Solar Calculator

These are the instructions for the gear4solar calculator.

This Excel spreadsheet automatically calculates battery, solar panel, and inverter sizes for your motorhome, boat or other off-grid installation.

This calculator is meant to simplify solar for competent persons building DIY, or allow unqualified users to show consistent requirements to contractors.


  • Sample data is already entered to get you started
  • Simply replace the current values in the cells that are pale yellow with your own data - here, and here
  • DO NOT edit any other cells
  • Work your way down the page. As you enter your data you will see other values elsewhere on the sheet are recalculated
  • Note that you can mouseover the little red triangles to see tooltips with useful advice
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The first step is to enter a list of your devices in the table under the Devices heading.

Once you have entered the name of the device in the description column, enter the wattage. Be as accurate as you can. You may need to look up the wattage for each appliance from it's user manual, or from a specification plate fixed onto the item itself.

If we just assumed that we ran our appliances all day, we would end up over-sizing our batteries and panels, which is expensive and space intensive. So, we should make an allowance for how many hours per day you expect your devices to run.

Have a think about how long in hours per day you will realistically operate each device.

In the sample data you will see entered a small 12V fridge runs constantly, whereas the 230V microwave is only used for 3 minutes a day.

In a typical motorhome or boat installation you will use a mixture of 12 volt and mains appliances. The distinction is important because you can save money on your inverter. Only your mains appliances need an inverter, which can be an expensive bit of kit.

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Battery Sizes

The next section calculates the size of battery required to power your loads.

It is assumed you will use 3.2V Lithium-ion Phosphate cells connected in series to create 12V, 24V or 48 Volt systems.

Note that we round off when talking about the voltage of our systems The nominal voltages are slightly higher. For example the nominal voltage of a 12V Lithium ion phosphate bank with 4 x 3.2V cells is 12.8 volts

It can be confusing that the industry has chosen to refer to batteries in amp hours (Ah) when Kilowatt hours (kWh) would be more logical.

  1. a 100Ah battery can deliver a theoretical 100 amps for one hour.
  2. a 100 Ah battery rated at 12 V, the power is 100Ah * 12V = 1200Wh
  3. a 1200Wh battery rated at 12V, the current is 1200Wh / 12V = 100Ah

No battery is 100% efficient so we need to take that into account for our calculations. 90% is typical for deep cycle LifePO charge/discharge. Remember, capacity is further reduced if you cycle from 10% to 90% (to prolong battery life) but that is a decision for later.

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Solar Panel Sizes

The next section calculates the size of the solar panels required to charge your batteries

Unless your location is in the sun belt of the southern US or Europe and it is summer, your system will not be 100% charge efficient. How many cloudy days off-grid you expect to rely on the system, with no or reduced sun, must be taken into account. How long you can remain autonomous is also dictated by how many sun hours per day you can expect. Average solar hours per day are determined by your latitude and time of year which may take a quick Google to lookup - but you can leave that at 5 for now.