I am building my camper and want to know how to calculate exactly what batteries I need, and what solar panels to meet my needs. Here is my list of the main and most powerful devices that I will use, and from their documentation I found out their power:

Things that I will use constantly: iPhone, MacBook Air with Apple Studio Display, few LED lamps... They can be recharged and used for some time from the built-in battery, but they must also be included in the calculations.

Now I need a formula that would help calculate the required parameters. But here I see many additional factors, such as: gloomy winter days, it is not desirable to discharge batteries to zero, additional energy losses during energy transmission, etc.

I will not use all these devices at the same time, but sometimes I need to turn on several devices. I have too little knowledge in electricity, so I cannot understand how it is calculated, I need an explanation.

Can you show me the formula to calculate the electrical system required for my needs?

  • \$\begingroup\$ Total energy captured by your solar panels needs to exceed your total energy consumption. Sum up your Wh of consumption. What did you end up with? How many hours of solar irradiation are you willing to bet on per day? \$\endgroup\$
    – winny
    Commented Jan 24, 2023 at 10:07
  • \$\begingroup\$ Where are you located and where will the camper be used. This affects solar panel issues. |See www.gaisma.com for sunshine hours per day by location. || Do you have any battery type preference (lead acid, Lithium Ion, ...?) || How many sunless days do you want to run on solar only?|| Will you have a backup generator or mains power topup? || Anything else that will help us? \$\endgroup\$
    – Russell McMahon
    Commented Jan 24, 2023 at 10:35
  • \$\begingroup\$ While I can see problems with mirrors in stationary application, I frequently muse about their use in mobile applications. Sun not shining bright enough to keep everything fed? Start a battle for sun at the campsite! \$\endgroup\$
    – greybeard
    Commented Jan 24, 2023 at 11:24

2 Answers 2


This is "out of my head" with sleep calling.
By all means point out any errors for my attention.

That is probably an unrealistic expectation for solar powering a mobile camper - it MAY be OK for a camper in a fixed location.

A very rapid summing of your watt-hours per day requirement plus incidentals gives about 7000 watt-hour per day.

In winter in many locations winter equivalent full sun hours = sunshine hours == SSH is about 2 hours.
Some areas are far less and some rather more, so we need to know areas of use.

Average sun per day by month by geographical location can be found at http://www.gaisma.com.
SSH in my text can be found under Gaisma's kWh/metre_squared/day figure.

At 2 hours per day you need 7000/2 = 3500 Watts of solar panels. A PV panel will give ABOUT 200 Watts per square meter of panel in full sun so you need 3500/200 ~= 18 square metres of panels.
That's 2m x 9m or about 6'6" x 30 feet.
A VERY large solar array.
Packing that up into a mobile unit would be interesting.
Setting it up permanently to supply a fix location camper would also be interesting.

It's likely that you can get a far more sensible, albeit not as cheap to operate, but cheaper to buy, system by using eg LPG for all heat sources practical.

Items that could realistically be replaced by gas powered equipment include:


  • Induction Cooktop - 1800W (1 hour a day)
  • Space Heater - 1200W (few hours only in winter)
  • Hot Water Heater - 1440W (1 hour a day)
  • Kettle - 1500W (1 hour a day)
  • Mini Clothes Dryer - 1300W (30 min - 1 hour)


  • Hair Dryer - 1875W (5-10 min a day)
  • Clothing Steamer - 600W (30 min - 1 hour)


  • Blender - 175 W (30 min)
  • Water Pump - 90W (30 min)
  • iPhone, MacBook Air with Apple Studio Display, few LED lamps...

You could use as much solar as practical to offset some gas use.
Thermal solar heating may be useful.


Per item: Load = watts x hours per day

Sum all loads.

PV panel needs for total solar operation Load_sum / Winter hours of sun per day x days without sun

Battery capacity = Load x days without sun x Safety multiplier

Safety multiplier

  • for lead acid is 2 or more.
  • For Lithium Ion is 1.2 - 1.5

Solar input:

SSH - Sunshine hours per day - average by month - for Prague

Total daily Watt-hour load / Full sun hours per day = solar panel watts.

The Gaisma page for Prague shows October to February SSH of under 2 hours per day. April to August is about 4 or more.
SO on an average day in given month divide the daily Watt-hours load by the SSH figures (kWh/m^2/day from table) to see solar panel watts needed OR Watt hours / day load = SSH x PV panel watts.

Example: In March SSH = 2.5 hours full equivalent sun per day, on average.
At 100% efficiency and load matched to panel and or battery:

  • A 1000 Wh/day load would need a 1000/2.5 = 400 Watt panel.
  • A 300 watt panel would provide a 2.5 x 300 = 740 Wh load.

BUT efficincy is always under 100% and load to panel matching is never perfect. An MPPT (Maximum Power Point tracking) controller will help optimise panel energy transfer to the battery.

Assume that achievable energy available compared to theoretical maximum is 50%-90%.

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  • \$\begingroup\$ My location here: gaisma.com/en/location/prague.html And then I will travel to the south of Europe. I do not understand chemistry, so I do not know which type of batteries to prefer. I want to be able to live for 1 day on solar energy and have the batteries charged again tomorrow. I plan to rent appartment once a week, so I will be able to charge from the wall power. If I only have one device on at a time, does that make the idea more realistic? \$\endgroup\$ Commented Jan 24, 2023 at 18:00
  • \$\begingroup\$ @TamilaAmbeon Total daily Watt-hour load / Full sun hours per day = solar panel watts. That Gaisma page shows October to February SSH of under 2 hours per day. April to August is about 4 or more. \$\endgroup\$
    – Russell McMahon
    Commented Jan 25, 2023 at 11:28

This article addresses your requirement in some detail including the calculations involved.

You may go through article and calculate according to your own requirements. It's not appropriate for me to just copy the answer in detail but, in summary, it describes, load sizing, panel capacity calculations, and battery and & inverter issues.

Note that this is more simplistic than will be required in practice, but is a good start.


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