The electrical sizing of an offgrid installation

An offgrid electrical sizing starts with a wish for a 12 or 24 volt installation, which is possible and can be scaled up to 230V via an inverter so you can operate the TV, razor, communication or whatever you might need on 220 Volt.

You need to start by figuring how much power (watts (W) or ampere (A)) you need, when you use the most power in a day  (Peak Watt on both 12/24 V and 230 V). So you have to read on the back of the fridge, on the bulb, navigation equipment or whatever you need to install and navigate to the Watt numbers that tell you the power consumption.

More interesting is that you should calculate how many watts you use, how much time in a day (in watt hours (Wh) or ampere hours(Ah)). There must indeed be enough Ah in your battery to cover your consumption.
But read more about all this below.

The next thing you need to consider is how many days you will be sure to have enough power. It may happen that it is both calm, dark and cloudy for more than a day or two. It is perhaps not so often, but how much security do you need? We recommend that you have a battery bank for min. 3 days’ consumption and 5 days will be good. Here it’s important that you determine what your needs are. There is often a big difference between a motor that can start the engine in an emergency, and if you live permanently in a cabin without other power sources.

The electrical sizing of an offgrid installationCharging Regulator and cable thicknesses

The electronics must be installed correctly. The correct cable sizing are important and it’s here some DIY’s often does it wrong. However, the cables are important in sizing offgrid installations.

Cable thickness or square as it is properly called, is important to get right! You work with a low voltage installations based Direct Current (DC) and it can yield tremendous sparks and is not to be toyed with! 12 and 24 Volt does not sound as much, but the current in these systems is very high and you have to take your precautions against injuries and fire.
A low voltage DC system therefore need strong cables (and less strong fuses). Thick cables is necessary on one hand in order not to melt when pulling high currents in the cables, but also to reduce the power loss in the cable. Think of the cable in your car, that goes from the battery down to starter … It is not just simply a tiny extension cable … I think you get it now; o)
Cable sizes look like this and if you’re in doubt about your cable is thick enough, then you are not in doubt! Thicker cables!

Here is an overview of cable thickness (square) to be used from a given wind turbine and to the regulator.

WindchargerCable run (M)12 V (mm2) 3 core cables!24 V (mm2) 3 core cables!
Rutland 12000-502,52,5
WP DataTables .
WindchargerCable run (M)12 V (mm2)24 V (mm2) only 914i and FM 910
Rutland 504, 504 e-furl, 914i and 9100-202,51,5
WP DataTables .
WindchargerCable run (M)(mm2) 3 core cables!
Rutland 18030-1002,5
WP DataTables

Now you have connected your wind, now we have to connected to your solar cells. Here you can see which type of cables you need:

Up to 10A DC Solar - Cable RunCable size (mm2)
0-5 Meter4
5-10 Meter6
Up to 20A DC Solar - Cable Run
0-5 Meter6
5-10 Meter10
WP DataTables

It has up to now in this guide been of no circumstances whether you have solar, wind or both in your setup, but now we have the power in the charge controller (maybe you have 2 charge controllers / systems, see below) from one or more sources and then the power flow becomes unified. After the charge controller comes the battery and here we must take account of a sometimes very high current. The cables must be able to handle full power from both solar cells and wind turbines simultaneously. Here it’s important that you have thick and short cables!
My recommendations will look like this:

Output from Charge controllerMinimum cable size (mm2)Cable MUST be rated to:Max length
Max. 35 A DC635 A1,5 meter
Max. 55 A DC1055 A1,5, meter
WP DataTables


Battery size.

