Solar Battery Bank Calculator

Graham Bogie

This solar battery bank calculator will help you correctly size your batteries for the daily load and energy usage in your camper solar setup.

It’s simple and straightforward to use. We’ve included a section below to answer some questions you may have.

This calculator forms part of our camper solar setup series and the wider subject of campervan electrics

And it’s just one of our electrical calculators for RVs and campervan conversions too.

Calculating the minimum size of your solar battery bank is the first step in planning your electrical design. 

Get this right and you can complete your DIY campervan conversion with confidence.

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Includes 110v & 240v, solar, B2B, batteries, inverters, 12v, 24v & 48v systems, wire gauges in AWG & mm² & much more!


How do I use this solar battery bank calculator?

  1. Enter every electrical device and component you’ll use in your camper
  2. Enter the watts each device uses
  3. Enter the maximum number of hours each day you’ll use each device
  4. Select whether each device is DC or AC
  5. Enter the amount of contingency you want to add

How do I know all the electrical devices I’ll use?

Getting a complete list of electrical appliances, devices and components you’ll use in your RV is the most critical part of sizing the electrical system.

Underestimate it and you may run out of power.

Overestimate and you’ll probably spend more money and make your setup more complex than necessary.

Work through your typical day, considering all the appliances you’ll use along the way. 

Don’t forget any gas or diesel appliances that need power to start them up.

And if you intend to use any AC appliance, remember to include the right size inverter on the list too.

I won’t use all my devices everyday. What should I enter in the calculator?

The aim of the calculation to size your camper electrical setup so you have enough power storage every day.

With this in mind, it’s important to consider your most heavy usage day.

Maybe you intend to live in your camper during the winter. 

Your heating will run for longer and probably your ventilation fan too (it’s the best way to stop condensation). 

You may be living indoors much more and with fewer hours of daylight than during the summer months. All this adds up to a greater energy demands.

Think it through carefully. It’s not going to be 100% accurate but you can get close enough.

How do I find the watts of each device?

The watts of most devices can be found either printed on the device, in the operating manual or the manufacturer’s website.

Sometimes, products list the power usage in current (i.e amps).

In this case, use the wattage calculator at the top of the solar battery bank calculator above to convert the current and volts to watts.

The way the power is stated on devices isn’t standard. Look out for things like:

  • Power: 12V/2.5A
  • Voltage: 12v Wattage: 25w
  • Amp Draw 12V/DC: 4.2

For some devices, in particular those with battery charging adaptors like camera batteries and laptops, use the input values specified not the output values.

How do I know my appliance is AC or DC?

The labels on specification of most appliances state whether it is AC or DC.

Failing that, if it has plug you could use in your home sockets, it is probably an AC appliance.

For calculation purposes, if in any doubt, assume it is AC as this produces a higher number so errs on the side on caution.

I have more than 5 devices. What shall I do?

If you have more than 5 devices to list follow these steps:

  1. Enter the 1st 5 devices
  2. Press the “add more devices” button
  3. A further 5 rows are displayed to add more devices and another “add more devices” button

You can add upto 25 devices in total.

I have more than 25 devices. What shall I do?

If you have more than 25 devices to list follow these steps:

  1. Enter the 1st 25 devices
  2. Change the contingency value to 0%
  3. Make a note of the Daily Power Usage figure
  4. Refresh the page in your browser to reset the calculator
  5. Enter the 1st device as the total of the first 25 devices
    1. Watts = Wh you noted down
    2. Hours = 1
    3. DC or AC = DC
  6. Add your additional devices

Why is the calculated watts higher for AC?

AC appliances need AC power but camper batteries produce DC power.

The DC power is converted via the inverter to AC power but there’s a cost. AC appliances use a greater power factor in the solar battery bank calculator.

I already know my total load in watt hours. Can I still use the calculator to size the battery?


If you already know your total load in watt hours, enter 1 device with the total watt hours in the watt field, 1 hour and list it as DC.

If you list it as AC, the calculator will increase the watt hours by the AC power factor.

How much contingency do I need?

How much contingency you include is completely upto you.

We’ve always used about 20% and it has served us well.

If you don’t rely on your electrical system, consider reducing it a little.

If unsure of your power use or appliances, consider increasing the contingency – you’ll never complain about having too much battery.

You can deduct contingency too by using a negative number. The calculator will warn you the value should be a positive number but will calculate accordingly.

What is the difference between Gel, AGM, FLA and Lithium Batteries?

Battery technology varies widely and advancements are being made all the time.

Currently, the most common batteries are Gel, AGM or Lithium. FLA use dated technology and we recommend steering clear of them.

Lithium batteries are significantly more efficient than Gel or AGMs so can be discharged to a much lower level.

This explains why you need a smaller Lithium battery than either AGMs or Gels.

But this doesn’t mean a lithium battery is always the best option. 

To assess which battery is most cost effective and best suits your needs, use our battery life calculator and comparison table.

More information, read our detailed post on campervan batteries.

How much battery should I install?

The calculator works out the minimum size solar battery bank you need to power your deivces.

However, if you only install the minimum amount of battery, you must have the ability to fully recharge it every day. 

Otherwise, you’ll discharge your batteries faster than you can top them up and end up with flat batteries pretty soon.

If you have a reliable source of recharging, like a hook up facility, you may not need any extra battery.

But if living off grid and rely on charging the batteries from solar panels or the alternator, you may need to consider increasing the amount of battery.

Whatever you decide, there is no point installing a battery bank greater than you can reasonably expect to charge.  

Do I need an inverter?

You only need an campervan power inverter if you want to use AC appliances powered by your deep cycle battery.

If you use AC appliances only when hooked up and use AC sockets on the mains circuit, you don’t need an inverter.

What size inverter do I need?

Check out our inverter size calculator

Need more help calculating your campervan electrical components?

Here’s the links to our other calculators:

Graham Bogie

Graham Bogie - Expert in RV & Campervan Electrics - Mowgli Adventures

Graham is a seasoned marine electrical engineer with two decades of experience designing customized electrical systems for plant machinery and converting campers and overland vehicles. His expertise has led him to author the reputable Campervan Electrics Handbook and become the chief designer of our Campervan Electrical Design Service. As a knowledgeable figure in the field, his YouTube channel, blog, Facebook group, and newsletter, offering electrical advice and product reviews, reach more than a million users each year.

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