What Size Solar Panel and Battery Do You Need

A common question we get asked is what size solar panel and battery do I need for a fully off the grid set up? 

When it comes to installing your own solar set up, you first need to determine your power requirements. This will depend on what appliances/items you need to power and how long for.

To work this out, you need to know the volts and watts or amps for each appliance. This information can usually be found on the appliance or in the product specifications online.

Not at this stage yet? Read our other blogs on solar here

To work out your power requirements:

1. First, list all the items you need power for.  

2. Then fill in the watts for each item. If you don’t have the watts, you can work it out by multiplying the volts and amps.

3. For each item, multiply the watts by the number of hours the appliance is used per day.

4. Finally, add up the watts per day for each appliance to get your total.

To work out the solar panel size:

1. Divide the total watts used per day by the number of sunlight hours.  The number of sunlight hours changes throughout the seasons, so use the “worst-case” scenario which is winter. In NZ, this is usually around 4 hours (but can be as low as 2 hours depending on location, panel angle etc).

2. This gives you the minimum watts your solar panel needs to be so round it up to the nearest available solar panel size. You may need multiple panels to meet the minimum watts. 

To work out the battery size:

1. Divide the watts used per day by the voltage to get the amp hours (a/h) used per day.

2. Then, divide the amp hours by the maximum battery discharge percentage to get the required battery amp hours. For Lead Acid and AGM batteries this is 50%, however Lithium batteries can be discharged to 90%, meaning you can get away with a smaller and lighter lithium battery than conventional batteries.

3. This gives you the minimum a/h your battery needs to be so round it up to the nearest available battery size and keep in mind you may need multiple batteries. 

Based on the examples used in the solar panel planner, a 80W solar panel and 75a/h battery would be suitable.

Keep in mind that this is for a completely off the grid system where you are fully charging your battery every day, and this is the minimum requirements for a winter’s day. You don’t want to run out of power so it’s better to round your numbers up and have a system that is slightly bigger than you need rather than not big enough. 

This information is to be used as a guide only. Always seek the help of a relevant professional or tradesperson if you have a technical query.

Read our other blogs on solar here. 


For more information about our range of solar products for boats and RVs, phone us on 0800 102041, email us or Livechat with one of our staff via the website. Better still, visit a store for personalised advice about your solar needs.

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We provide general information on products, not personal advice.  Always seek the help of a relevant tradesperson if you have a technical query.

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