What size battery do I need to run my set up and for how long?

What size battery do I need? This is a really common question we get asked and it's a lot of the design work we do, calculating a customers estimated power needs and pairing the right batteries, chargers and solar panels to match. 

You can check out our batteries here.

Battery basics

There are lots of different types of battery (AGM, GEL, wet lead acid, EFB, lithium etc.) We generally use wet batteries or AGM batteries. Sometimes for higher powered conversions, we would use lithium batteries.  The content and explanations below wouldn't include lithium batteries as they behave differently, we are writing some articles on these separately.

Battery capacities are generally marketed in Amp hours (Ah), this is a measure of how much power is in the battery, depending on the brand, this figure will be marked by C5, C20 or C100, meaning the test discharge was done over 5, 20 or 100 hours. We try to always use the C20 figure as it tends to be the closest to how most people use their batteries. 

But what we prefer to use is the usable Watt hours (W/h), The number of Watts of power a battery can deliver us over the 20 hour period. We find this is really good at comparing product loads and average uses with batteries. It also allows us to standardise batteries with different recommended discharge levels (which can lead to inaccurate readings as the ampage will change as the voltage decreases).  We test all of our batteries in house, on a standardised test, allowing us to make direct comparisons between all of the batteries we stock. You can see the results of our most popular batteries below in the battery table below. 


Here we've got our power consumption calculation formulas with some common items you might find in a camper,  the formula below explains how the calculator will work.

Download our battery calculator here.

Power required per product = Power of the product x duration of usage x quantity of that product

Example: LED down lights (4) = 1.8W x 4 (hours per night) x 4 lights = 1.8 x 4 x 4 = 28.8W/h

Typical power consumption of popular products

ComponentPower (W)Duration of usage (h)Quantity (Q)Watt hours (W/h) (W x h x Q)
LED down light (each)1.84416
LED reading lights (each)1224
Waeco CRX-50 Fridge (average over 12 hours empty)17241408
Dometic CRD-20 Fridge (average over 12 hours empty)10000
12V water pump 200.112
USB port charging a mobile phone 123136
12V TV271127
1000W inverter in standby mode 11000
240V travel kettle (600W) run through an inverter 660000
Total Watt hours per day


Notes for the above table: When running an item through an inverter, multiply it's quoted wattage figure x1.1 or add 10% to allow for the inefficiency of the inverters process.

The table text in green is filled in for an example of a Swb T5, for 24 hours between arriving at your stop and leaving, based on typical usage. The fridge in our test was empty, it's likely that it will be more efficient if it's pre-packed with cold items and not opened regularly. (we opened it once per hour to check the internal temperature). 

Power required in the van = Sum of all products' power x desired number of days between charges

Our most popular batteries

BatteryLow height lead acidLow height 95Ah AGMLow height 100Ah EFB
Amp hours capacity C2082Ah95Ah100Ah
Watt hours capacity C20450W/h850W/h900W/h
Battery technologyWetAGMEnhanced flooded battery
Warranty4 years 3 years2 years
Requires ventingYESNOYES
Dimensions (WxLxH)354x175x190mm354x175x190mm354x175x190mm

Which battery is for me? 

Our battery calculator will recommend the minimum battery you could use, but only considers power consumption, so please use the table above to help you decide. 

As you can see there's a bit more information here than just the power a battery will give you.  Warranty, venting requirements, size etc. 

The one that's very important to a lot of people is the venting. liquid filled wet lead acid batteries and enhanced flooded batteries need to be vented to the outside (why? click here). This is done via a small hole at the top of the battery. No problem at all if it's under the bonnet or in a locker that's vented to the outside but sealed from the inside. However, for putting the battery under your drivers seat this can be a bit more of a pain. So it's worth considering. 

We hope this helps you on your way to working out what batteries you'd like to use and if your requirements don't quite meet up with our calculator or the batteries we supply, then we'd love to hear from you so we can design a more bespoke set up to suit. 

Thanks for reading. 

Have a great day.