How long does 400 watts last

As any parent with multiple children can attest, having portable AC power available for long road trips is a blessing. While many new vehicles now come with built in power inverters, there are millions of older family vehicles that do not have this option.

Keeping the family occupied with portable movie players, video game consoles, mobile devices is an absolute must, however, many of these accessories do not plug in directly to 12V outlets. If we have a water pipe; Volts would be a measure of the water pressure force in the pipe, Amps would be a measure of the current or flow through the pipe. A Watt would be the measure of of what you can do with that water, like turning a water wheel.

So, how do we determine Watt hours? Watt Hours are calculated using a similar equation when dealing with batteries. A motor start current, or surge current, is about 3 times the running current. An average current is needed to work out how this type of appliance would drain a Ah battery. Source: euenergylabels. Each fridge label carries an estimate of the energy it will consume on average over the year in kilowatt-hours kWh.

The first step if to work out how much energy it takes per hour:. How does this relate to battery amp-hours? For an appliance such as a W refrigerator, we can equate the hourly watts used to battery capacity by converting amp-hours Ah into watt-hours Wh :. Really, we only have 50Ah available, or Wh. I'll ignore the inverter losses to keep it simple:. A word of warning: Look at the chart below and you'll see that the battery volts goes down as it discharges.

Because of the above, I would revise the comfortable running time down to 10 hours. How do you know how much capacity is left in your Ah battery? There is a rough relationship between lead-acid terminal voltage , at least, good enough for an estimate of how much power you have left. The calculator below finds the run time for appliances running on 12V DC supplied from a deep-cycle Ah battery. Enter the wattage and see the run time:. Calculating run time for AC appliances is a bit more complex, because an inverter will be used and also some AC appliances have compressors installed.

The calculator below doesn't account for refrigerators and similar appliances freezers, air conditioning, heat pumps as I covered that quite extensively earlier on in the post:. A Ah 12 volt deep-cycle lead-acid battery will power a W AC fridge for 30 hours, drawing power at a rate of 40 watts per hour.

A Ah battery will run a W DC appliance for 1 hour. An AC appliance rated at W will only run for 0. A Ah lead acid deep-cycle battery will run a 30lbs thrust trolling motor on Speed 4 for 4 hours. It's often said that a Ah amp-hours battery can deliver 1 amps for hours or amps for 1 hour. This is not really an accurate description of battery operation. Batteries should never be discharged completely so the rated capacity isn't available for use.

When higher currents are drawn the battery capacity reduces significantly , so the amps vs hours relationship isn't linear at all.