Running A 100 Watt Solar Panel, & Charging A 12 Volt Deep Cycle Battery With It

Below is a three part guide about running a 100 watt solar panel, and charging a 12 volt deep cycle battery with a 100 watt solar panel.

The three specific parts of the guide are:

– How much power a 100w Solar panel produces 

– What a 100 watt solar panel can power

– How to charge a 12 volt deep cycle battery with a 100 watt solar panel

Below these three parts, we also include a separate and final part about charging a 100ah battery with solar panels.


Note that this information is general information only. You’ll need to refer to manufacturers instructions and the advice of a professional for qualified opinion on the correct solar panel setup and practices for you.


Let’s take a look at the information in the guide …


How Much Power Does A 100w Solar Panel Produce?

You might use the information below as a general estimate in figuring out how much power a 100 watt solar panel produces (i.e. how many watts per hour and per day a standard 100w solar panel will produce)

100 watt solar panels are usually used for off-grid type situations or powering standalone DC or AC items, rather than powering a house.

Solar systems can come in 12, 24 and 48 volt systems – but, we’ve calculated based on 12 volt because it’s the cheapest, and it’s what many people use.

Some steps to consider:


1. Consider and calculate power loss

A solar panel company may indicate that the panel is a 100 Watt panel, but 100 Watts doesn’t take into consideration how a solar system works in reality with inefficiencies and power loss.

Power loss and inefficiencies can be caused by:

Efficiency/conversion of the particular panel when sunlight hits it, and then converting that sunlight into energy

The solar controller you use when the power comes from the panels through to the battery (used when charging most 12 volt batteries)

The solar inverter you use when you convert from 12 volt to say 115 volt (an inverter is used for AC household items or charging back to the grid)

You also lose some power across the length of your connectors or cables

+ other minor factors and variables


How to calculate power loss and inefficiency?

This one is pretty simple.

You might subtract 20% from your panel – so, a 100 Watt panel actually becomes an 80 Watt panel (100 – 20 = 80).

So, we now have a 100 Watt panel which is producing 80 Watt per hour.


NOTE: some people might subtract 10% now for the inefficiency of the panel, and then choose to subtract 10% later when they figure out their final load or charging numbers to account for the controller, inverter, cable and other inefficiencies, but it’s up to you.


2. Consider and calculate number of available sunlight hours

The second thing you want to consider is how many daylight hours you get in a day where you will be using your solar panel.

The absolute maximum sunlight hours you might get will usually be around 8, but in most areas, you might get around 4-5 hours of direct sunlight on your panels.


How to calculate available sunlight hours?

Let’s say you decide you get on average 5 sunlight hours in your area.

You would multiply 80 Watts x 5 hours = 400 Watts per day

So, your 100 Watt solar panel might be producing 400 Watts of power a day

Note though, that if you have a cloudy or rainy day – you might get zero direct sunlight hours, and your power produced could be zero for that day.


3. Summary

So, that’s how much power a 100 watt solar panel produces …

A 100 Watt solar panel actually becomes an 80 Watt panel when you factor in inefficiency and power loss

That same 100 Watt panel, with 5 hours of direct sunlight a day will produce 400 watts a day of power


4. What To Do Next?

To calculate your whole solar system, next you’ll want to know

What a 100 watt solar panel can power (how many and what items can a 100 watt panel power, and how to calculate the watts that each item uses)

How many 100 watt panels you need for your system

How long and often to charge your 12 volt deep cycle battery to keep your solar system charged


What Can A 100 Watt Solar Panel Power?

Below we will show you how you can estimate what a 100 watt solar panel can power, & how to estimate those loads

Specifically – how you can estimate what DC battery, and AC household items a 100 Watt solar panel can power.

We will also give you an idea of how you can calculate individual item loads.


1. Consider and estimate how much power you’ll be using

Above, we estimated that a 100 Watt panel, with 5 hours of direct sunlight available, will produce 400 Watts of power for us per day.

