Designing an Off Grid Solar System

The aim of this post is to give you the basics for designing and creating your own system and hopefully enough of an understanding of what an off grid system is, the main considerations when designing one, system components, our recommendations, and anything to avoid!
So let’s explore the essential steps to design a robust and reliable off grid system.

An off-grid energy system is akin to having your own power source right at home. To achieve this, it relies mainly on solar panels to capture energy from the sun. This collected energy is then stored in a specialised battery, ensuring it’s readily available when you need it, such as during nighttime or on cloudy days. This comprehensive solution empowers you with electricity autonomy, freeing you from reliance on conventional power companies. As a result, you gain control and a dependable backup power source, whether it’s for your home office or workshop.

Our first step is to understand the feasibility of your system. Realistically, if you’re hoping to go completely off grid but, for example, you need to run a heat pump for space and water heating, and you only have a minimum amount of roof space for solar panels, you’ll need to readdress your heating requirements, or else reconsider if being truly off grid is for you. However, if you have the space (roof or ground) and you’d like the freedom of generating your own power read on..

Let’s explore the essential steps to design a robust and reliable off grid system.

Step one is figuring out what appliances you want to run to calculate your daily and weekly energy usage.

To do this we need average power usage for various household appliances. Our off grid calculator will help to point you to one of our off grid solar kits, based on your average usage.

Now, you’re not likely to use the same amount of load everyday, and in the Summer months you’ll have longer days, using less lights, maybe drying clothes outside. In the Winter you’re likely to use more energy for the opposite reasons. Especially so if you need to use electric space or water heating. So it mostly averages out, after removing any outliers who, for example, may be away in the winter months.

So, now you can look at sizing a battery bank.

So using the previous example, a bare minimum battery bank size needed to provide this usage if the sun isn’t shining is 10 kWh’s.

If you want a robust system, that’s able to cope with a few days of low sunlight we’d recommend adding another days self sufficiency to this battery size, so 10 × 2 = 20 kWh’s (This is optional of course!).

Let’s look at Choosing your battery later, for now we need to decide on system voltage.

Offgrid Western recommends a 48V system for most off grid homes, due to the greater number of options for inverters available and LiFePO4 batteries that have industry leading warranties.

There’s a rule of thumb we use for UK based off grid solar systems;

The average UK power output annually from 1 kWp of solar is 865 kWh’s. ¹

This means an average of 2.37 kWh is generated daily. (Yes, you will generate more in summer and less in winter, but the average helps size the array to take into account the seasonal changes).

So, to fully charge our batteries every day using the average amount of solar generation we can use this calculation;

Battery size / average daily generation = ideal PV array size.

It’s good practice to add 20% to the array size to ensure a robust system. In practice we’ve seen functional off grid systems without this 20% buffer, but there needs to be some reduction in winter energy use to avoid running out of power. If you’re thinking of installing a backup diesel or LPG generator you should be able to reduce this PV array size by 20%. Giving you an array size of 7.6 kW

¹ This is from published PVGIS data and assumes a south-facing panel at 37 degrees.

So, now you know your target array size. You just need to fit it somewhere…

Using our compact JA Solar 405W panels, a 9.5 kW array will require 24 panels. With each panel being 1.86 m², that’s 44.6 m² of space you’ll need.

Got that amount of space? Yes? Then you are good to go. Our solar only kits come complete with the mounting option of your choice and a DC-coupled solar charger ready to add in to your Victron system.

No? If you don’t have the space for your ideal array, look at the space you do have and what size array you can fit. You can then work backwards to find something suitable, perhaps you can boost winter generation with a generator or wind turbine?

Or a grid connection if you’ve got one! You may also need to reduce your electrical energy usage, we’ve got a great post in the works that covers reducing your usage to cover your needs with solar and battery storage.

Positioning your solar panels to make the most of the UK’s sunshine is fairly simple. It needs to be as close as possible to a 35-37 degree angle and perfectly south-facing. Any shading will affect your generation, so keep the array away from trees, hedges, other buildings and roof obstructions. Winter generation can be degraded by trees or hedges that only shade in the winter when the sun is low!

