Full Off-Grid Solar System

A Complete Off-Grid Guide to Solar Power

It’s hard to go wrong with solar power when starting any off-grid renewable energy project. Their ability to reliably generate electricity even in the most overcast weather makes them incredibly popular in a broad range of applications and the availability of low cost components has made this technology remarkably accessible. However, the design of solar systems can vary a lot, and it is therefore important to understand how to choose the correct components for any intended application and budget. This article has been created as a complete off-grid guide to solar power and aims to give an in-depth view of the considerations required for any project whether this be a campervan conversion, a sailing boat, an outbuilding or even a house.

Types of Solar Panel

The two most common types of solar panel technology are monocrystalline and polycrystalline and choosing between the two can often be the first stumbling block when designing a solar system. However, the two technologies are actually the same in the way they both use crystallised silicon to generate electricity. The main difference between them is the quality of those silicon crystal structures, where monocrystalline solar cells have a more uniform crystal structure and are therefore more efficient than polycrystalline but may also be slightly more expensive as a result.

An alternative to crystalline technologies are thin film panels that are designed to bend and mount flat along uneven or curved surfaces. Thin film solar panels are less common than rigid panels and suffer a lesser efficiency but make up for it in practicality and cost.

Series vs Parallel Arrays

Any individual solar panel will have a limit to the power it can deliver on its own, but connecting two or more panels in an array will allow for a greater power delivery to any renewable energy system. The simplistic design of solar panels means there are only two ways in which we can connect them together, series and parallel, but what does this mean?

A series solar array will connect the positive cable of one panel to the negative cable of the next, this is normally the easiest way to connect solar panels using male and female MC4 plugs. Connecting two or more panels in series will add the voltages generated from the connected panels and increase the overall power available to the system. It is therefore important to be aware of the voltage rating of any forward equipment before connecting your array in order to avoid damage.

Note: It is also important to recognise the electrical shock risk associated with stacking voltages like this!

A parallel array can be considered safer than a series array because it adds electrical current instead of the voltage generated by each connected panel. This is achieved by connecting the respective positive and negative cables of the array together which often requires the use of parallel connection cables. Solar panels connected in parallel also require the use of blocking diodes to stop the array discharging in reverse through any shaded panels. Finally, as we are adding electrical current in a parallel array setup, it is important to be aware of the maximum current capacity of each panel (in Amps) an ensure any cables and equipment are rated appropriately. This can be calculated by adding the short-circuit current of each connected panel together.

The Importance of Charge Controllers

Charge controllers continue to be the most overlooked component in many off-grid solar system designs, despite their potential to notably affect efficiency and yield of any renewable source. Although the decision is often determined by budget and system power ratings, it is important to understand how they work in order to spot any false economies.

The main function of a charge controller is to convert the higher voltage of a solar panel or array into a lower, more useable voltage, that can be used to charge a leisure battery safely. The two main types of solar charge controller are Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).

PWM charge controllers periodically pull power from the solar panel or array in pulses of varying time periods, this process creates the lower average voltages needed by a leisure battery. PWM charging is simple and therefore a cost-effective option for any solar system, but is not the most efficient as a result.

MPPT charging on the other hand, is more intelligent in the way it tracks the output voltage of a solar panel or array and, using a known correlation, determines the point at which it can pull the maximum amount of available power from the system.

As with any power system, it is important to be aware of the maximum output power of any connected renewable source, and to choose an appropriately rated charge controller accordingly. The maximum power capacity of any solar panel should be clearly stated by the manufacturer, usually on the reverse of the panel, and calculating the total power of an array is simply a matter of adding the power in watts of each connected panel together.

Charge controllers also have voltage and current ratings alongside the total power they can manage. Calculate these ratings for a series array by adding the open-circuit voltage of each panel together and finding the panel with the highest short circuit current rating. Similarly for a parallel array, find the panel with the highest open-circuit voltage and add the short-circuit current of each panel in the array.

