How to set up an Off-grid Solar System

How to build an off-grid solar system explained as concisely as possible from start to finish.

How to set up an Off-grid Solar System
Photo by Bill Mead / Unsplash

With rumours of power outages coming in diverse places, including Germany, which is shutting down Nuclear and Coal power stations.  They will become almost solely on Russia for natural gas supplies, what could possibly go wrong.  The UK has also flip-flopped on the issue of new nuclear power putting more and more eggs into the wind basket.  That decision embarrassingly failed during the COP climate change summit in Glasgow, when gas power stations needed to be brought online because of not enough wind.

The thing with power is you shouldn't be dependent on anyone else, or they are the ones in true power.

You can make yourself self-sufficient in power by reducing your power needs and by increasing your generation capacity.  And, as with everything I suggest, you don't want to be dependent on one source, even grid power.

So what are your easiest options:

  1. Solar power
  2. Wind power
  3. Hydro-electric power
  4. Grid power

This article will focus on solar power, although we'll be considering the whole system from power generation to power storage and conversion.  The storage and conversion parts apply to other off-grid power generation systems also.

Although we're talking about off-grid, it is totally possible to combine self-generation with grid power, although the regulations will vary from place to place and with Grid power, you pay whether you use it or not.

Loads and Devices

Load is the technical term for what will be running on the system.  We can call them devices or appliances to make the topic easier to understand.

It is possible to run an RV or campervan using bottled gas and 12V D.C. without needing to convert to 120/240V A.C..  I'll assume that at this point you're wanting to operate a somewhat normal home, at least that's what I plan.

What can you not live without, at least in the short term to maintain some normality?

  • Washing machine
  • Coffee machine / Kettle
  • Vacuum cleaner
  • Air-conditioning or Desktop fans
  • Fridge / Freezer
  • Oven and Hobs
  • Tumble dryer
  • Electric shower

If you can, you should consider reducing the requirements here.  Even replacing high-power devices with lower power devices.

Vacuum cleaners can be battery-powered which will be much lower power, or cheaper plug-in ones can be had that run at 500W.

Induction hobs use a lot less power than normal ceramic hobs. Oven usage should be limited, with slow cookers, gas ovens, or wood stoves used instead.

Tumble dryers and air conditioning usage should be minimized.

The washing machine will probably be less than 2500W peak power and the coffee machine will probably be about 1000W.  Fridges are normally 250W.  Unfortunately, a power shower requires too much power at sometimes 10kW.  It would be better to run this from gas or to have a water tank and slow heater.  Solar water heaters can be made very easily in an emergency using plastic bottles or installed professionally on a home.  Although they can be considered more efficient than solar electricity (solar photovoltaic systems), their actual utility, I believe, is less, because the energy captured by the system cannot be used beneficially.  But that's a whole other subject.

Once you've worked out what devices you will be running, consider if you want to run them all at the same time, or if it's possible to run them at different times throughout the day.

Total Power Consumption

For this post I will tailor it to my particular interest which is creating a 2500W system capable of running the washing machine, or a few other devices but only as a last resort or money-saving option, rather than on an ongoing basis.  The plan will be to gradually upgrade the system as it proves itself useful and effective.

Whatever your load is, you need to also consider your peak power which for washing machines can be higher than the average power.  Check your particular devices for the constant power and the peak power.

Knowing that the device will consume this much power allows us to select the Inverter, which will convert the DC current to an AC current.

How to select the cables for the Solar system

Normal housing regulation wiring should be used to connect the devices to the inverter, the calculation to work out the current required by the devices is:

So for a 2500W device operating at 240V, the current is approximately 2 amps.

Then you can use a wire guage chart to work out what guage wire you require which is dependent on the distance between the inverter and the device.  Best follow building regulations for this unless it is purely an off-grid build.

Now between the inverter and the battery, the calculation needs to be done again. Usually the batteries are 12V, 24V or 48V.  The higher the voltage the lower the current and the cheaper the wires.  If you're just starting out and want to see how it goes then start with 12V and from there you can gradually upgrade to 24V and then 48V.  Many devices can run of both 12V and 24V.

So for 2500W to the inverter operating at 12V DC the current would be 208 Amps! That is significant and so the distance wants to be reduced as much as possible. According to the charts you could use 8 guage wire with up to 50cm lengths.

Between the charge controller and the battery you want to match the maximum output of the charge controller.  When buying the charge controller you can select the maximum output and the cost increases accordingly.  I'll deal with how to select the charge controller in its own section.  If you keep the distance under 1 metre then 10 or 12 guage wire will work here.

Between the solar panels and the charge controller it depends on both the types of panels and the configuration.  It they are in parallel then the current of each panel gets added together, if they are in series then the voltages get added together. Which configuration you choose depends on how many panels you have and what charge controller you have.  As an example we'll assume two 100W panels connected in parallel.  These panels operate at 20V DC and have a maximum current of 5A, which we can read in the specification for the panel, and so when connected in parallel the maximum current will be 10A at 20V.  This cable run from the panels to the charge controller is going to be the longest so ensure the cable is selected to minimize losses.  For this current over a length of 10m, then 10 guage wire is sufficient.  If you have only 2m then 18 guage wire is sufficient.

