Building a 12V off-grid solar power system

This is how I built a 12V off-grid solar power system in my back garden and what I have learned through the process.

Conception and Initiation

All started with a 300W auto inverter I had laying around, 2 x 6V batteries (joined in series) from my daughter’s electric car (faulty) and a 250W garden light I wanted to be able to power using the power of the sun. In the end the goal has changed. I wanted to be able to power my photo-video setup off-grid, where power is not available. How expensive could it be? To find out continue reading and hopefully, you will be able to follow my example or avoid some of the mistakes I’ve done.


I started my planning by watching videos on YouTube about off-grid solar systems and the more videos I watched the more confusing it all become.

In the end, after a month of research, I decided that I don’t want to invest lots of money into this and since I have most of the items, I will spend £55 for a 30w solar panel and a 10amp PWM solar controller.


It was the middle of winter in Essex. First, I connected the fully charged battery to the PWM solar controller. The controller wouldn’t turn on. I went to my car and connected the controller to the battery and the controller came on. I deduced that my battery was the issue.

Then, I invested in a basic multimeter to help me troubleshoot further similar situations. My multimeter confirmed that my batteries were dead hence the reason why my controller wouldn’t start.

Next, I bought two second-hand car batteries from the Facebook market place for £40 hoping that at least one was decent.

Wired one of the batteries to the PWM solar controller and finally, it came to life. Fixed the solar panel on the roof of my shed and run the wires inside. Finally, I connected the inverter and tested the 250W garden light. The light turned on for a few seconds then went off and the inverter started beeping.

After some reading, I understood that the car batteries I got (flood acid car batteries) are designed to deliver a lot of power for a short amount of time, then the car inverter would quickly top up the battery in transit to have it ready again when needed.

Side note:

That’s why your battery will run flat if you forget the lights on or listen to music when the engine is not running.

To address this issue, I thought that if I buy a smaller light the issue will be solved. So, I spent another £27 for a 20w garden light, and some wires and a UK plug and learned how to wire all together hoping that this will be the end of it. Well, it wasn’t!

Needless to say that it worked a bit better, but again after a few minutes, the inverter would start beeping again and had to shut everything down.

I then decided to connect the second battery in parallel to keep the Voltage at 12V, but double the Amps. In order to do that I needed cables and adapters which are not cheap.

However, I managed to connect everything together, let the batteries charge for a few days and everything kind of worked until the sun stopped shining and my batteries wouldn’t charge enough.

To address this issue I thought I need bigger solar panels and a better MPPT inverter because apparently, the PWM is not very efficient.

I started with a 150w solar panel from Dokio which perfectly fit on top of my shed. I took this as a sign to move forward with this project, so I decided to replace my PWM solar inverter with the best one out there (the most expensive) – Smart Solar controller MPPT 100 | 30 Bluetooth from Victron. Coincidently the sun was shining a bit more than usual, so my system started to deliver more stable results.

After months of reading the stats on my Victron app, I realised that the 150W solar panel usually delivers half the watts partly because of its installation and the sun’s elevation in the sky – the rays wouldn’t hit my panel at 90 degrees for the best outcome.

The next logical step was to add a second panel in parallel to help collect as much sun as possible and charge the batteries as quickly as possible.

At this point, I spent around £600 and am still not able to keep my 20w light on all the time without triggering the inverter’s alarm.

I thought now that the inverter is not efficient so I bought the best of the best, a Phoenix 12 | 1200 pure sine inverter from Victron and replaced the 20w light with a string of 5w LED bulbs.

Now the inverter was too powerful and sucking all the power from the batteries and shutting down my lights in the middle of the night. At least the inverter warning message was only showing when connected to the app and just shut down the inverter if the battery level went below a specified voltage limit.

At this point my initial goal has changed. Now I want to be able to power my photo/video setup off-grid.

I decided to invest in a proper LiFePo4 battery, a 300Ah from Roamer which was on offer for “only” £1305.

After 2 months and a half of waiting the battery arrived. It was 33% full and I was supposed to charge it first before adding it to the system. Of course, I was too excited and started wiring everything together.