Storage Battery Chemistries

There are generally two main types of battery chemistry that are used in the context of off-grid renewable applications, lead-acid and lithium. In both cases, there are benefits and shortfalls to consider and so it is important to understand the differences between the two.

Lead-acid battery chemistries have been around since 1859 and as a result have a reputation for simplistic and rugged reliability that is popular in combustion engines. Their wide use has made them an extremely cost-efficient option but are often dismissed for having a shorter operating lifetime and lower energy density compared to the equivalent lithium chemistry.

Note: Despite the stigma, the progress of lead-acid chemistries can still make them extremely competitive with lithium for off-grid applications, as well as being a lot more resistant to abuse.

Lithium based chemistries perform very well on a number of fronts, they have a longer operating lifetime and a much higher energy density than their equivalent lead-acid chemistry. Naturally, this makes them optimal for use in electric cars and home storage but, as the more modern technology, does makes them significantly more expensive.

Note: I have observed that in the current state of battery progress, the return on investment for lithium storage is actually about on par with lead-acid but this will likely improve as the technology matures.

It is also worth noting that due to their specific charging requirements, lithium chemistries are not as well supported in the context of off-grid renewables compared to lead-acid and are more difficult to interface them with common types of charge controllers.

DC-AC Inverters

The last component to consider when designing any off-grid solar system is the inverter. Inverters convert the direct current (DC) of your storage battery into the alternating current (AC) used in the vast majority of domestic electronics like microwaves, computers and televisions.

There are two main types of inverter to choose from, modified sine wave and pure sine wave, where both perform the same function but will differ depending on the demands of your system. There are generally three factors to consider when designing for each case, budget, efficiency and power requirements.

Modified sine wave inverters are a simple and cost-effective solution for generating the alternating current needed in many off-grid applications. In many cases, it will be indistinguishable from what is ordinarily found in any household mains socket and can be used to power many non-critical devices like laptops, televisions, microwaves, electric kettles and mobile phones. Modified sine wave inverters are however known to be electrically noisier than pure sine wave and therefore may not be suitable for any medical equipment or other critical applications. Having said this, power supply circuitry on modern electronics are so good these days anyway it is unlikely that you will notice any functional difference.

Note: It is often claimed that the “harmonic noise” that occurs in modified sine wave inverters can cause functional anomalies in some devices but I find this slightly dubious, especially in the context of upselling.

Pure sine wave inverters, as the name suggests, generate a pure sine wave that is identical to what can be found in any domestic electrical socket. As you might expect, any inverter that is designed to connect to the national grid will always be of this type and there are obvious advantages to using them. For one, efficiency is a big factor with some sources quoting gains of up to 30% over the equivalent modified sine inverter! This is not an insignificant figure, especially if you are designing a solar system for maximum yield, as it will ultimately be consuming less of your stored power but the inevitable downside of using pure sine wave inverters is that they often cost twice the price!

Lastly, understanding the power requirements of the devices you plan to connect to the power system is critical and working backwards is one technique to help us choose an appropriate inverter to use. This can be calculated by adding the individual maximum power ratings of each device that will be connected to the output of the inverter. This is usually found on a sticker applied on the appliance or device by the manufacturer and is stated in watts, to give an example, electric kettles are usually rated at 2200 watts.

The sum of these power ratings will give a good idea of how big our inverter needs to be, for instance if we wanted an electric kettle (2200W), a microwave (2200W) and a TV (200W) our inverter would need to be rated for at least 4600W so a 5000W inverter would be appropriate.

Conclusions

Hopefully this article has been helpful in guiding you through the considerations needed when designing any off-grid solar system. There is a lot to absorb, so it can be useful to go through the guide step by step until you know what to look for. It may also be useful to work backwards if you are designing a solar system around a specific set of devices and need to figure out how much solar power you need.

There are a few more details that I would like to add to this guide so I will likely be splitting the article out into multiple parts in the near future to make it more digestible. However, if you liked what you’ve seen so far then please consider joining my mailing list for more article updates, read the blog or visit the shop.

Thanks for reading!