How to select the Solar system voltage

The best select for higher power users would be 48V, it allows higher power loads whilst reducing the cost of the inverter and cables because of the lower currents.

The downside of the 48V system is the initial cost.  The battery will be four times the cost of the 12V system.  In order to charge the 48V battery then the panels should always be configured in multiples of 4 so this is also likely to increase the cost.  

When starting out with a small system then we can take advantage of lower cost charge controllers and inverters and fewer batteries and panels.  If you're budget allows then maybe a 24V system is a good starting system that allows some medium to large loads, the battery will be expensive but not prohibitively so and the cables can be those more widely available.  The larger guage cables are hard to find although of course smaller cables can be doubled up to double the cross-sectional area.

I will be designing my system to use a 24V battery because my plan is to use the system to reduce some running costs around the house.  That means running some large consumers.  The first target is Sunbed which is 1200W, gets used almost daily in the Winter for up to 1 hour (multiple people) and is in the loft near to the solar panels.  The downside with this approach is that the most energy will be generated summer when the sunbed is used the least.  At this point I will probably have to carry the battery and inverter down to the cellar where hopefully I could run the washing machine from it.  This will allows us to save some money on our bills whilst ensuring we're somewhat prepared for a sudden loss of electric grid power.

How to select the right solar power inverter

Firstly select one that runs at the correct voltage, this will be 12V, 24V or 48V. Many of the 12V ones will also run at 24V so if you are starting small but plan to grow larger then look for one that can operate at both voltages.

Secondly look for the continuous output power.  This is very likely not the advertised power; the advertised power will be the peak power.  This peak power is very likely two, if not four, times as high as the continuous power output.

Thirdly make sure the output power is suitable for your appliances, e.g. 240V 50Hz in Europe or 120V 60Hz in North America.

Fourthly select a pure sinewave inverter to protect your equipment as the cheaper modified sinewave inverters include a lot of harmonics which can damage motorized equipment.

Finally come the nice-to-haves.  Choose one with a reasonable efficiency.  Look for one with good reviews.  Do you want some form of remote control to turn it off, as leaving it on can drain the battery?

How to select the right solar battery

There are many options when it comes to batteries, but let's assume you're going to go one of the two most common routes which is either Sealed Lead-Acid (SLA) or Lithium Iron Phosphate (LiFePo4).  SLA batteries you can buy at any car parts dealer and find at a junk yard.  For better value you could seek out used Caravan batteries or Fork lift truck batteries.  For LiFePo4 batteries you'll probably be buying them new.

The pros and cons I will deal with very quickly.  SLA batteries are not intended to be constantly drained, that's why a so-called "leisure" battery for a caravan is a good choice because it can handle a bit more abuse.  However, don't expect them to last more than about 3 years.  Also what they call the Depth of Discharge (DOD) should only be about 50% of the total capacity so for a 100Ah battery only about 50Ah is useable.  It can be discharged more but the lifetime will be reduced.

Also the constant current supply needs to be considered.  This is often measured in "C's" where a C is a multiple of the amphour rating.  So 1C would be 100amps. Except with SLA batteries the rating is measured with a low current discharge and they will actually last a much shorter time with a higher current discharge, maybe 50% shorter.  So now that 100Ah battery has 50Ah useable but only 25Ah when discharging at a high current.  In order to reduce the current required from a single battery, it is worthwhile considering putting two or more batteries in parallel to share the current across the batteries.  These considerations increase the cost.  If you are really looking for 100 amps for 1 hour then you now need four batteries.

LiFePo4 batteries on the otherhand are almost perfectly designed for energy storage.  They can provide up to 100% DOD and still last for 10 years or more. They can also do so whilst discharging at 1C.  So to get 100 amps for 1 hour then you only need to buy a single 100Ah battery.  They also weigh about 1 third of the weight of SLA batteries and are safe from toxic chemicals and spontaneous combustion.  The downside is the cost, although when you start looking at 100 amps for 1 hour then it really becomes the only viable option in my mind.

To decide how much capacity you need is simply the power of your loads for the time you will use them.  Then add 25% for some conversion losses.  I'll work on 2500W for 1 hour.  That's 3125Watt-Hours (Wh) including the extra.  Then to convert this to amp-hours for the battery you simply divide by the voltage of your battery.  So 3125/24 = 130 Ah.

Consider also whether you'll want extra capacity to account for days when they cannot be fully charged due to cloud cover.  The costs quickly mount up but capacity can always be added at a later date.

How to select the right Solar Charge Controller

Solar charge controllers generally come in two types: the Pulse Width Modulation (PWM) type, and the Maximum Power Point Tracking (MPPT) type.  PWM is the cheaper option and when buying from certain chinese suppliers they will often advertise as MPPT whilst actually it is a PWM type.  So check the reviews and use the "too good to be true" rule, meaning if it looks too good to be true, then it probably is!

