volts. Therefore, always use chargers with a charge controller to prevent overcharging and battery damage.

100ah battery 2

Can a 100W solar panel charge a 100Ah battery?

A 100-watt panel is conveniently sized in the solar power market, increasingly standardized and relatively easy to transport. This has made them a popular choice for portable power kits, especially for camping, RVs, and general outdoor and/or off-grid activities.

But remember that a single 100w panel can produce only around 30Ah of power per day, assuming 5 – 6 hours per day of sunlight. If boondocking or camping, you may easily need 80Ah, making one 100w panel insufficient to charge the battery quickly in average weather conditions.

You would likely need two 100 watt panels or more to recharge your battery on a normal day. However, if you’re using energy more intensively (e.g. running an air conditioner, boondocking or camping off-grid for extended periods), you could need 1800w of panels or more.

The portability of 100w panels makes them an excellent choice for short-term or light usage. This could be powering a few small appliances in an RV or providing light while camping.

However, if you install a new solar electricity system in a house or run a range of appliances or devices, you will probably want to think bigger than 100 watts.

100ah battery 3

What devices can a 100w panel power?

A single 100w panel could power one of the devices listed for anything from a few hours (laptop) to a full day (Wi-Fi router):

–     Laptops – 60-watt hours

–     Ceiling fan – 35-watt hours

–     Certain lamps and lights – 14-watt hours

–     LEDs – 10-watt hours

–     Wi-Fi Router – 6-watt hours

–     Smartphone charging – 5-watt hours

100ah battery 4

How long will a 100W solar panel take to charge a 100Ah battery?

Assuming 50% discharge in an AGM battery, charging could require up to 8 hours of sunlight with a clear sky. But, of course, this would take longer on a dull day or in winter with fewer hours per day of peak sunlight.

A lithium battery could charge more quickly.

How many solar panels do I need to charge a 100Ah battery in 5 hours?

We first divide 100 Ah by 5 hours to get 20A of current per hour to work this out.

Then, using the equation Power (watts) = Voltage (volts) X Current (amps) and assuming a 12v battery, we get: 20 amps x 12 volts = 240 watts of solar panel.

For 240w of solar power, you could use any of the following sizes of solar panels wired in series or another similar combination:

–      2 x 120-watt panels

–      3 x 100-watt panels

–      5 x 50-watt panels

Conditions are rarely completely ideal, and you should therefore also factor in the loss of sunlight due to clouds or shadows. Allow for 10-20% more wattage than ideal conditions to compensate for this.

Adding on 20% of 240w gives the desired wattage of 288 watts, which rounds up to 300 watts of power. You could install a single 300-watt panel or other combinations, including:

–      3 x 100w panels

–      6 x 50w panels

100ah battery 5

How long will a 300-watt solar panel take to charge a 100Ah battery?

A solar panel of 300 watts should charge a 100 Ah battery in around 5 hours. If it is only partially depleted, this could be quicker, possibly just two hours, for example.

100ah battery 6

What type of battery can I charge with solar panelling?

A 100Ah rechargeable battery is usually a “deep-cycle battery” designed to discharge regular recharge and provide power for extended periods.

Deep cycle batteries are batteries that are designed to be discharged deeply on a regular basis. Deep cycle batteries are most commonly used in solar power systems, backup generators and electric vehicles.

There are two types of deep cycle batteries: flooded lead-acid (FLA) and sealed maintenance free (SMF).

Solar panels can be used for charging the four main categories of these batteries:


Lithium batteries are a newer technology with significant advantages over older battery types. The biggest benefit is perhaps longer-lasting power, with lithium batteries lasting 5 to 10 times longer than other types listed here.

Lithium batteries can store four times the energy of lead-acid batteries and have a longer lifespan. As a result, they can be used as needed, run down to almost zero power without risk of damage, and don’t need to be constantly charging.

Wet cell / flooded battery

You can only discharge this widespread type to 80% of capacity. In addition, wet cell batteries require regular to function correctly and must be regularly refilled with water.

Although wet cells hold charge well when stationary, they can be vulnerable to shaking and knocking. Rough journeys or dropping could cause them to lose charge.

Gel Cell Battery

Gel cells are sealed lead-acid batteries using sulfuric acid and fumed silica to form a gel-like substance that does not slosh around like water. Gel cells can function in different positions and undergo rougher travel without losing charge, unlike wet cell batteries.

Absorbent Glass Matt (AGM)

AGM batteries are also sealed lead-acid batteries. Invented in the 1970s, they contain electrolytes with a mix of sulfuric acid and fibreglass.

AGM batteries can be discharged to around 50% of capacity without damage. In addition, they are spill-proof and can be moved around without damage or impairing function.

Containing no water, AGM batteries are significantly less heavy than wet cell batteries. They also hold a charge longer and power devices more quickly.

What is a 100Ah battery?

Ah is short for “amp hours”, the unit of measurement for capacity. A 100Ah battery can deliver up to 100 amp-hours of power to your appliances.

For example, if a 1 amp lamp runs for 5 hours, five (4 X 1) amp hours will be used. This represents 10% of a 100Ah battery or 5% of a larger 200Ah store.

