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Not all 400-watt solar kits come complete with batteries.
So if you end up with one of those, you may need to work out not only:
- What size of battery bank it is suited to charging but
- Also how many batteries you need and
- What size of inverter (watts) to use if you plan to run any AC appliances.
Should you find yourself in this position then this post will help you get started.
Here, you’ll find information on how many batteries, their capacity (Ah), charge controllers, and inverter to get you started on the right foot!
What Size of Batteries (Ah) and Number to Charge with 400-watt Solar
How many batteries you end up using to store the estimated 1,600 watt-hours (assuming 4 hours of sunshine/day) depends mainly on the voltage of your battery bank and the chemistry of the batteries (AGM/LiFeP04) you choose.
Even then there other other nuances involved such as how much energy the connected appliances draw that should be factored into the decision but the guidance below is a good place to start.
AGM Batteries
12V Battery Bank
Consider a battery bank of 300 Ah at 12V AGM. This can be made of 2 x 150Ah or 3 x 100Ah batteries connected in parallel. This battery bank can store the estimated 1,600 Wh of energy generated by the solar panels each day.
24V Battery Bank
Consider a battery bank of 100 Ah at 24V AGM. This can be made of 2 x 100Ah batteries connected in series. This battery bank can store the estimated 1,600 Wh of energy generated by the solar panels each day.
Note: This assumes your location receives about 4 hours of sunshine each day and that the batteries are not discharged beyond 50% of their capacity.
Using LiFePO4 Batteries
12V Battery Bank
You can use a 200Ah LiFeP04 battery bank at 12V made up of 2 x 100 Ah LiFePO4 batteries connected in parallel.
24V Battery Bank
In the case of a battery bank at 24V, you can use a 100Ah LiFeP04 battery bank at 24V made up of 2 x 100 Ah LiFePO4 batteries connected in series.
Note: The indicated battery capacities below are only a guide, depending on how much sunshine is available at your location, and the battery capacities you select.
You’ll also need to consider how much energy your appliances draw on a daily basis. You may choose different battery capacities and the number of batteries, and your solar system will work well too!
How to Connect the Battery Bank (Safety!)
Make sure to connect the battery bank to the charge controller before the solar panels.
Connecting the solar panels to the charge controller first before the batteries can damage the charge controller!
Battery Capacity Monitoring
Knowing how much runtime you have available with the connected appliances and inverter avoids being suddenly surprised that your inverter has shut down because the batteries are deeply discharged.
Invest in a battery charge monitor for your solar system. Check if your charge controller supports a battery charge monitor and possibly an app feature to keep track of your energy usage and use your connected appliances for as long as possible.
Note: Battery monitoring is more accurate with LiFePO4 batteries.
This is the one added benefit of using LiFePO4 batteries!
Must you Use this Battery Bank Capacity (Ah)?
Not really. However, doing so ensures that most of the energy that is generated on a daily basis is stored.
If you only use the solar system occasionally, you could use a smaller battery bank but be mindful that more of the generated electricity will not be stored.
You may also be interested in this post: How many watts of solar to charge a 400 Ah battery bank?
Charge Controller for a 400-watt Solar System
The peak current from a 400-watt solar kit is about 22 amps. This assumes that each of the 100-watt/ 18V solar panels is connected in parallel.
At a minimum, use a 30 MPPT charge controller or larger (if you plan to add more solar panels) to protect the battery bank from overcharging.
The charge controller battery type should be set to AGM or LiFePO4 depending on the actual battery type connected to the charge controller so that the battery is charged to the recommended voltage settings.
Failure to do this risks shortening the life of the batteries!
Should you Use AGM or LiFeP04 Batteries?
If the budget is not an issue, I would recommend you go with LiFeP04 batteries. They are lighter, last longer in some cases 5-7 years compared to about 3-4 years for AGM, and on top of that, have more usage energy per battery.
Battery monitoring is also more accurate with LiFePO4 batteries!
You could still use AGM batteries, but keep in mind that they don’t last as long and have comparatively less usable capacity so you tend to end up with more units to attain a battery bank size!
How Long to Charge the Battery Bank
If the batteries are drained to 50% of the capacity (AGM) and 80% (LiFeP04), the estimated time to charge fully can range from 4-7 hours depending on how deeply discharged they are.
In addition to the extent to which they are discharged, the time to charge depends on how much sun is available.
What Size of Inverter (Watts)?
The size of the inverter you use depends on the combined power consumption (watts) of the connected appliances and not the solar panel wattage or batteries per se.
That said, you may consider a 1,000-watt or a larger pure sine wave inverter (preferably) if you plan to connect more appliances in the future.
Check the battery voltage that your inverter supports: Only connect a 12V inverter to a 12V battery bank and similarly, a 24V inverter to a 24V battery bank to avoid damage to the inverter.
What Appliances Run Off a 400-watt Solar System?
Depending on your circumstances – whether in an RV, remote cabin, tool shed, or at home with the 400-watt solar system providing backup power, the range of appliances may be different.
With that said, the listing of appliances with their indicative run times can paint a good picture of the capabilities of the 400-watt solar system that you may then adapt to your specific case.
Note: This assumes a 400-watt solar system whose estimated 1,600 watt-hours are stored in a battery bank and that there’s an inverter available and connected.
Assumed daily energy production of the system 1,600 watt-hours
- A small fridge, 80 watts for 10 hours, about 800 watt-hours
- 10pcs of 10-watt LED lights for 5 hours, about 500 watt-hours
- 40-watt LED TV for 3 hours, about 120 watt-hours
- 15″, 60-watt Laptop for 3 hours, about 180 watt-hours
Total daily energy consumption is 1,600 watt-hours.
Fuse protection for a 400-watt System
You’ll need to install fuses to protect the solar system wiring from damage. The fuse rating (amps) will depend on the wire gauges used.
An example of fuse sizes you’re likely to find used for protection against short-circuit and overcurrent faults in a 400-watt solar system are:
- between panels and controller, 30A
- between controller and battery, 30A
*Note: Fuse sizes above are indicative. The actual fuse rating depends on the wire gauges used. Always contact a qualified solar professional.
Closing Thoughts
To get started with using a 400-watt solar system optimally, you can use a battery bank of 300 Ah at 12V AGM. This can be made of 2 x 150Ah or 3 x 100Ah batteries connected in parallel.
If using a LiFeP04 battery, you can use a 200Ah LiFeP04 battery bank at 12V made up of 2 x 100 Ah LiFePO4 batteries connected in parallel.
Plan to use a 30-40 amp MPPT charge controller.
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