If you’ve got a 300 Ah battery system and plan to build an off-grid solar system using it then you need to work out what size (watts) of solar panels to use for your system to power the connected appliances and keep the batteries charged.

Beyond buying the right size of panels (watts), you’d need to figure out how many of them you need, to check if there’s enough roof space to install them.

This post will hopefully help you work out what size and number of solar panels you need, the charge controller rating, and what appliances you can run off it.

## How Many Solar Panels you Need?

The number of solar panels you need can vary depending on whether you use AGM or LiFePO4 batteries.

**AGM batteries**

If using AGM batteries in your battery bank then plan on a 1 kW solar array. The actual number of solar panels depends on the solar panel size (watts) used as a building block in the array.

For example, you’ll need 4 x 250 watt solar panels or 10 x 100 watt solar panels for a 1 kW solar system.

*Note: This assumes your location receives about 4 hours of sunshine per day.*

**LiFePO4 batteries**

With LiFeP04 batteries plan on a 2 kW solar array instead. Lithium batteries have more usable capacity (80%) compared to AGM and can be discharged more than the AGM batteries.

The actual number of solar panels depends on the solar panel size (watts) used as a building block in the array. For example, you can use 8 x 250 watt solar panels or 20 x 100 watt solar panels for a 2 kW solar system.

* Note: This assumes your location receives about 4 hours of sunshine per day. *

**The calculation**

The estimated solar panel system size is based on the formula,* (battery capacity (Ah) x battery bank voltage (24V) x recommended depth of discharge of battery)/ average sunshine hours for your location.*

The recommended depth of discharge is 50% for AGM and 80% for LiFePO4.

Assuming a 300Ah, 24V AGM battery bank at a location that receives on average 4 sunshine hours, solar panel size is (300Ah x 24V x 50%)/4 hours =900 watts of solar or 1 kW solar system.

## How Long to Charge the Battery Bank?

On average, if the battery is discharged to its recommended depth of discharge using the solar system sizes above, it will take approximately 4 hours to fully charge the system.

Actual time to charge depends on the extent to which it is discharged and how many hours of sunshine are received at the installation location.

## How Long the Battery Lasts?

How long the battery lasts depends on the charge level of the battery bank and the combined energy consumption of the connected appliances.

Assuming a fully charged 300Ah, 24V AGM battery bank as an example, with 3,600 wh of energy, you could run the appliances:

- Fridge, 150 watts for 10 hours ~1,500 watt hours

- Microwave. 1,000 watts for 15 min, ~250 watt-hours

- Coffee maker, 1,000 watts for 15 min, ~250 watt-hours

- TV, 50 watts for 4 hours, ~ 200 watt-hours

- Lighting, 10 LED lights each rated 10 watts for 5 hours, 500 watt-hours

*Total energy consumption is 2,700 watt-hours which leaves 900 watt hours to spare, from (3,600 watt-hours – 2,700 watt-hours)*

### What Size of Charge Controller to Use?

If you’re using a 1 kW solar system connected in a 24V configuration then you can use a **60 A charge controller or larger**. For a 2,000 watt, 24V system use a **100A charge controller or larger**.

## What size of Inverter to Use?

Inverter size depends on the combined power consumption of the connected loads.

With that being said, some 1 kW solar panel systems come installed with a 3,500 watt inverter but you could as well use smaller capacity inverters such as a 1 or 2kW inverter as an example.

Keep in mind that you are limited to the connected loads that match the power handling capacity of the inverter.

## Closing Thoughts

If you’ve got a 300Ah battery bank, the number of panels to use (and for any system) depends on both how many sunshine hours the location receives and the energy drawn by the connected appliances.

Assuming a location with 4 sunshine hours, then you can consider a 1 kW system for a battery bank with AGM batteries or a 2 kW system for LiFePO4 batteries.

Hope this helps!

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