Not all off grid systems use the same components, for instance, you can link an AC-coupled PV inverter to a Victron system, hang on for a detailed blog post on that.

We’ll focus on a standard DC-coupled, single-phase Victron system for this example. Working from generation to storage to AC output;

Wiring your panels, strings, parallel or both?

When connecting up panels there are a few things to be aware of;

Solar panels have a rated output in (STC) or standard test conditions.

That is the advertised power output of the panel in Watt’s. This is what the panel will produce under laboratory conditions, you can assume that generally, the power output will be lower. However, on some cold and sunny days in the right conditions, a panel can overproduce, so it’s important to calculate your solar array’s voltage and current and correctly match it to a solar charger. Luckily, Victron has a handy solar charger calculator. You can use with their solar panels, it will also allow you to input your own panel’s specs to help calculate your solar power.

Your solar panels are connected up into strings. A string is a series of panels connected, typically in multiples of two or more.

When a string of panels is connected in series their voltage is multiplied by the number of panels in the string, this can quickly result in a very large voltage that will damage a DC solar charger if connected.

If a string of panels is connected in parallel the voltage remains the same, however, this time it’s the current that is multiplied. Again, if not carefully calculated this can result in a large amount of current damaging the DC solar charger if over its rated limits.

Now, here’s the trick.. optimising your strings to keep the voltage and current within the limits of the solar charger.

We’d recommend using Victron’s calculator to double-check any variations for your solar array, we’ve simplified the calculations here, but, your system voltage, temperature, cable length and CSA also play a part in sizing the solar charger correctly.

We only use market-leading DC isolators from IMO there are a few reasons for this, but ultimately safety is the number one reason.

Take a look at this video which better explains why a specialist DC isolator should be used instead of an AC isolator.

DC isolators come in 2 or 4 poles, which corresponds to the amount of terminals inside the unit.

• A 2 pole will allow one positive and one negative solar cable.
• A 4 pole will allow two positive, and two negative.

This lends itself nicely to the PV strings we looked at earlier, if you have two strings of panels you can use a 4 pole to link everything up inside the isolator, removing the need for costly MC4 joiners.

Your solar cables should be terminated with bootlace ferrules, with only one cable being fitted into each terminal. Don’t be tempted to piggyback multiple cables into the isolator terminals, you’ll pay for it later!

You should also ensure you select an isolator that is sufficiently rated to the correct current and voltage that your solar array will generate.

Victron offers three variations of inverter, all three will produce AC by converting DC from a combination of battery and solar charger. However, each has its advantages.

Designing an off grid power system requires careful consideration of your energy needs, and sizing the inverter is a crucial step in this process. The inverter converts DC power from your battery bank into AC power for your appliances. Here’s a step-by-step guide to help you size your off-grid inverter:

Assess Your Power Consumption:

List all the appliances and devices you intend to power with your off-grid system. Note their power ratings in watts (W) or kilowatts (kW). Include both continuous and occasional-use devices.

Calculate Total Power Consumption:

Sum the power ratings of all your devices to determine your total power consumption. This figure represents the maximum power demand your system should be able to handle.

Consider Surge Power and Operating Temperature

Some devices, especially those with motors or compressors, may have a higher power demand during startup. Factor in this surging power when sizing your inverter. All Victron inverters can supply inrush currents above their advertised ratings.

Select an Inverter Size:

Inverters are rated by their continuous power output. Choose an inverter that can handle your total power consumption comfortably. If your total power consumption is 4000 W, consider an inverter in the range of 4500 W to 5000 W for a safety margin.

Plan for Future Expansion:

If you anticipate expanding your off-grid system in the future, consider choosing an inverter with a capacity that accommodates potential additions to your power demand.

Alternatively you can add another inverter later on to create a more powerful single phase system, or a three-phase system. A detailed blog post on that is coming soon.

Unsure?

Reach out to us, we can help correctly size your system, better yet, take a look at our off grid kits, all based on your energy usage.

Take a look at why we use LiFePO4 batteries.

