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A 400 watt solar panel system is becoming more commonplace, with kits for RVs and off the grid setups more affordable than ever. However you still need a battery to store that energy. The question now is how many of those batteries you should have and what size.
A 400 watt solar panel can produce 1200-2400 watts a day depending on how many hours of sunlight are available. To save that power for later use, you need a 200ah AGM or lithium battery.
How Many Batteries Does a 400W Solar Panel Need?
First you have to calculate how many watts your solar panel system produces in a day. That will tell you how many batteries are needed.
Solar panel output x sun hours = watts
Watts / battery voltage = battery size or number of batteries needed
Let us start by figuring out how much power your 400 solar panel produces. Solar panels need sunlight to produce energy, so the number of sun hours determines how many watts it can produce. You also have to factor in system losses, passing clouds and weather fluctuations.
If your area gets 5 hours of sunlight, a 400 watt solar system may produce up to 2000 watts a day:
400 x 5 = 2000
If your region receives 7 hours of sunlight, the output can reach a maximum of 2800 watts:
400 x 7 = 2800
These calculations assume the solar panels generate 400 watts every hour. In reality the output will be a little lower. Passing clouds, overcast sky, system losses, solar irradiance affect the output.
Assuming good but not perfect conditions, you might get 360 watts per hour. Using this example, if you get 5 hours of sun per day:
360 x 5 = 1800
Your solar panel generates 1800 watts with 5 hours of sun. Now that you know how many watts your solar panel produces, you can get the right battery size for it.
Battery Size for 400W Solar Panels
To keep things simple you should match the voltage. If you have a 12V 400W solar panel the battery should be 12V too.
The formula is:
Watts / battery voltage = battery size in amp hours (ah)
Continuing with our example: if your system produces 1800 watts a day, divide it by the battery voltage:
1800 / 12 = 150
A 150 ah battery is the minimum battery size required. However it is better to get a 200ah battery like the Renogy AGM in case your solar panel produces more power than usual.
During summer your solar panel will generate more energy than any other time. You get more sunlight hours which means more solar power. Rather than let it go to waste, get a bigger battery.
If you typically get 1800 watts a day, it could reach 2000 watts or more during summer. A 200ah 12V battery has a capacity of 2400 watts so no energy gets wasted.
It is always better to have a larger battery than a smaller one. Because solar power production is imprecise you must always have space available in case energy production goes up.
How Long Does a 400W Solar Panel Take to Charge a Battery?
A 400 watt solar panel can fully charge a 200ah battery in 5-6 hours. This assumes the battery is 50-75% discharged and there is 5 hours of sunlight. Charging time will take longer if the battery is fully discharged and there are fewer sun hours available.
The following calculations use a 200ah battery, but it also applies with 300ah batteries and other sizes. You just replace the figures with your own battery specifications.
There are three factors that affect battery charge time by solar power:
- How much energy the solar panel produces
- Available hours of sunlight
- The battery depth of discharge rate (DOD)
The DOD tells you how much of the battery capacity can be used before it has to be recharged.
A 200ah 12V lead acid battery has a 50% discharge rate. Meaning, you have to recharge the battery when capacity drops to 100ah. Only 50% is usable at a time.
100ah is 1200 watts (100 x 12 = 1200) so your 400W solar panel must produce 1200 watts to fully charge the battery.
If your system produces 400 watts an hour, the battery will be fully charged in 3 hours:
400 x 3 = 1200
If your system only generates 360 watts, it will take about 5 hours:
360 x 5 = 1800
AGM, gel and lithium batteries. AGM and gel batteries usually have a DOD rate of 70-75%., while lithium batteries such as the Ampere Time LiFePO4 have a DOD of 85-100%.
You would think that lithium batteries will take longer to charge. In fact, a fully discharged lithium battery recharges faster than a 50% full lead acid. Lithium batteries are more sophisticated and advanced, capable of absorbing more power without incurring damage.
The depth discharge is just one factor though. The available sun hours and solar panel output also play a part. If you charge multiple batteries it will take more time.
What Size Charge Controller For a 400W Solar System?
A 50A charge controller is required for a 400W 12V solar panel with a 200ah 12V battery. To find the charge controller size, multiply the total solar panel watts by the battery voltage and add 25%.
If you have a 400W solar panel and a 200ah 12V battery:
400 / 12 = 33.3
33.3 + 25% = 41.6
Some would suggest adding 20% to the total, but 25% provides more cushion and reserve capacity.
Given a choice between MPPT and PWM, an MPPT controller is the better option. With the Victron Smart Solar controller for instance, you get the highest possible charge from your panel into the battery.
The problem with PWM controllers is it reduces solar panel voltage to correspond with the battery. 60 cell 12v solar panels are usually capable of 36 volts. A PWM controller will reduce this to 18 volts to match the battery output (12V batteries can reach 18V). Almost half the solar power is wasted in the process.
With an MPPT charge controller it will set the voltage needed by your battery at a higher efficiency level. The voltage can be increased or reduced depending as is required.
How Long Will Batteries Last on 400W Solar Systems?
Battery life depends on its depth of discharge and how much load it is carrying. The heavier the load, the faster it will lose capacity.
Briefly, solar panels generate DC and it is stored in the battery bank. The charge controller keeps the charge at the maximum safe level. Then your inverter converts the DC solar power into AC so appliances can use it.
If you have a fully charged 200ah 12V lead acid battery, it has 2400 watts (200 x 12 = 2400). But with a 50% discharge you can only use 1200 watts. After that the battery needs recharging.
If your battery is lithium the DOD is 80-100%, while with AGM it is 70-75%.
Multiply the battery capacity by its DOD in decimal form. If you have a 200ah battery with a 70% DOD:
200 x .70 = 140
The usable capacity is 140ah (1680 watts)
How long will those 1680 watts last? In theory it seems simple enough. Run a 1680W load and the battery should last an hour before the DOD drops to 50%. If you load 840W (half of 1680) the battery should be good for two hours right?
Not quite. Due to system losses, inverter inefficiency and Peukert’s Law, a battery will discharge and lose capacity faster when running a large power load. A 200ah battery fully discharged over 96 hours is going to last longer than one discharged in an hour.
What Battery Should I Choose?
Lithium deep cycle batteries are the best for solar panels. They are efficient, fast charging, need no maintenance and have an 80-100% discharge rate.
AGM batteries are a good alternative if you are on a budget. Their DOD is not as good as lithium but still more efficient than older lead acid batteries. It really comes down to your budget and how much depth of discharge you want to use.
The battery must be large enough to store the maximum capacity of your solar panel. With a 400W system, 200ah is ideal. A 12V 200ah battery has a 2400 watt capacity, enough for most 400W systems. Of course you can also get a larger battery bank which will not cause problems.
If you want, you can get two 100ah batteries or one 200ah for example. The advantage of the 200ah is it takes less space, and that is something that will become a factor if your power requirements increase. As it is, 200ah is enough for a 400W solar system.