# What Size Solar Panel To Charge 400ah Battery?

Curious to know what size solar panel you need to charge 400ah battery. Use our solar panel size calculator or follow the steps given below to find out.

## 6 steps to calculate IDEAL solar panel size for 400ah battery

There are many ways to calculate the size of solar panels for your battery but most of them lead to inaccurate results.

In my experience, this method will provide you with a close estimate of the solar panel size required to recharge your 400ah battery.

### 1. convert battery capacity in watt-hours

To convert battery capacity in watt-hours multiply the battery capacity in amp-hours by its voltage.

Formula: Battery capacity in watt-hours = Battery amp-hours (Ah) Battery × volts

Abbreviation: Wh = Ah × V

Let's say you have two 12v 200ah lead acid batteries connected in parallel, which will make a total of 12v 400ah.

`400ah battery capacity in watt-hours: 400 × 12 = 4800 watt-hours`

### 2. calculate the battery usable watt-hours

Every battery type is recommended to be discharged at a certain level, which is called its depth of discharge (DoD) limit.

Here are the most common battery types and their recommended DoD limit.

• Lead acid, AGM, and Gel batteries: 50%
• Lithium (LiFePO4) batteries: 100%

Multiply the battery capacity in watt-hours by 0.5 for lead acid.

`4800 × 0.5 = 2400 watt-hours`

Note: Ask your battery manufacturer or look at the battery specs sheet for the discharge limit.

### 3. take into account the battery charge efficiency rate

• Lead acid battery charge efficiency: 85%
• Lithium battery charge efficiency: 98%

Multiply the battery usable watt-hours by 1.15 for lead acid type battery or by 1.02 for lithium type.

`2400 × 1.15 = 2760 watt-hours`

### 4. take into account the charge controller efficiency rate

The job of charge controller is to stabilize the output voltage from solar panels to safely charge the battery.

A 12v solar panel will produce about 18 volts when exposed to the sun. The charge controller will drop the voltage from 18v to 12v in order to safely charge the battery.

There are two common types of charge controllers available in the market right now, PWM and MPPT.

PWM charge controller will drop the voltage but will not increase the amps which as a result will cause total power loss.

Because total power (watts) = Amps × volts. But an MPPT charge controller will decrease voltage but will increase the amps to cover up the losses.

For example, you have a 100 watt solar panel and it will produce 100 watts, 18 volts, and 5.5 under ideal conditions (18 × 5.5 = 100 watts).

When you use a PWM charge controller, the voltage will drop to 12v but the amps will stay the same (5.5). As a result, 34% power loss (12 × 5.5 = 66 watts). But an MPPT charge controller will increase the amps to 8.

PWM vs MPPT charge controller efficiency:

• MPPT efficiency: 98%
• PWM efficiency: 75-80%

Let's say you're using an MPPT charge controller and this is one which you should use for this large PV system.

Multiply the above value by 1.2 for PWM charge controller or by 1.02 for MPPT charge controller.

`Total solar power required: 2760 × 1.02 = 2815 watt-hours`

### 5. Divide total solar power required by desired charge time (in peak sun hours)

Peak sun hour is when the solar radiation averages about 1000 watts per meter square for an hour.

In simple words — 1 peak sun hour = 1kW/m2 solar radiation.

`Solar power required per peak sun hour: 2815 ÷ 5 = 563 watt-hours`

Read the below post for an in-depth guide on what are peak sun hours, their importance, and how to calculate them.

Must read: Peak Sun Hours: Explanation, Importance, How To Calculate?

### 6. take into account solar panel output efficiency

Solar panels are designed to produce their mentioned wattage rating under standard test conditions - STC. Which includes: 1kW/m2 solar radiation (also known as peak sun hour), 25oC temperature, and 1.5 air mass (AM).

But in real world conditions, you will rarely experience 100% output from your solar panels. 80% is the most common percentage. For example, a 100 watt solar panel will produce about 80 watts per peak sun hour.

Now the last step, multiply the solar power required per peak sun hour by 1.2.

`563 × 1.2 = 675 watts`

Turns out, you need around 700 watts of solar panels to fully charge a 12v 400ah lead acid battery from 50% depth of discharge in 5 peak sun hours.

Related post: Solar Panel Output Calculator - What's the average solar panel output?

## What Size Solar Panel To Charge 400ah Battery?

Here are charts on what size solar panel you need to charge your 12v, 24v, or 48v 400ah battery in desired peak sun hours.

### 12v 400ah Battery

Table: what size solar panel to charge 12v 400ah lead-acid or lithium (LiFePO4) battery

#### Summary

• You'd need around 550 watts of solar panels to charge a 12v 400ah lead acid from 50% depth of discharge in 6 peak sun hours. And 950 watts of solar panels for lithium (LiFePO4) battery from 100% depth of discharge.

### 24v 400ah Battery

Table: what size solar panel to charge 24v 400ah lead-acid or lithium (LiFePO4) battery

#### Summary

• You'd need around 1.32 kWh of solar panels to charge a 24v 400ah lead acid from 50% depth of discharge in 5 peak sun hours. And 2.3 kWh of solar panels for lithium (LiFePO4) battery from 100% depth of discharge.

### 48v 400ah Battery

Table: what size solar panel to charge 48v 400ah lead-acid or lithium (LiFePO4) battery

#### Summary

• You'd need around 2.65 kWh of solar panels to charge a 48v 400ah lead acid from 50% depth of discharge in 5 peak sun hours. And 4.65 kWh of solar panels for lithium (LiFePO4) battery from 100% depth of discharge.