4WD Power Consumption Calculator

How it works

Every appliance does the same simple sum. Its watts multiplied by its hours gives watt-hours. So a 45 watt fridge running ten hours uses 450 watt-hours. Add up every appliance and you get your daily total. That is the energy your battery must refill each day.

Amps are the same idea shown at 12 volts. Divide your daily watt-hours by 12 and you get amp-hours. A 450 watt-hour fridge draws about 38 amp-hours. Your battery is rated in amp-hours too, so the match is direct. This is how you see if a battery can carry your load.

Battery size then depends on chemistry. A lead-acid battery should not drop below half charge. Lithium can go far deeper and stay safe. The tool splits both figures for you. That way you can weigh cost against real usable capacity before you buy anything.

A worked example

Take a common weekend set-up. A 40 litre fridge runs at 45 watts for ten hours. That is 450 watt-hours. A strip of LED lights draws ten watts for three hours, so 30 watt-hours. A phone and a GPS use 18 watts for two hours, or 36 watt-hours.

Add those together and you get 516 watt-hours a day. Divide by 12 and that is about 43 amp-hours. A lead-acid bank would need roughly 86 amp-hours to stay above half charge. A lithium bank near 54 amp-hours covers the same day with room to spare.

This is the reason many tourers fit a 100 amp-hour lithium cell. It clears the daily load with a healthy margin. That margin is for heat, for extra fridge runs, and for wiring that has seen a few outback seasons.

How to use this calculator

Start with your fridge. It is almost always the biggest draw in a camp set-up. Enter its rated watts and a realistic run time. A good compressor fridge cycles on and off, so ten to twelve hours of run time is a fair summer guess.

Next add the small loads. Lights, phone chargers and a 12 volt fan are quick wins. Keep each entry honest. Do not count an hour you will never really run. When you are unsure, round the hours up rather than down. That builds you a safe margin.

Then read the three results in order. Check the daily watt-hours first. Then the amp-hour figure. Then compare the AGM and lithium bank sizes against what you can fit and afford. Press reset to clear the form and try another set-up a different way.

Interpreting your result

Amp-hours are the number that matters most here. Your battery and your solar panels are both rated in them. If your daily figure sits close to your battery's usable capacity, you have no room for error. Heat, a half-open lid and a long charge lead all push the real draw higher.

The solar figure is a rough guide as well. It assumes a good day of strong Australian sun. Cloudy coastal mornings and low winter angles cut that harvest short. Treat the panel number as a floor, not a ceiling. Add margin if you rarely get a plug-in.

If your result feels tight, the fix is usually simple. Run the fridge on a warmer safe setting. Switch to efficient LED lighting. Keep the fridge full and shaded, and out of direct sun. Each one trims your daily load without any new cost at all.

Accuracy, assumptions and limits

This tool uses rated watts, not measured ones. A label plate often shows peak draw, not the average. Real fridges sip power once they are stable, then spike on hot days. Your true figure can sit well below or above the estimate on any given trip.

Battery chemistry sets the safety limits in the maths. The AGM figure assumes a 50 percent depth of discharge. Lithium assumes 80 percent. An old or sulphated battery holds less than the sticker says. Inverter idle losses and thin cable also eat into what reaches your gear.

So treat the output as a planning map, not gospel. Confirm the exact figures against your own appliance and battery manuals. For any mains or fixed wiring, follow the AS/NZS 3000 Wiring Rules and use a licensed auto-electrician for the install.

Power in practice: heat, inverters and hidden load

Real trips rarely match a tidy spec sheet. Heat is the biggest single variable. A fridge in a hot rear cabin works far harder than one sitting in shade. The compressor runs longer, the duty cycle climbs, and your daily watt-hours can climb close to double.

Inverters quietly steal power too. Even with nothing plugged in, a small inverter idles away a few watts around the clock. Run it only when you truly need mains. Better still, charge low-voltage devices straight from the 12 volt side to skip that loss entirely.

Water and air add hidden load as well. A pump, a fan and a UHF radio draw modest current one at a time. Over a long hot night, they still add up. Build headroom into your bank, and the maths stays honest even on your worst afternoon.

None of this needs fancy gear. It needs honest numbers and a little slack. Plan for the worst realistic day, keep your loads tidy, and a sensible bank will carry the whole trip without drama.