The popularity of a 400-watt solar panel system—which may generate up to 2,400 watts per day depending on how many hours of sunlight are available—is increasing. Kits for RVs and off-grid installations have become more affordable than ever before. A 200-ah AGM or lithium battery is needed to store power for later use, but you will still need batteries to hold the energy. Therefore, how many and what size of those batteries should be used?

## Batteries Needed For A 400W Solar Panel

To begin, determine how many watts your solar panel system generates daily. This will indicate how many batteries for 400 watt solar are required.

Watts = solar panel output x sun hours.

Watts/voltage = battery size or the number of batteries required.

Let’s start by calculating how much power your 400 solar panel generates. Because solar panels require sunshine to generate electricity, the number of sun hours influences how many watts they can generate. You must also account for system failures, passing clouds, and weather swings.

If your area receives 5 hours of sunlight per day, a 400-watt solar system can generate up to 2000 watts per day: 400 x 5 = 2000

If your area receives 7 hours of sunlight every day, the output can reach 2800 watts: 400 x 7 = 2800

## Size of Battery for 400W Solar Panels

Just make sure the voltage is the same. It stands to reason that a 12V battery would be the best choice for a 400W solar panel.

The formula is as follows:

Watts multiplied by battery voltage equals battery size in amp hours (ah)

For example, if your system generates 1800 watts per day, divide it by the battery voltage:

1800 / 12 = 150

The minimum battery size required is a 150 ah battery. However, a 200-ah battery is preferable if your solar panel produces more power than usual.

During the summer, your solar panel will produce more electricity than at any other time of year. Greater sunlight hours result in more solar power. Rather than wasting it, acquire a larger battery.

If you usually get 1800 watts per day, it might rise to 2000 watts or higher during the summer. A 200-ah 12V battery has a capacity of 2400 watts, which means that no energy is wasted.

A larger battery is usually preferable to a smaller one. Because solar power output is imprecise, you must always leave room for increased energy production.

## How Much Time Can a Battery Be Charged With a 400W Solar Panel?

A 400-watt solar panel can charge a 200-ah battery completely in 5-6 hours. This assumes that the battery is 50-75% charged and that there are 5 hours of sunlight. If the battery is entirely depleted and fewer solar hours are available, charging time will be longer.

The calculations below assume a 200-ah battery, although they also apply to 300-ah batteries and other sizes. Simply substitute your battery parameters for the figures.

Solar power battery charging time is affected by three factors:

- The amount of electricity produced by the solar panel
- Sunlight hours available
- The depth of discharge rate of the battery (DOD)

The DOD specifies how much battery capacity can be utilized before it must be recharged.

The discharge rate of a 200ah 12V lead acid battery is 50%. You must recharge the battery when its capacity falls below 100ah. Only half of the available space is usable at any given time.

To properly charge the battery, your 400W solar panel must output 1200 watts (100 x 12 = 1200).

AGM, gel, and lithium batteries are all options. AGM and gel batteries typically have a DOD rate of 70-75%, but lithium batteries have a DOD rate of 85-100%, such as the Ampere Time LiFePO4.

Lithium batteries would appear to take longer to charge. A depleted lithium battery recharges faster than a lead-acid battery that is 50% full. Lithium batteries are more complex and advanced, allowing them to absorb more power without damage.

However, depth discharge is only one factor. Other factors include the number of available sun hours and the output of solar panels. It will take longer to charge many batteries.

## How Big of a Charge Controller Do I Need for a 400W Solar Panel System?

A 50A charge controller is needed for a 400W 12V solar panel and a 200Ah 12V battery. Multiply the wattage of your solar array by the voltage of your batteries, and add 25% to get the amperage of your charge controller.

The formula for calculating the amount of energy produced by a 400W solar panel and a 200ah 12V battery is

- 400 / 12 = 33.3.
- 3 + 25% = 41.6

Some may advise a 20% increase, but a 25% increase gives additional cushion and reserve capacity.

When choosing between MPPT and PWM, an MPPT controller is the superior choice. For example, you can get the most charge from your panel into your battery with the Victron Smart Solar controller.

## How Long Do 400W Solar System Batteries Last?

Batteries’ lifespan depends on the strain they’re put through and the depth to which they’re discharged. The greater the load, the quicker it loses capacity.

Solar panels create direct current (DC) stored in a battery bank. The charge controller maintains the charge at the highest safe level. The DC solar power is then converted to AC by your inverter so that appliances may use it.

A 200-ah 12V lead acid battery can produce 2400 watts when ultimately charged. However, you’re limited to 1200 watts when using a 50% discharge. After that, you’ll need to charge the battery.

The DOD of a lithium battery is 80-100%, whereas an AGM battery is 70-75%.

Multiply the capacity of the battery by its DOD in decimal form. If you have a 200ah battery with a 70% DOD, you can do the following:

- 200 x .70 = 140
- The battery’s useable capacity is 140ah (1680 watts)

## The Bottom Line

In most cases, a single battery can power a 400-watt system. Smaller solar panel systems benefit significantly from lithium 100Ah (amp hours) batteries. Depending on how often they are used, a 400-watt solar panel can provide enough energy to run a vacuum cleaner, dishwasher, electric blanket, computer monitor, and processor for several hours or even days. The popularity of 400-watt solar panel systems is growing, with new owners citing their versatility and ability to power homes and appliances for several hours as reasons they chose this type.