The MPPT type has been demonstrated by many people to be about 30% more efficient.  That means your battery is going to be 30% more charged than using a PWM charge controller and the same solar array.  The solar array of each charge controller should be configured differently though.  A PWM charge controller works better at lower voltages, so panels in parallel connection.  Whilst the MPPT charge controller works better at high voltages, so panels in serial connection.

Ensure that whatever the type of charge controller, it is suitable for charging LiFePo4 batteries as these typically charge at a lower voltage than an SLA battery.

In the grand scheme of things the charge controller is a cheaper component that fits between the battery and the panels.  You could simply select the one which fits best.  Or you could calculate how many panels you need to charge your batteries to 100% and then choose your charge controller accordingly.  Let's consider this latter approach.

LiFePo4 batteries can be charged as fast as 1C, let's round this down to 100 amps for our system.  For a 24V nominal system, the charging voltage is approximately 28V and the MPPT charge controller can be assumed about 95% efficient or higher.  So that means about 2800W of panels are required for get to 100 amps.  That's not going to happen any time soon for me.  With 100W panels, I'd need 28 of them.

I'm going to start with four 100W panels because we're in rented accommodation and that's about all I can safely hang out of the loft window.  So I'll have 400W being converted to 28V which gives a current of just 15 amps (rounded up).

Usually the smallest charge controllers are 30 amps so I'll just go with that then, assuming if I upgrade to a whole house type system then many things will be replaced including this charge controller.

Also keep an eye on the maximum input power if you plan on buying several solar panels.

How to select you solar panels

Having got this far you know that I'm constrained by having a rental house and needing it to be removable.  Other people will be constrained by having an RV or campervan.  Some others may be budget constrained.  These will all reflect on your decision here.

You want to buy the highest total power you can afford that fits your space. Everyone should be able to purchase 100W panels as a minimum, these are approximately 1 metre long by half a metre wide.  If you have a bit more space and can afford them then consider getting 300W or even 400W panels.  The power per square metre will go up and the cost per Watt will come down.

If you have a 12V system then you can start with 1 panel and increase according to your budget.  If you have a 24V system then you will need at least two panels. And if you have a 48V system then you will need at least four panels.

When you have multiple panels then you connect them in series to get to a suitable voltage for your charge controller and then connect them in parallel which will increase the current.

The solar panels should come with a specification stating what the open circuit voltage (Voc) is.  This is the voltage measured using a multimeter when nothing else is connected.  For a 100W panel this is usually around 20V.  Then the short-circuit should be stated as Isc or Imp.  If the voltage is about 20V then this current will be about 5A for a 100W panel.  

Connect the panels in series to get the voltage as high as is acceptable by the MPPT charge controller.  They work more efficiently with higher voltages.  Let's assume the charge controller maximum solar input voltage is 90V then we can safely connect four 100W panels in series.  If our MPPT charge controller has a maximum input power of 500W then thats as far as we can go.  If it's 1000W then we could in theory add another 4 panels in parallel.

The Solar Power Reality Check

Now we've reached the end we've selected an inverter to power our devices, we've got a battery that can supply the required current at the required voltage for the required length of time.  We've got a charge controller and some panels.  However, now we realise that if we want to consume 2.5kW of power every day even in Winter then we need a way of generating at least 2.5Kw of power every day even in Winter!

Before we got the sunbed our electricity usage for a five person household was about 8kW per day.  This was averaged over the whole year which is reasonable to assume because we didn't use any electric heating or air conditioning unit.  We did have a tumble drier, washing machine and dishwasher.  I cannot remember whether we had the induction hobs at the time or not.  Suffice it to say that looking at last year's electricity bill should give you an idea of how much power you need to power a house.

A few years ago I found that TESLA were recommending 15kW per day for the average 3 bedroom American house.  Alongside this was a 14kWh battery presumably running at 48V.  It also used an SMA Sunny Boy Storage 2.5kW inverter and charge controller that was approved for grid feed-in connection.  This means that the battery can truly be used as a backup system and to reduce electricity costs.

If I was in my own grid-connected home then I would definitely look at going for a more expensive grid connected inverter because it allows the energy generated to be used by all household appliances year round and reduce your electricity bill.

Back to my startup system.  400W now looks pitifully undersized.  At this point I could reduce my expectations, assume I only ever run my laptop charger and shrink all the other components accordingly.

I may realistically assume that whilst in rented accommodation I might be able to reach 1000W.  I could then reduce my parts list to match this expectation.    An alternative, and in my mind better solution, would be to choose a charge controller that accepts an AC input so that the solar panels supplement the grid.  Or for some people a petrol generator might be an option for powering the larger equipment and the solar generation used for the smaller equipment.

Summary

I include the diagram here to summarize how everything hangs together.  It's quite simple if you work through it one item at a time.  Start at the appliance and fill in the missing parameters in the table.

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