So, we have an estimate of how much daily power we will be producing.

But, now we need to find out how much power we will be using.

To do that, we need to look at each item we are using, and add up to total power load.


How to calculate how much power you will be using?

You can find out how much power each item uses in two main ways:

1. Look on the product label and find the Wattage – that is how many watts the item will use per hour


2. If you can’t find the Wattage, see if the label states how many amps and volts it is.

You can multiply amps by volts to get watts e.g. a 1 Amp, 115 volt item will use roughly 115 Watts an hour


NOTE: if you can’t find this information on the item, visit the manufacturer’s website and find the product details/specifications – they should be available there.


Now, let’s say we have three items we want to run daily – 1 x 50 Watt TV, 1 x 48 Watt Laptop & 1 x 15 Watt LED Light.

50 + 48 + 15 = 113 Watts an hour we will be using for those three items.

Now you need to make an estimation of how long you will run those items per day (you can calculate each item separately or together – we calculated them together to make it simple)

If we say we will run those items together, all for 3 hours a day, we get 113 Watts x 3 hours = 339 Watts total power we will be using per day


2. Confirm your solar panel production is enough for your power usage

Above, we estimated that a 100 Watt panel, with 5 hours of direct sunlight available, will produce 400 Watts of power for us per day.

And, we just calculated that we will be using a TV, laptop and LED light 3 hours a day, creating a power load/usage of 339 Watts a day.

400 Watts power production is greater than 339 Watts power usage, so we should be OK.


3. Summary

So, that’s what a 100 watt solar panel can power…

Figure out the Wattage load per hour each item you want to use or charge will take up

Estimate how long you will use those items per day

Get a total power usage for all items

Confirm the power production of you 100 Watt solar panel is more than your power usage. If not, you’d consider additional 100 Watt panels


4. What To Do Next?

To calculate your whole solar system, next you’ll want to know:

How long and often to charge your 12 volt deep cycle battery to keep your solar system charged


How To Charge A 12 Volt Deep Cycle Battery With A 100 Watt Solar Panel? (How Long & How Often)

Below we will show you how you can estimate what how long to charge, and how often to charge your 12 volt deep cycle battery with one or multiple 100 Watt solar panels in your system.

You can make the following considerations and calculations:


1. Consider and estimate the capacity of your battery

Ultimately, you may have 1 or several batteries you need to charge.

Several batteries creates a bank of batteries, but to keep this simple, we will estimate the capacity of 1 x 12 volt deep cycle battery.


How to estimate the capacity of your battery?

You’ll need to find out how many amp hours your battery provides, and multiply that number by 12 volts (the voltage of the battery).

To do this you’ll need check the label on the battery, or you can check the battery manufacturer website for the information.

Let’s say you have a deep cycle battery that provides 100 Amp hours – we get 100 Ah x 12 volts = 1200 Watt hours.


2. Cross check battery capacity, with solar panel production, and power usage

This is the last estimation you need to make.

Above, we estimated:

  • That a 100 Watt panel would produce 400 Watts of power per day with 5 hours direct sunlight available
  • That running TV, Laptop & LED Light for 3 hours a day at 113 Watts an hour puts our power usage at 339 Watts a day

Now, there are several reason why, but you should look to keep you battery above 50% total charge, which is 600 Watt hours for our battery above.


How to cross check battery capacity, with solar panel production, and power usage?

With your current setup, you are using 339 Watts a day from your 1200 Watt capacity battery, but you are replacing 400 Watts every day – so that setup works as long as you are charging every day.

However, if for example we choose to not charge for 2 days, or it’s cloudy or rainy for 2 days and we get close to zero charge – we subtract 339 x 2 (678) from 1200, and we find that our battery is now at 522 – which is below 50% of 1200 Watt hours battery (600 is 50%).

So, what you need to do is make sure you charge long enough each day, and often enough weekly, to keep your battery above that 50% mark – while also taking into account your power usage.


