I am having some trouble understanding if I am doing my calculations correctly when trying to calculate the battery size / solar panel wattage for our RV.

I started off small by just calculating for my laptop.

My laptop draws about 5 amps on 120V (mains) which is 600W.

So next I did the calculations for the battery and solar panels. I am assuming that because I am using a 12V battery I will need to change the amps by a factor of 10 to get the same wattage.

Battery (12V) - 50 amps * 12V = 600W (So if I use my laptop for 8 hours then I am looking at 50amps*8hours = 400 amp hours)

Solar Panel (18V) - 50 amps * 18V = 900W

This seems a very big setup for just running one laptop, can someone tell me if this is actually correct or where I am going wrong. Does the inverter affect this or is my math just way off?

This is my amp measurement while my laptop is on. enter image description here

enter image description here

  • 2
    \$\begingroup\$ 600 watts is a lot for a laptop; my laptop is currently consuming 6.1 watts. Granted it's not doing that much, since I'm just on this website with a few things running in the background, but I think you should double check where you got that 600 watt figure from. \$\endgroup\$
    – Hearth
    Jul 14, 2021 at 2:33
  • \$\begingroup\$ @Hearth It does say 65W on my charger, but when I measured the amperage at the plug using an amp meter it says between 3-5amps. Maybe the amp reading is wrong? I'm a bit new to measuring current. \$\endgroup\$ Jul 14, 2021 at 2:43
  • 3
    \$\begingroup\$ You're probably measuring it wrong, then, because 600 watts is utterly absurd for a laptop. \$\endgroup\$
    – Hearth
    Jul 14, 2021 at 2:44
  • 2
    \$\begingroup\$ With your new image, can you show us what the thing you're clamping the meter around is? Some of those have multiple turns to increase sensitivity, and you have to divide the reading by the number of turns. You also shouldn't be using the 200 amp range to measure such a low current. \$\endgroup\$
    – Hearth
    Jul 14, 2021 at 2:52
  • 2
    \$\begingroup\$ Edit your question to add a picture of that, just to be sure. But "x10" in this context almost certainly means "times ten". \$\endgroup\$
    – TimWescott
    Jul 14, 2021 at 2:54

1 Answer 1


Laptops do not draw 600W. Chargers are typically 10% of 65W rated for max brightness with battery charging and maybe 80-100W max on the AC side while charging with max display brightness. Do not be confused with fuse Amp ratings.

A typical car battery is only 50 Ah. 700 CCA for 30 s.

Solar power is far less than rated due to less than optimal usage such as placement, direction, weather and lack of MPPT controllers etc.

So your numbers are a bit off...

Do more reading be more concerned about energy supply and demand averaged over several days with efficiency.

  • \$\begingroup\$ Hmmm. I do see on my charger it says 65W. So 65W / 120V = 0.5A is the correct answer?? When I measure the amps at the plug using an amp meter it says between 3-5 amps though. \$\endgroup\$ Jul 14, 2021 at 2:40
  • 1
    \$\begingroup\$ looks like a measurement error somewhere \$\endgroup\$ Jul 14, 2021 at 2:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.