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Page Contents
- How many solar panels does it take to charge a 200ah battery?
- Difference between deep cycle and regular car battery
- Battery SOC vs DOD – What is SOC and DOD?
- How to estimate remaining battery capacity
- How to calculate watt hours of a battery – Ah to Wh conversion
- Daily solar irradiance data by location
- Do I need a solar charge controller?
How many solar panels does it take to charge a 200ah battery?
We e need to provide a bit more detail about the battery before answering this question. The capacity is 200Ah, but will the battery be fully discharged?
This would be extremely unusual – I can’t remember ever discharging a battery 100%. Some lithium-based batteries can be 100% discharge, but acceptable discharge levels vary depending on battery type and design.
The most common high-power battery in use nowadays is the lead-acid type, so I’ll base my answer on this.
As a general rule, a 200Ah lead-acid deep-cycle battery would need a 300 watt solar panel to fully recharge from 50% Depth of Discharge (DOD) assuming 4 peak-sun-hours per day. Charging would be complete in one day with a clear sky.
Difference between deep cycle and regular car battery
There are various types of lead-acid cell, from wet-fluid electrolyte to AGM, but there is another distinction that is more important.
Batteries can be classified as auto type (car batteries) or deep-cycle, often called ‘marine’ or ‘leisure’ batteries.
Auto batteries can deliver very high current for short periods, while deep-cycle can deliver low to medium level current for an extended time.
Car batteries shouldn’t be discharge more than 20%, while a deep cycle lead acid battery can be discharge regularly to 50% and to 80% from time to time.
Why is the difference between car battery and deep-cycle important?
If the 200Ah battery in question is an ‘ordinary’ battery, then normal recharge capacity needed would be:
200Ah x 20% = 40Ah
If it was a deep-cycle battery, then the recharge might be:
200Ah x 50% = 100Ah
The recharge capacity for the deep-cycle battery is more than twice as much, which will affect the size of the solar panel required and/or charging time.
Our calculations will include both types of batteries.
Battery SOC vs DOD – What is SOC and DOD?
Battery State of Charge (SOC) and Depth of Discharge (DOD) are basically two ways of looking at the same thing – how much battery capacity has been used?
SOC is a measure of how much capacity is left in the battery, while DOD is a measure of how much has been taken out. We’ll be focused on DOD as this is the amount we want to replace with a solar panel.
How to estimate remaining battery capacity
You can estimate how much capacity a lead-acid battery has left by measuring the terminal voltage with a multi-meter.
This should be done after several hours of inactivity, when the battery is neither charging or discharging, so that there is almost no chemical changes taking place inside the cells.
Lead-acid battery depth of discharge chart
Once you have a stable voltage reading, check the table below to work out how many amp-hours your battery has left:
State Of Charge % (12 V Pb) | Battery terminal voltage |
100 | 12.73 |
90 | 12.62 |
80 | 12.50 |
70 | 12.37 |
60 | 12.24 |
50 | 12.10 |
40 | 11.96 |
30 | 11.81 |
20 | 11.66 |
10 | 11.51 |
How to calculate watt hours of a battery – Ah to Wh conversion
We can measure battery capacity in two ways, either in ampere-hours (Ah) or in watt-hours (Wh). Both have their uses.
Watt-hours is good for expressing the amount of energy to be replaced, which is convenient as we use the same units to measure solar panel energy generation over time.
Ah is useful when we know how many amps a solar panel can deliver. We can then work out how many hours the battery will take to charge.
However, this depends on constant power output from the panel. If a cloud passes over, then it doesn’t work out so well, as current output of the solar panel falls off.
For the two types of lead-acid battery previously discussed, the probable recharge capacity needed in watt-hours would be:
Auto battery: 40Ah x 12volts = 480 watt-hours
Deep-cycle battery: 100Ah x 12 volts = 1200 watts-hours
Now we need to work out how much energy a solar panel can generate in any location.
