When sizing up a system people often ask how long they will be able to use a particular appliance before running their battery flat.

To work this out you will need to think about:

• What appliances you are using and for how long

• What other constant loads you have - for example fridges, lights, water heaters/pumps

• The charging source you have - generator/solar/alternator

• The age of the battery/batteries.

Next, let's clarify some terms.

Watts (W) is the name given to the amount of power an appliance uses and is usually found on the back or underside of an appliance.

Amp hours (Ah) is the amount of power your battery holds.

To work out how much power an appliance will draw from your battery we first need to understand the following calculation:

V (Voltage of battery) x A (Amps of current draw) = Power (Watts)

To work out how many amps an appliance draws, we switch this around:

Amps = Watts / Volts 

These Amps are the per hour draw from continuous use, so to calculate the amp hours consumed from the battery, we then add in a time based factor as well.

This is usually per minute of use for most devices. We convert this to minutes of usage by dividing the above amps by 60 mins then times by the number of minutes of use to get amp hours consumed from the battery.

Let's run through a common example, the coffee pod machine. 

A small coffee pod machine tends to range between 1300-1400W. Power conversion losses from converting 12v DC battery power to 230v AC mains power in an inverter uses about 10% more power than the actual appliance draws, so expect around a 1540w draw from the battery (1400w x 1.1 = 1540w).

Assuming a cup of coffee takes about a minute to make:

1540w / 12 volts = 128 Amps

128 Amp / 60 mins = 2.13 Amps per minute

2.13 Amps x 1 min use = 2.13 Amp hours consumed from the battery per cup of black coffee.

If you’re more into white coffee, then a milk frother will consume around 540W. Add your 10%, gets you to 594W. Using the same calculation above…

594w / 12 volts = 49.5A

49.5A / 60 mins = 0.825 Amps per minute

0.825 x 2 minutes to heat milk = 1.65 Amp hours consumed from the battery

2.13 Amps for the black coffee plus 1.65 Amps for the milk = 3.78 Amp hours total

So, for a hot cuppa (with milk), you’re looking at removing approximately 4 Amp hours from your battery.

If you have a 100 Amp hour lithium battery (with a recommended draw of 80ah for best longevity), this means you can consume 20 cups before your battery reaches 20%.

BUT…this assumes you are not putting anything back into your battery.

A decent 200W solar panel (in good conditions) will add back in about 10-14amps, so unless you are drinking 20 cups in a row, you’ll barely notice the odd cuppa throughout the day.

As a general guide, here’s some typical power draws from your average appliances again using the above calculation:

• Caravan microwave (1200W) - 10 minutes of use = 18.3 amp hours.
• Toaster (800w) - 3 mins use = 3.6ah
• Kettle (2200w) – 3 mins to boil = 10.1ah
• Induction cook top (2000w) – 10 mins cooking = 30.5ah
• Laptop (80w) – 120 mins of charging (not usage) = 14.6ah
• TV (40w) – 120 mins = 7.33ah 

So, what does your inverter use when you aren’t using it?

A good inverter like the AllSpark Pure Sine Wave Inverters will have a very low no-load/idle power draw (0.3-0.6 amps), which means that while your inverter is sitting idle but still turned on, it will not be running your batteries flat.

However, it is worth mentioning that even some well known/premium brands do not always meet this criteria and can consume substantial battery capacity while sitting at idle – as much as 2 amps per hour or over 40 amp hours per day.

It is always good practice to switch off your inverter when not in use to avoid any waste and this is where a remote switching panel really comes in handy, especially if your inverter is mounted under a bed or couch or in the front boot.

What about other constant loads?

People often forget to take into account constant loads like fridges, water pumps/heats, and lights.

This is why a battery monitor is a great addition to any system.

A battery monitor allows you to plan what you can and cannot run based on your current setup and provides a data point for any upgrades you may be considering.

Once you’ve set up your constant loads, being able to measure the power draw of your system means you can then work out how long you can run your extra appliances.

In the end, the amount of power your inverter uses is a direct result of the appliances you are using. Knowing how to calculate the correct size for what you need to run, and the amount of power your appliances draw, means you can have more control over your battery usage.

This will give you the confidence that your system can handle the way you travel.