I am converting my 2011 Nissan Frontier Pro-4x to an overland-ready setup. For this, I want to set up a separate electrical system to power certain items. I need some help calculating the requirements and consumption for my system.

Here is what I have planned so far for a dual-battery power supply setup:

  • Stock alternator to charge both batteries, 130 amps
  • 12v Deep cycle battery, need to know the right aH requirement for my setup
  • Battery isolator
  • Inverter (probably a 300w inverter)

And here are the power consumers I have planned:

  • Laptop MacBook Pro, connected to an inverter. Needs around 60 watts per hour per average (95w max). Needed for 8 hours per day
  • 4g modem & Antenna (Peplink Pepwave Max Transit Mini), connected to 12v DC. Needs 12 watts per hour. Needed 8 hours per day
  • Diesel Heater 12v DC, needs 72 watts per hour (only needed occasionally at night, in very cold weather)
  • Fridge, 12v DC, needs ~35Ah/day, runs all day

I'd expect the second battery to be fully charged by the alternator every 2 days at the worst.

I need help calculating the total aH requirements that I have, especially with the inverter it becomes a bit confusing.

Thank you!

  • Inverters are pretty efficient, so just go with Watts and get amps on the 12V side from that (i.e. 11W for the laptop is 1A@12V). Also remember that even for an AGM battery you should not discharge much below 50% of the rated Amp-hours, lithium may be better but has its issues as well. Finally, a dc-dc converter (e.g. RedArc) may act as a battery isolator as well (and some stand alone battery isolators won't work well with modern alternators, which would include your vehicle).
    – Jon Custer
    Nov 10, 2020 at 20:01
  • Also, you may be able to get an adaptor for your laptop to run directly from 12V. I know I can for a Mac laptop.
    – Jon Custer
    Nov 10, 2020 at 20:01
  • 1
    You did not specify the engine size, but 4 cyl should have 110A alternator, the V6 is 130A.
    – EᑎOT
    Nov 11, 2020 at 0:02
  • 1
    Your laptop wattage assumption seems to be strangely low. I'd guess an average laptop charger is around 19V and 5A or so, which gives you a more reasonable 95W. The following calculations assume it is 5A @ 19V, which is 8A @ 12V.
    – EᑎOT
    Nov 11, 2020 at 0:17

1 Answer 1


Here is my calculation that I came up with (please check if this and my conclusion makes sense):

|               | (Average) Wattage | Voltage | Amps | Hours used on a cold day | aH usage per day |
| Laptop        |                60 |      12 |    5 |                        8 |               40 |
| Modem         |                12 |      12 |    1 |                       12 |               12 |
| Diesel Heater |                72 |      12 |    6 |                       10 |               60 |
| Fridge        |              27.6 |      12 |  2.3 |                       24 |             55.2 |
|               |                   |         |      |                          |                  |
| Totals        |             171.6 |         | 14.3 |                          |            167.2 |

My conclusion is that at the very least a 200aH deep cycle battery is required to keep everything running for a day with the above usage pattern.

However, this would drain my battery well below 50% which would decrease the life span of the battery significantly.

I am thinking of getting 3x100aH batteries and connect them in parallel. Additionally, I worry a bit that my 130amp alternator might take a long time to charge the batteries up again.

I'm thinking of getting a solar panel to help keep those batteries charged. I'd also get a DC/DC charger to make sure the batterie(s) receive the correct voltage to charge up to 100%.

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