We are moving nicely forward :o). Now we have the power flowing into the batteries and the consumption system is already being feed from the batteries with this new renewable energy … BUT! Can your battery system handle all the power now is produced? and are the battery the right size? We have not finished sizingyour offgrid system just yet 🙂
In basic I recommend that the size of your battery bank must be: in ampere hours (Ah) min. 10 X the maximum charging current in amperes (A).
Eg. If you have a 12V (13.8 V) Rutland 504 wind charger (about 65 watts at 20 m / s) and 100 Watt Solar Panel. That’s a total of 165 watts max.
At 13.8 V gives a charging current of maximum: 165W / 13.8 V = 12 Ampere. and Battery Bank should therefore be about 10 X 12 = 120 Ampere hours (Ah).
This also applies to the larger systems and this can be a considerable battery sizes, when the system grows. The larger the systems are, you may go with a lower number than 10, but never less than 5 and then you will experience that you have more power than you can store, because your battery is filled too quickly and it will also be discharged quickly.
The life of your battery depends on, that it is not discharged to a low level and even if you use deep cycle batteries, you are best served with a good big battery bank.
Here you need the figures from the top of the page. Namely how many Ah you use in a day. There must be enough Ah of battery for X number of days (I would recommend min. 3 days) AND then you should remember that even a deep cycle battery cannot tolerate more than 50% discharge. More than 50% discharge and even a deep cycle battery is damaged. Never run your battery empty! And especially starter batteries do not tolerate it.
I made a chart showing the calculation for Rutland in 1200 with and without solar cells:

Charge sourceMin. battery capacity (12V)Min. battery capacity (24V)Recommended battery capacity (12V)Recommended battery capacity (24V)
Rutland 1200175 Ah85 Ah350 Ah175 Ah
Rutland 1200 & 10 A solar225 Ah110 Ah450 Ah225 Ah
Rutland 1200 & 20A solar275 Ah135 Ah550 Ah275 Ah
WP DataTables

Two battery banks

Many of our charge controllers can support two battery banks. It is most often used sailboats and caravans. The aim is to have a main battery that’s recharged primarily and used to start the engine and a secondary battery bank for consumption for TV, kitchen equipment, PCs, etc.
So if you will have this, then you must ensure that the charge controllers(s) can support this.
You will with this setup need a good starter battery for the engine and a deep cycle for consumption.
When you calculate the battery capacity it’s the total capacity of your battery banks that you need.

Battery type

Batteries I can write about for hours, but I will drop the geek sector and make it an overview.
If your offgrid installation is “only” for consumption and should support consumption over a long period, with a moderate and relatively steady use of power (typically cabins, communication systems and consumption batteries for the boat and caravan), then you need a deep cycle battery as it can withstand multiple and deep discharge cycles.
For your starter battery (if you have 2 battery banks), use a common good starter battery as recommended by the manufacture of your engine.
There are a myriad of batteries and you can get them in many versions. Energig’s systems are built to handle all types of lead batteries. We recommend Wet, AGM and Gel batteries.

Two charge controllers. Several systems.

Do you have a combined solar and wind generator system with more than 160 Watt (HRSi and HRDi) or 20 A (Rutland 1200), you need to have a separate solar charge controller to be mounted in parallel to your wind charge controller. there is no problem in this, as you just think of them as two separate systems, right up to the battery. In the battery they are becoming united into one. Therefore, you must calculate the battery capacity from the total charge capacity.
Don’t take into account the power that may come from a running engine (boat, truck, caravan, etc.) or grid connection that can let your battery occasionally. These systems are controlled by relays and switches off automatically when there is no more need for them and that’s what we want, because they cost money in operation. Basically, we of course prefer to do without them altogether 🙂


You must always make sure that there are fuses mounted between the battery bank and your 12/24 Volt consumption system and these must again be aligned with your need for power and cable thickness. It makes no sence to put fuses that can draw more power than your cables are capable off handling Becource it is then the cables and not the fuse that is burned. We do not like fire such places!? …
There are fuses in most charge controllers and also diode protection in many solar panels and charge controllers (the diode protects against power loos to the solar panels, when it is dark/night.

What next?

You can always call or write us. We will always assist you with getting it all figured out right and can guide you so that you get the right solution.
We take a pride in that you get the right solution, since it is the only way we can hope that you will recommend us to your neighbour in the port and at the campsite.

If you need more information and want to dig more into offgrid installations, then you can find plenty of more information on exactly that topic which interest you on this page and let me recommend our blog on renewable offgrid installations. There you can find a lot of knowledge and also participate in the discussion / ask questions.