Thinking in Wattage

 

Confused about portable power storage for your camping applications? Why not go with the flow and think in wattage?

Words Rachael Doherty

Getting off-grid in comfort is now easier than ever before, with fixed and portable campsite power rapidly evolving. Trouble is, off-grid campsite power storage solutions are rated at various Amp Hour and Voltage combinations, creating confusion when comparing different systems. That’s why we’re flipping the sums to uncover what we use and what we need. 

Why Watts?
Electrically speaking, the power your appliances use at any given time is measured in Watts (i.e., the flow), as a product of Voltage and Amps. Watt Hours (Wh) represent that usage consumed for an hour. So, if a device uses 20 Watts for an hour, it has used 20Wh. Watts and Watt Hours are useful when comparing power systems as a battery’s Watt Hour capacity dictates your available power. Solar panel output is also measured in Watts and an appliance’s maximum Continuous Watt usage is usually easy to find or calculate. 

Multiple Voltages?
Manufacturers tailor lithium battery Voltage for specific needs. Lithium camping batteries usually consist of 3.2V or 3.6V cells, depending on the chemistry. Batteries rated between 12.8-14.4V are often for cars and compatible caravan setups. Some 18-32V batteries are optimised for power tools. While other batteries use Voltage to optimise longevity and charging capabilities.

Thinking in Watts
To calculate an appliance’s Continuous Wattage, multiple Voltage (V) by Amps (A). For example, a fan that uses a maximum of 0.35A on 12V (.35 x 12) will use up to 4.2W continuously.

Battery Wattage
To calculate your battery’s Watt Hours, multiply the Voltage (V) by Amp Hours (Ah). For example, a 23.2Ah x 43.2V lithium battery stores 1002Wh, which in theory lets you use 1002W for an hour–or 501W over two hours or 250.5W over four hours and so on. Large battery banks store multiple Kilowatt Hours (kWh), each equalling 1000 Wh. In practice, a battery’s chemistry and other power system variables reduce the usable power available which we’ll explain later.

12V & 240V Devices
Voltage is all about power potential. A 10A 240V domestic outlet, for example, can deliver 2400W compared to 180W for a 12V 15Ah merit plug in your car or RV. Some appliances use more wattage than others; electric heating and cooking require lots of power due to the heating element, which is why toasters, electric kettles, heaters and hair dryers are usually 240V.

Inverters 
Not all 240V appliances use more wattage than their 12V cousins. But you still need to convert the power source to 240V to plug it in off-grid–which is where pure sine wave inverters fit in. Inverters are rated by their maximum Continuous Wattage, which can be as low as 150W (less than that merit plug) or up to 5000W. As they’re rated for Continuous Wattage, pure sine wave inverters allow for split-second spikes in Wattage that occur when switching on appliances.

Where did my power go?
So you’ve crunched the numbers but you’re not getting enough power from your portable or fixed power system? Here are common factors that influence usable power–whether it’s a battery box, fuel generator, solar generator or complete retrofitted RV power system:

  • Inverters have a ‘conversion rate’, typically 90 per cent but can be as low as 80
  • Lithium batteries typically have an 80 per cent working capacity
  • AGM batteries have a working capacity of about 50 per cent 
  • Voltage is vital when you’re charging a battery, 12V AGM batteries for example require 14.7V to charge fully, which is why DC-DC chargers exist 
  • Fuel generators across the board don’t advertise their continuous wattage. This figure is usually published in the specifications 
  • Weather conditions and shade heavily influence solar panel performance 
  • Power systems have inherent ‘resistance’. The heat caused by wires that are too thin or poor connections, for example, dramatically reduces a system’s efficiency. The AS/NZS 3001.2 explains how to minimise this