DIY Off Grid Solar Project

First budget set up £1000

Normally I start with waffles so let's dive straight into the technical details first this time. Any feedback and warning welcomed as this is my first attempt.

I installed two 375W Eurener Zebra solar panels: one on my south facing conservatory roof and one on the back wall. Solar output peaked at around 600W combined (parallel). I overspec-ed it due to the flattish wall placement and in anticipation of reduced winter sunshine. 

[Update: October autumn day averaging only 1.5kWh -- 0.3/0.6kWh when cloudy and 2.5kWh when full sun.]

[Update2: As the winter sun gets lower, the panels fall into more shadows.]

The panels were chosen for the low weight (19.2kg) to efficiency (20.6%) and ones that I can collect by car; quite often panels are delivered in pallets costing around £50. The 3.3m aluminium mounting rails were hard to source but I was lucky to talk them into supplying two with the panels and fitted nicely into the car, just.

Important solution maximise the solar output are two things: (a) lithium batteries and (b) an MPPT controller and in this case EPEVER 4210N that can output 40A.

MPPT basically converts extra solar voltage, e.g. 40V 20A into 14V of battery charging currents up to 40A which would otherwise be wasted if lithium batteries were not used. Ok going Lithium means increasing the budget.

40A at 14V basically puts a cap on the PV I need to supply as anything above 560W cannot effectively be used. This would be an issue in the summer but in autumn this limit is ideal.

While my 12.8V Lithium is capable of taking 40A charging current and output 150A, it's both a challenge and undesirable to max out.

Most electrical devices with motors has phase lag causing high current surges. My 3000W 12V inverter will draw 3000/12 = 250A!

Even 2kW (166A) is enough to trip after a short while. Ouch.

Fuses, circuit breakers and RCD

MPPT needs a battery to operate and calls for batteries to be present when PVs (photovoltaics) are connected. So batteries are simply connected to MPPT directly since MPPT has many built-in safety features. Solar panels on the other hand are connected to a double pole 50A circuit breaker acting more as a on/off switch when servicing the rest of the system. I bought a mc4 inline fuse but left unused. 

The batteries are connected to the inverter via single pole 150A circuit breaker.

The inveter output is connected to a integrated RCD wall socket.

Overcoming high current draw 12V or 24V

I started with one 100Ah 12.8V Lithium battery (because it is the most costly item in the system). And very quickly I realised three things:

(a) it runs low pretty fast, enough to cause inverter to stop and moan not much below 12V, (DoD ~50%? Not much danger of excessive draining battery DoD >80% if the inverter is having none of it!) So your actual usable storage capacity is not 1.2kWh but more like 60%?

(b) battery output cable runs hot and the 150A circuit breaker keep tripping when my vacuum operates above mid power for a few seconds,

(c) as a pure off grid system batteries gets full and lots of solar power are wasted on sunny days even in autumn. At 500W, a 60% discharged 100Ah can be topped up in 1.5 hour. A 4 hour autumn sun can fully charge 300Ah easily.

I'm using 25mm² cables for battery and 10mm² /16mm² for MPPT input/output.

There were many reasons for choosing a 12V but not necessarily if I were to start all over again.

At the time, 24V pure sine wave inverter were hard to find and 24V hybrid MPPT/inverter were confusing to me. They either don't officially supports lithium batteries or only come in modified sine wave or very expensive. Mainly I didn't know such device existed when I started.

The good news is I overcame the problems above with the system I already purchased with an additional Lithium battery.

Solution

Most advice on the web and Youtube connects two batteries in parallel using jump leads. 

Take a general example:

PS: The diagram actually shows a hybrid inverter with PWM controller.

What didn't make sense to me is to load the single wire pair between the batteries and inverter. Thicker and chunkier to carry >200A, sure, but in my view better to use twice the same cabling and individually switchable breakers. Not least to utilise what I already have.

A typical 2kW draws 166A and that generates heat regardless of wire thickness. My solution is to use two separate 150A circuit breakers for each battery wired directly up to the inverter input.

I'm happy to say it has worked well so far. No more cable heat or breaker tripping while drawing >2kW for long periods. Each battery cabling should draw no more than 100A continuously. Honestly this is not measured or failed yet (in the sense of stress testing). And of course more (indirect) paths now exists between the each battery and the inverter.

The MPPT is now able to draw full 41A from the solar panels most of the time and the battery voltages has not dropped below 12V.

Being off grid, it is tricky to use the available power effectively and efficiently. I started with powering  my fridge and vacuum and NAS. Currently I have added a TV and microwave.

My hope is to supply 200kWh/year fridge and 100kWh/year TV and 100kWh/year of microwave and vacuum. At 50p/kWh that should contribute £200 a year. 

Unfortunately this extra battery blew my budget:(  And cost-wise no longer compares favourably with professionally installed 6kW system typically at £10-12k. Meaning it'd now take 8 years to breakeven.

The back story

Sure I'm not the only one thinking to cut my energy bills by going solar in the climate of "cost of living" crisis and global warming/draught in summer of 2022. Solar equipment demand and waiting time is up.

Having said that, be able to afford a solar system clearly doesn't qualify me as "in crisis" and then as soon as hosepipe ban come into effect in SE England, autmn rain arrived, grass turned green and water butts are full again. Ukraine war actually pushed gas prices up double that of electricity although UK still generates roughly 50% of electricty from CGGT (Combined Cycle Gas Turbine) when wind is low.

Part of the fear was blackout over the winter months. Let's hope it never come.

I harbour the thoughts of solar panels as sunblocks since I moved in with the south facing conservatory unbearably hot in the summer topping 45C. Since then I have tried different solutions which failed like wall fan and extractor fans but ultimately aluminium foils work best when installed on the outside! On the inside, the reflection into the double glazing generate significant heat although it stays high on the roof and still effective in cooling the room.

Finally before installing the solar panels I decided to paint it white and that seriously worked. I'd highly recommend it instead of spending serious money on conservatory blinds.

Update:

As I feared, it is looking like cloudy autumn skies are not good for deceptive heaving hitters like microwave. Rated at 900W but drawing 130A and consuming 1700W (seems real and hungry).

"Within the microwave oven’s high voltage circuitry the transformer does the job of changing, or transforming 240 volts AC into 4000 volts AC.  This high voltage is needed to make electrons leave the cathode in the magnetron and move them towards the anode to generate microwaves."