Now my system got 2 x 150w solar panels (300w), a DC switch, a Victron MPPT 100 | 30 smart solar charge controller, a Roamer 300S ah battery, and a 500 Amp smart shunt from Victron, a Victron Pure sine Inverter, Phoenix 12 | 1200. After a full day in the sun at 30 degrees, the battery levels only increased to 53%, so in theory, if the battery is completely empty I could fully charge it in 5 days.

After more reading I have discovered that the cables from the battery to the inverter should be thicker at least 40mm (mine was 24mm), and even the cable from the solar panels to the controller and from the controller to the battery should be 6mm to be able to safely carry 30amps (mine was 4mm).

I also ordered 2 more solar panels bringing the total number to 4x150w=600w, a battery disconnect switch, and the correct size cables and went to TLC Electrical Supplies store in Basildon to buy the lugs required.

I wasn’t sure what exact size lugs I need. For example, the battery end would require an M8 thread, the shunt would require an M10 and the battery disconnect switch (to stop seeing the spark when working on the system) had a M10 thread.

Initially, I thought that 25mm, 10 thread lugs would suffice. In the end, I bought 35-12 and 25-10 lugs and I am so glad I did that. Once I removed the insulation from the thick 40mm cable the 35-12 would fit like a glove, so I used those for all the M10 threads, 6 in total. I used 2 x 25-10 lugs for the 6mm cables from the solar charge controller to the Shunt and the battery respectively.

Monitoring and Controlling

I have been monitoring my system for a few days now and everything is connected and working as expected. The battery has reached a 100% charged level. The maximum power I was able to generate was 1.5kWh in a day.

What I have learned?

  • It is important to have a clear goal, a written plan, and design the equipment requirements in advance. I did it towards the end of the project. See poster image.
  • Changing the scope of the project in the process can be expensive. From being able to light a bulb for a few seconds at a time to being able to power flash heads, laptop and monitors off-grid is a different kettle of fish.
  • Being over-optimistic about how much a solar panel can produce and the weather in the southeast UK. In my case, I can only produce 50% of the advertised wattage.
  • It was a fun project, but contacting a specialist in advance could have saved me some headaches and money.

Total cost so far £2871 out of which £2766 actual cost deducting the cost of the items no longer in use.

Winter update

Is the end of November now and the sun is very low in the sky and raining most of the time during the day, which means that the battery is not being charged by the sun. To resolve this issue, I bought a Victron Energy Blue Smart IP22 Charger 12/30(1) 230V and top up my battery during the night when the tariff is cheaper.

Cost breakdown

PcsAffiliate LinkItem descriptionPrice per unitTotal
4 150W 18V Monocrystalline Solar Panel to Charge 12V 119.99479.96
1Home Bargain24-hour timer switch2.992.99
1 Breaker 17.2917.29
1 Slotted Aluminum DIN Mounting Rail4.994.99
1 Energy SmartSolar MPPT 100V 30 amp 12/24-Volt Solar Charge Controller (Bluetooth)200200
1 Energy Phoenix Inverter 12/1200 230V VE.Direct UK330330
1 Energy VE.Direct Smart Dongle (Bluetooth)41.6541.65
1 Energy SmartShunt 500 amp Battery Monitor (Bluetooth)107.95107.95
1BTSKY 1.5M/5 Feet 10 AWG Solar Extension Cable with Female and Male Plugs for Solar Panels11.9911.99
1 LITTON Battery Disconnect Switch13.9513.95
2 Metre Black Battery Starter 40Mm Cable8.4916.98
1 Metre Red Battery Starter 40Mm Cable8.498.49
1 Solar Panel Branch Connectors 4 Way Y Branch Parallel Adapter Cable17.9917.99
1 300AH LIFEPO413051305
10² Crimp Cable Lugs – 10mm Stud Hole0.313.1
10² Crimp Cable Lugs – 12mm Stud Hole0.414.1
1 Wifi Extension Lead 3 Gang White2525
1 Energy Blue Smart IP22 Charger 12/30(1) 230V175.93175.93
Cost Breakdown 12V off-grid solar power system

Disclaimer: This post may include affiliate links. If you click one of them, I may receive a cute commission at no extra cost to you. Thank you in advance!

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