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Powering RV's and Power Usage

What do you do to power your RV when you have no mains power supply?
How to calculate the battery size to run lights and compressor refrigerator in my RV?

Battery State of Charge

Power for RV’s

Caravans, 5th wheeler caravans, camper trailers, motorhomes and campervans can all obtain their power from a mains/shore supply.  We all love to camp in national parks, bush camps or remote places where mains power may not be available but we would still like to have lighting in our RV or even be able charge our mobile phone, watch TV or a DVD or use a laptop computer.  But what do you do to power your RV when you have no mains power supply?

For bush/independent camping or ‘free camping’, as it is sometimes known, some RV’s are wired so the lighting and any 12V compressor refrigerator will be powered directly from the power stored in the battery in your RV.  The battery or battery bank (multiple batteries connected to increase the available capacity) is then topped up by the battery charger when connected to mains power, running a generator or by solar panels.

Motorhome power diagram


Note:  This article has not been written by an electrician, battery or solar expert & should not be considered a replacement for specialist advice.  The articles are provided for information purposes only.

Always seek professional advice for any battery or solar problems.  While great care has been made in the creation of these articles, we cannot guarantee the accuracy or omissions on these pages.  If in any doubt whatsoever, seek professional advice.


Power usage

Batter SOCFirstly, you need to consider what electrical appliances you expect to run from the battery storage system and calculate how much power (Amp hours or Ah) is required to run all the items over a 24 hour period.  The period you decide on is dependent on how much charging capacity you have to restore the power used from your batteries. After you have made that calculation, you then factor in about a 20-25% safety margin.  This calculation will assist in deciding what amount of amp hour battery bank capacity (or battery size) you require to meet your power needs. 

There are a number of calculators available on the web to help you calculate the minimum battery size and solar panels you require to meet your power needs and Bainbridge Technologies website has an excellent one. 

Some calculators show the amount of power expressed as Watts.  If this is the case, divide the wattage by the system voltage and this then gives you the current capacity in amp hours (Ah).  For example, an estimated usage would be roughly –

- 90 Ah battery could supply 1 amp for 90 hours, or 9 amps for 10 hours, or 5 amps for 18 hours

- 120 Ah battery could supply 1 amp for 120 hours, or 9 amps for about 13 hours or 5 amps for about 24 hours

An example of a calculation table of battery usage for a 12V system may be:-

Electrical Equipment

Power Consumption (or Watts)

Current (Amperes)

Hours used each day

Total Ah per day

Compressor fridge/freezer #





Interior lights
2 x 8w fluros *





Recessed exterior light – fluro










Lemair Washing
Machine via Inverter

270W (on spin)




Xantrex 1800


  1 A (idle)


   5 A

Water pumps
(ShurFlo Extreme)


10 (max)



Bose SoundDock





Computer and phone chargers





Fan «










Add 20% safety margin





Total Ah needed





# Vitrifrigo 230ltr - the refrigerator cycles.  This is average Ah per day and this could change  depending on how long the compressor is actually running.

*  Little Lunar fluro 0.75 amps x 2.  LED lighting would use less power

«  Bora 12V fan Model 748CA – 0.19 amperes on high

As it shortens battery life to discharge the batteries below 50% then you would not want to discharge the batteries below that point too many times (see battery manufacturer’s charge/discharge curves), so based on the above calculations, roughly your power requirement would be 188.35 x 2 = 376.7 Ah of battery capacity.  Of course, the other options are to limit your power usage by not running some appliances as long or as often as shown above or don’t use appliances which have a large power consumption.

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