The heart of your system, it controls, monitors and communicates. It’s an essential part of your off grid install. All the GX devices will communicate with Victron’s excellent VRM platform. They will require Internet access though, most have built-in WiFi, so it’s just a matter of connecting to your existing network.

Don’t have an internet connection, you can purchase a 4G LTE modem for your GX device. This requires a SIM data contract in addition to the modem, you’ll then need to run a DC power cable to it.

There are a few different variations of GX device, and we’ll start with the simplest and most popular;

The Lynx system consists of a Distributor, Power In, a Shunt and a BMS.

The Lynx is a complete distribution system for larger systems that require a higher rated busbar, 500 Amps up to 1000 Amps.

Lynx Distributor

Likely the most used part of the Lynx system, the Lynx Distributor is essentially a 1000 Amp-rated DC busbar. It comes with M8 bolts to secure your DC cabling, protection to separate the positive and negative cable lugs, and fusing for Victron Mega Fuses (sold separately). Look out for a detailed blog post on system fusing and cable sizing.

Lynx Power In

The Power In is a Distributor without fuse holders. It’s mainly used for ‘power in’ that does not require fusing. (Fuses would be on the ‘power out’ in the Distributor)

A good example is making a high-integrity battery bank. Where you can wire in multiple 12V batteries in series to create a 24 or 48V system, or parallel to keep the same voltage while adding storage capacity. A more detailed blog post coming on this soon.

Shunt VE.Can

This shunt bolts in between the Distributor and Power In. It’s not a BMS but a battery monitor, and doesn’t have a Bluetooth connection so it needs to be connected to a GX device. It reads battery voltage, current draw, and temperature, to estimate the state of charge of the battery bank. If you’re using a battery system with a BMS, there’s very little use for it.

However, it does act as a main fuse for the system via an ANL or CNN Fuse. If you’re using a battery bank with built in BMS, you’ll have to add in some battery fusing. We use Mersen 2 pole MultiBloc fuse carriers for protecting our 48V systems.

Smart BMS

The Lynx Smart BMS bolts are in place between the Distributor and Power In, much like the Shunt. However, this smart BMS is designed to manage Victron Smart LiFePO4 batteries. It won’t work with any lithium batteries with a self-contained BMS. It’s also limited to 500A, unlike the rest of the system which is rated to 1000A. Keep this in mind when putting it to use.

The Takeaway

We generally only use the Distributor for our projects, when you’re building higher voltage systems (48V) with currents exceeding 250A it is a much safer and cheaper option than multiple smaller rated busbars which tend to be exposed. The distributor can be endlessly linked for really large systems and battery banks, and is cheaper than custom busbar creation.

All our off grid kits come with everything you need for a safe and robust off grid system. Including an earth rod kit, AC consumer unit, RCD and SPD protection. And of course a full easy-to-follow wiring schematic.

It is vital to incorporate an earthing system into your off grid installation.

If your off grid system is mobile, such as a camper van, motorhome, caravan or boat then your earth will be the chassis of the vehicle, and this section won’t apply.

Grid-tied homes have the benefit of a DNO earthing arrangement, TN-C-S or TN-S. An off grid home requires a TT-system, if an electrician has wired your home they should have installed one, and it will generally be located close to your consumer unit. And labelled as a safety earthing connection.

A TT-system consists of a copper plated metal rod, from 9 mm diameter upwards (size dependent on the potential fault current of the system). The earth rod allows any potential fault current generated to flow safely to ground and dissipate, instead of damaging your system, or worse giving someone an electric shock!

The earth rod is driven into the ground, taking care to avoid any underground cabling, pipework or other services. If you’re unsure about what may be located underneath the ground always consult a professional. An electrician will be able to either scan the ground for any services or liaise directly with the DNO to check for any HV cables. If the earth rod is not suitably separated from other underground services, there could be a risk of voltage being transferred to the earth rod, potentially creating a harmful electrical current on your earth supply.

With the earth rod in place, you can now run earth cable from the rod to the installation. We recommend a minimum size of 16 mm² to ensure a sufficiently low resistance for any fault current to flow. This earth cable can now be connected into your consumer unit, or a dedicated earth block. This will act as the main earthing terminal (MET).

