Introduction to Off-Grid Solar

As the solar industry becomes increasingly competitive, the ability to install off-grid solar systems opens up new possibilities for both new and established installers.  Our specialised team is here to assist you with any off-grid projects you would like to take on.

This guide has been created to help installers know where to start when thinking about off-grid projects. There is a lot of crossover between off-grid and on-grid solar installations. For example, off-grid systems will include a PV array, an inverter and a battery. A key difference is the importance of energy and power calculations: without the grid to provide supplementary power, off-grid systems have to be sized correctly in order to meet all the energy requirements of the project. Calculating the size of the required system is therefore usually the place to start.

Starting considerations when thinking about off-grid PV installations:

1. Is there a grid connection?

2. How much solar do I need?

3. What size of inverter do I need?

4. What size of battery do I need?

5. What size MPPT regulator do I need? 

    

       1. Is there a grid connection?

Having a grid connection present can make a system design easier whether the client wants to use grid electricity or not. If this is the case, let us know and we can help factor it into your quote.

If there is no grid connection, it means that the system will need to run entirely off of solar (with the help of a generator too if needed / desired). This means it is very important to know how much energy is going to be needed, and create a system that can meet these needs.

       2. How much solar do I need?

For this question, you need to think about your daily energy consumption. Because it is an off-grid system, the output of the array needs to be able to fully cover the daily energy consumption.

This means you will also need to think about how much space is physically available for solar. Sometimes this will be the deciding factor in whether a system can really hope to be self-sufficient or not.

To calculate the daily energy consumption, you can use our handy Solar Panel Calculator. See example below. 

* Some appliances are rated in terms of yearly consumption. If you use it less than normal this may be adjusted. 
** You can also adjust for consumptions which are not daily, such as a washing-machine running 1.5 hours every 3 days gives you 0.5 hours a day.

You can also consider the effects of the seasons on solar output. The productivity of the panels will vary throughout the year. See the general rules of thumb below about how array size relates to output throughout the year:

        3. What size of inverter do I need?

To select the right size of inverter, you must calculate the peak load of the system.

The peak load refers to the maximum load that will be required by the system at any one time. If we go back to the table above, let's say that our peak load is when we run all these items at the same time. 

Let’s say that we will run the following together:

Then we will need an inverter that can output at least 2220W. You can check out our range of pure sine wave inverters here, and our range of inverter-chargers here. 

           4. What size of battery do I need?

The voltage of your system is dependent on your batteries.

Typically vehicles (boats, motorhomes, etc) will have a 12V system. This works in these situations as most of the consumers (the onboard appliances) will also be 12V DC items. These vehicle are often using lead acid batteries which are also slightly hardier in adverse weather conditions.

Fixed structures may be 48V as this means that fewer Amps need to be transported, which enables the use of smaller gauge wires etc as there is less current to heat the circuit. These circuits are better suited to larger systems where most of the consumers will be run from inverted AC loads. They are typically lithium ion batteries which can be discharged to a greater depth without suffering damage, and tend to last longer.

Battery bank requirements depends on two things:

• Having batteries capable of delivering the power draw of the inverter. For example a 6400W inverter in a 48V system will require (6400 / 48 =) ~134 Amps to provide its output. So the discharge rate of the battery bank needs to be able to deliver this.

• Having energy storage large enough to run loads for a full day plus any backup for cloudy days (2-3 days is recommended). 
  
  
  

           5. What size of MPPT regulator do I need?

It is important to correctly size the MPPT. In order to do so, you must consider the open circuit voltage of the solar array, and the battery charging amperage.

If we take a Victron MPPT charge controller as an example, it will be called something like an MPPT 100/50 (XXX/YY).

The first value refers to the upper limit of the input panel/string open circuit Voltage (Voc).

The second value is the upper limit of the output charging amperage.

• Calculating the upper Voc of the array:

• When connecting the panels in series the Voltages will add up

• When connecting the panels in parallel, the Voltage stays the same and the current adds up

• Calculating the max available current:

• Array wattage / voltage = max available current

For example, using 6x 425W panels, each with a Voc of 39V, a single string of those panels in series would give you 234V.

In a 48V system, the maximum current the string could ever output is 6 x 425 / 48 = 53.125A.

In this case, you could use a 250/60 controller which would be safe as the input Voltage is below the limit, but a little oversized as you'd never hit the 60A output.

Alternatively you could run 2x parallel strings of 3x panels which would mean the Voc would only be 117V. You could then use a 150/45 which would be safe, you'd hit the charging output more often, and it would likely be much cheaper too!

If you send us the specifications for the desired system, we are happy to quote you for the correctly sized components

What does a basic off-grid system look like?

Our off-grid team has created various wiring schematics for different off-grid setups, which you can find here.

Below is an example of a basic DC-coupled off-grid system.

The DC power flows from the panels into the solar charge controller, which converts it into regulated DC. This regulated DC power is then fed into the batteries and any DC appliances. 

In order to power AC appliances, you can add an inverter or inverter/charger. The Phoenix Smart Inverter in the example below is one of Victron’s sine wave inverters.

• Victron Energy are a leading manufacturer in the off-grid world, having first started in 1975, creating power conversion equipment for the marine and automotive industries. We are a proud distributor of Victron, believing that they are top of their game in the solar industry, and leading the way with a strong, technology focused R&D team.

If your array can’t generate enough energy to meet your needs all year round, then you can add a generator. This connects directly to the inverter/charger (e.g. the MultiPlus-II below), which automatically regulates the turning on and off of the generator in times of low power production. 

You can also do an AC-coupled system - these are used for larger off-grid systems, where the larger loads makes it more efficient to convert to AC power first. This functions similarly to a grid-connected AC-coupled system, where you have your PV inverter, and then an AC-coupled charger to re-invert the power to DC to be stored in the battery. 

The Midsummer Off-Grid Team

Midsummer Energy originated in 2005, with a focus on designing off-grid solar systems. As Midsummer has grown, our specialist off-grid team has remained an integral part of the company.

How we can help:

• Design systems and provide quotes

• Create custom wiring diagrams for your projects

• Program products before we send them to you

• Offer installation and post-installation support

Get in touch

Email: sales@midsummerenergy.co.uk
Telephone: 01223 858414

Let us know:

• Roof dimensions and panel layout

• Daily energy consumption

• Peak energy usage

• Whether you want a generator

• Whether there is a grid connection