Note that effective capacity in real life can differ from nominal values, and a 100Ah battery may deliver less power than specified. This is more true for lead-acid battery types where capacity is ‘lost’ when it discharges faster.

How do I find out my battery capacity?

Battery capacity is normally clearly stated on the packaging and container.

In lead-acid batteries, it is also indicated by ‘C’ with a number (e.g. C1, C5, C20) giving the C-Rate, the number of hours in which the power contained can be delivered.

For example, if you see a description of C20=100Ah, this battery can deliver a total of 100Ah if it is discharged in 20 hours. Lead batteries are often C20 by default.

With a faster discharge, the total battery capacity drops radically. For example, if you disconnect in 5 hours, it will only deliver 70Ah.

This is because the internal resistance increases when the battery is discharged faster, and heat loses capacity.

How often should I charge a 100ah battery?

This depends on factors including battery type, efficiency and how much power you use.

In general, lead-acid batteries should not be discharged below 50% – 80% (according to type). On the other hand, lithium batteries can be almost fully discharged and recharged without the same issues.

For example, we should not use more than 80Ah from a 100Ah wet cell battery before recharging. Discharging below 80% depth of charge could cause damage and reduce overall battery life.

It is important to monitor your battery’s level of discharge to avoid being unexpectedly left without power.

How can I check the charge level remaining on my 100ah battery?

The quickest way to determine the remaining charge is to check the display on a device connected to your battery. If you don’t have a device connected, you can check the approximate charge status with a voltmeter or multimeter set to DC volts.

While the relationship isn’t exact, the voltage can give a reasonable approximation of the charge level. You can place the meter leads on the positive and negative terminals to take a reading

12v batteries in good condition and fully charged will have a resting voltage of approximately 12.6 -12.8 volts. As the battery charge is used, the voltage will drop.

If meter readings for a lead-acid battery fall to 12.50v-12.10v (representing 80%-50% of full charge levels), it should be charged. Lithium-ion batteries can be completely discharged without the same risks.

For a 6-volt battery, you should halve these figures.

How quickly can I charge a 100Ah battery?

Speed of charging is dependent on battery type, usage and starting level of discharge, charger efficiency and the current output rating of the charger.

For example, a 100Ah lithium battery connected to a 60 amp charger should recharge in around 1.7 hours (100 Ah / 60 amps per hour). However, as charging nears completion, there is a gradual reduction in charge current, and the real-life total charging time would be around two hours.

Under the same conditions, a lead-acid battery would take between 6 hours and 8 hours to reach full charge.

How does a solar panel work?

A solar panel converts light into electricity. A solar panel’s power, or wattage, depends on its physical dimensions and efficiency, with voltage a key characteristic for battery charging.

Solar panels are generally part of a system rather than connecting directly to batteries. Before reaching a battery, solar energy from the panels will flow into a regulator or solar charge controller (SCC).

In many solar panels, we can observe an open circuit voltage of about 22 volts, and the SCC is there to prevent overcharging and battery damage. For example, a solar panel in full sunlight connected to a 12v battery could potentially charge at levels well above 12 volts.

The solar regulator detects the existing voltage and adjusts panel output when it approaches 14.1 volts to prevent damage from this. It also maximizes the current delivered by the solar panel and thereby keeps the charging time short as possible.

Voltage does not vary with light intensity in the same way as current. You should therefore avoid overcharging by using a charge controller or moving to a lithium battery.

Charge controllers don’t have their own inbuilt power supply, and the system relies on battery voltage to function. These chargers must therefore be connected to the battery before connecting to the solar panel itself.

Once connected, the charger will display battery voltage. At this point, you can connect the charger input terminals to the solar panel.

How much do solar panels cost?

You can either buy a 100-watt solar panel alone or in a kit with a solar panel, charge controller, cables, and brackets.

For portable solar power kits, a standalone 100-watt panel can be purchased for 0 or less, with a complete 100-watt solar starter kit costing +$300.

Volts, Amps, Watts and solar panel calculations – help!

Forgotten all your high school physics? This short reference will make it easier to follow guidance on solar panel size and charging.

–      Volts (v): the measure of electrical current flowing to or from a device or power store.

–      Amps (A): the measure of the flow of electrical current.

–      Amp Hours (Ah): the measure of capacity, the available charge stored in a battery.

–      Watts (w): the measure of energy used for solar panel and electrical device power.

To make solar panel energy charging calculations, you can use the equations below:

–      Watts = Amps X Voltage

–      Watt-hours = Watts X Hours of sunlight per day (= total wattage of panels required)

–      Amp-hours = Amps X Hours of use (= total charge)

A final word on solar panel charging…

The sun is a renewable energy source that can make your power consumption more environmentally friendly and sustainable, wherever you are. So whether your usage is high or low, whether you have one hour of sunlight or seven, there will be some advantages to solar energy charging.

As long as you have the right equipment and a few clear hours of sunlight, you can make electricity from the sun and fully charge your battery.

We hope we’ve helped you figure out how solar panels work, what battery capacity means, how many watts of power you use and which solar panel system to buy.

Now it’s time to get out and make the most of the sun!

Scroll to Top