To this MET, all your Victron equipment chassis’s must be bonded by connecting an earth cable of sufficient size to the supplied earthing points on each chassis. We recommend a minimum of 16 mm².

Residual Current Device (RCD) and Surge Protection Device (SPD) are both forms of electrical protection required in an off grid system.

A Residual Current Device (RCD) is a crucial safety device designed to protect against electrical shocks and fires. Its primary function is to monitor the balance of electrical currents flowing through live and neutral wires in a circuit. Under normal circumstances, these currents should be equal.

1. Monitoring Currents:
   • The RCD continuously monitors the flow of electrical current in a circuit.
2. Detecting Imbalance:
   • In a balanced electrical system, the current in the live wire should be equal to the current in the neutral wire. If there’s an imbalance, even as small as a few milliamps, it indicates a potential fault.
3. Tripping Mechanism:
   • Upon detecting an imbalance, the RCD swiftly activates its tripping mechanism. This mechanism is highly sensitive and responds within milliseconds.
4. Instant Disconnection:
   • When a fault, such as a leakage to the ground or through a person, occurs, causing an imbalance, the RCD reacts by instantly disconnecting the power supply. This rapid response prevents electric shocks and mitigates the risk of fire.
5. Protection Against Electric Shocks:
   • By cutting off the power supply in the event of a fault, an RCD significantly reduces the risk of severe electric shocks. This is particularly important in areas where water is present, such as bathrooms or kitchens.
6. Types of RCDs:
   • There are four different types of RCDs, each serving specific purposes. Some RCDs also come with additional features like time delay to avoid nuisance tripping.

For an off grid system to require an RCD, the inverter must be ‘transformer-less’, which means there is no separation between the transformation of DC to AC, therefore allowing some DC current to leak into the AC circuits. The cheapest RCDs are type AC, unfortunately, these are not suitable for an off grid solar system as they can become saturated with DC current and will then not function correctly or at all in some cases! A Type B RCD is the only type of RCD that should be fitted to your system. Type B can detect and handle both AC and DC residual currents.

We advise the use of a Type B RCD when supplying your AC circuits from your Victron Inverter.

While technically, the Victron inverters have a wound transformer and therefore the AC and DC circuits are separated, it is best practice to have a correctly functioning RCD as a backup safety device. If you’re using a different or cheaper inverter, it may not be separated internally, in which case an RCD is required.

A Surge Protection Device (SPD) is a vital component in electrical systems designed to safeguard devices and equipment from voltage spikes and transient surges. Here’s how an SPD works:

1. Voltage Monitoring:
   • The SPD constantly monitors the incoming voltage from the main power supply.
2. Detecting Voltage Spikes:
   • When there is a sudden increase in voltage, commonly known as a surge or spike, the SPD detects this anomaly.
3. Fast Response:
   • SPDs are engineered for rapid response. In a matter of microseconds, they react to the surge, preventing it from reaching connected devices.
4. Diverting Excess Voltage:
   • The key function of an SPD is to divert excess voltage to the ground, via your earth rod. It provides a low-resistance path for the surge, directing it away from sensitive equipment.
5. Protection Across Circuits:
   • SPDs are often installed at key points in the electrical system, such as distribution boards or close to sensitive devices. This ensures comprehensive protection across various circuits.
6. Preventing Damage:
   • By limiting the voltage that reaches electronic devices and appliances, SPDs prevent damage caused by overvoltage. This is crucial for preserving the lifespan and functionality of equipment.
7. Types of SPDs:
   • There are different types of SPDs based on their application, including Type 1 for main distribution boards, Type 2 for sub-distribution boards, and Type 3 for individual devices. Choosing the right type depends on the specific needs of the electrical system.

For your off grid system you’ll likely use a Type 1 SPD that will fit alongside your RCD which is fed from your inverter AC output. We strongly advise the use of an SPD, as it is a cheap way of protecting your home circuits and sensitive electrical devices from any surges.