Daily solar irradiance data by location
Although there are several other factors affecting solar panel output, such as tilt angle and orientation, by far the most important is irradiance, or peak-sun-hours.
This is a measure of how much sun energy falls onto a solar panel’s surface and varies according to location. It can be significant.
For example, in Nevada it’s almost 3 times higher than in Scotland, UK!
The quickest way to find the irradiance in your are is to simply enter you city into the site Global Solar Atlas and read it off from historical data – see the image below:
Once you have the number of peak-sun-hours, multiply the value by 100 and you will know how much energy in watt-hours you can get from a 100 watt solar panel.
A good rule of thumb is around 400 watt-hours/day, although it can vary considerably.
Using location irradiance to size solar panels
Let’s use this 400Wh/day average as an example. These are the number of 100 watt solar panels you would need for both our example batteries:
Auto battery: 40Ah x 12volts = 480 watt-hours.
This battery would need a little more than a 100 watt solar panel to charge this battery in one day. Either add an extra 40 watts, or another 100 watt panel to charge it quicker.
Deep-cycle battery: 100Ah x 12 volts = 1200 watts-hours.
This battery will need (at minimum) 3 x 100 watt solar panels to fully recharge in one day.
The above assumes a clear sky and no shading. In the case of the deep-cycle battery, it would be prudent to add 50 to 100 watts extra, as there are always losses associated with solar circuits.
The table below shows the solar power required to recharge the batteries from different locations and is an illustration how the irradiance value can make a big, big difference in the size of solar panels needed:
Location | Glasgow, UK | Chicago, Il | Lancaster, Ca | |
Peak-sun-hours/day | 1.86 | 4.03 | 8.07 | |
Solar panel rating in watts needed to fully charge each battery type in one day - full capacity 200Ah | Car Battery - 40Ah discharged (20%) - 480 Wh required | 258 watts solar panel rating required | 119 watts solar panel rating required | 60 watts solar panel rating required |
Deep-cycle - 100Ah discharged (50%) - 1200Wh required | 645 watts solar panel rating required | 297 watts solar panel rating required | 149 watts solar panel rating required |
I prefer this method of calculating solar panel size because it balances the energy required over time.
Instantaneous values of current and power don’t mean much in relation to solar panel output as it varies so much over the day.
The energy transmitted by the sun in the morning and evening is very low, wherever you are located. The most energy is converted in the 4 hours centered around mid-day.
Peak-sun-hours is an approximation of the total energy available on a clear day and is a more accurate assessment of the energy available over time.
Do I need a solar charge controller?
As a general rule, always use a solar charge controllar unless using a very low-power solar panel, less than 5 watts. A solar charge controller ensures that batteries don’t become overcharged. PWM (Pulse Width Modulation) controllers are very cheap, while MPPT (Maximum Power Point Tracking) solar chargers can give a significant boost in solar power output.
Our calculations have been quite crude, assuming maximum output from solar panels with no losses. Solar battery chargers should always use a solar charge controller and these have inherent losses.
There are two basic types of controller and they operate in different ways.
MPPT vs PWM – which is better?
A PWM (Pulse Width Modulation) solar charge controller samples the battery voltage and chops up its output voltage to a little above the battery voltage, delivering a charge current accordingly.
This design completely ignores the Maximum Power Point of the solar panels, which is the voltage optimum to deliver maximum current.
This situation occurs when the load resistance matches the panel’s Characteristic Resistance. The MPPT (Maximum Power Point Tracking) solar charge controller is a DC to DC converter and matches its internal resistance to that of the solar panel.
MPPT charge controllers are up to 40% more efficient than PWM.
How should two or more solar panels be connected?
Even low-power MPPT controllers normally allow up to 60V solar input. Each 100 watt solar panel Voc (open circuit voltage) is about 22 volts, up to 3 could be connected together in series.
If you need more than 3, then use two strings of parallel panels in series as shown below;
In fact, MPPT solar controllers are more efficient when the input voltage is high.