3. Summary

So, that’s how often and how long to charge a 12 volt deep cycle battery with a 100 Watt Solar Panel…

Estimate the Watt hour capacity of your 12 volt battery

Cross check battery capacity with solar panel power production, and your own power usage

Charge often and long enough to make sure you keep the charge of your battery above 50% of the total capacity


4. What To Do Next?

Read through the above guide again to get a better idea of estimations.


A Few Things To Note & Be Aware Of With The Above Information …

You can get 12, 24 and 48 volt solar systems.

These estimates are based on a 12 volt system, because it’s the cheapest and it’s what a lot of people start with.

If your battery bank changes to 24 or 48 volt – you’ll need additional panels, and suitably rated solar controllers and converters.

When using solar charge controllers (for DC charging) and/or solar inverters (for AC charging), make sure they are rated for the power and voltage you have running through your system

How your solar panels are wired or connected up i.e. whether they are stacked together, and/or if they are connected to individual or combined controllers, inverters and batteries effects charging setup

Always consult a professional, and the relevant authorities before installing and using a solar system. These guides are for estimation purposes only.


Charging A 100ah Battery: Size Of Panels Required, How Many Panels, & How Long To Charge

If you have a 12 volt 100Ah battery, the following information relates to the size of solar panels that might be required, how many panels, and how long the charge might take…


What size solar panel to charge a 100ah battery?

If you refer to the calculation below, of ‘how many solar panels do I need’, we use the example of a 100Ah 12 volt battery.

With 6 hours of sunlight that we estimate we get in the day, and by overrating the solar system by 20%, we estimate we will need 240 Watts total power.

For 240 Watts, you might try some of the following sizes of solar panels and combinations:

2 x 120 Watt panels

3 x 100 Watt panels

5 x 50 Watt panels


You’d want to make sure the panels you get can be stacked together, and that your solar controller is rated for the total wattage and Amperage you have running through your system.

Most standard PWM solar controllers (the cheaper ones compared to the MPPT controllers) are rated for 400 Watts, but check the controller you get for it’s ratings.


How many solar panels to charge a 100Ah battery?

Number of panels =

Battery Ah x Battery Voltage = Watt Hours

Watt Hours / Hours Of Sunlight You Get A Day = Total Watts Of Solar Panels You Require – you may choose to have several panels that make up this Watts number, or 1 panel

Total Watts x 10 to 20% = Actual Watts Of Solar Panels You Require (due to sunlight being inconsistent, panels not functioning at stated capacity and other inefficiencies like solar controllers not functioning at capacity, it’s recommended you overcompensate your requirements by 10 to 20 %)



We have a 100 Ah 12 volt battery.

100Ah x 12 = 1200 Watt hours

1200/6 hours of sunlight in a day = 200 Watts

200 Watts x 20% = 240 Watts

So, if I got two 120 Watt panels, or 3 100 Watt panels, I’d have enough solar panels for this battery, as an estimation


How long does it take to charge a 12v 100ah battery?

Time of Charge =

Total Wattage of Your Panels / the voltage your system is running at (usually the voltage of your battery) = Amps delivered

Ah of your battery / Amps delivered = time it takes to charge



We have the same 100 Ah 12 volt battery, and we are using 2 x 120 Watt panels

240 / 12 = 20A

100Ah / 20A = 5 hours to charge the battery (roughly – actual time taken does depend on variables like sun on the day, and efficiencies of your system)



These are estimation calculations only, to help you get started.

You should always check with manufacturers, and installation professionals as to your final requirements, and certainly before you install a portable solar system

Charging may also be affected by other factors like if your battery is deep cycle or a starter battery, lead acid or lithium ion, number of batteries in the battery bank etc.


More Resources On Calculating Battery & Solar Panel Requirements

These resources give more calculations and information on battery and solar panel requirements:

How [To] Size Solar Battery Bank and Solar Panels – How Many Batteries? How Many Solar Panels?

Solar Panel Power Design & Matching With Batteries




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