Off grid for under £2,000 – Case study

Fridge, lights, TV, computers? YES.   Kettle, toaster, hairdryer, washing machine, dishwasher NO. These were the parameters for a minimalist off grid PV system for a friend’s pod. Low power but low price. Tempted by an ad for some second hand panels from Canada we were soon knocking on the door of Bimble Solar near Brighton. We fancied 4 PV panels and were shown into a large barn with stacks of panels up against the walls. ‘Which ones are ours then?’ we asked. ‘Any ones you like’ was the answer. They were mostly second hand and we settled on the 300Watt ones from France; defunct solar farm casualties. Apparently there are many solar farms either going bust or upgrading so there will be heaps more cheap panels to come. As panels should last 20 years it makes sense to start off with cheap used ones – who knows what the possible upgrade will be in ten years or so.

20 minutes and £1,740 later we had the kit in the car with batteries to be delivered later. Here’s the kit list:-

4 x 300W PV panels, 4 years old  and £99 each.

So this is the 1.2kWp array

40A 12v/24v MPPT charge Controller – New Model Tracer4215BN

This charges the batteries efficiently and safely.

Optional MT50 MPPT Display meter for New Tracer BN Charge Controllers 10a 20a 30a 40a

Tells you what is going on with every part of the installation.

Victron Phoenix 1200W, 24V inverter

Much favoured by the boating community this turns 24V DC into 240V AC, up to 1.2kW. The output is pure sine wave AC so good for computers and TV.

Sterling 200Ah Sealed Battery 12v

Two of these in series gives 24V. Big and very heavy there is no doubting the quality and 200Amp.hours is at the top end end of the range.

All the connectors and wires were included and the wiring turned out to be very simple.

To keep the wiring neat and safe I like to use two consumer units; one for the low voltage DC side and another for the 240V mains side. The bus bars and the DIN rail in the consumer unit make wiring quick and convenient and when all the units are board mounted, with wires going behind the board, it all looks neat and tidy.

PV Off-grid wiring

Once the fridge was turned on the display showed a draw of just under 5 amps for a few minutes at a time. This was not going to tax the 200Ah batteries at all. Every bulb in the place is an LED so no problems there and there will be enough left over for TV and computer and to fire up a gas water heater.

The proposed Rinnai 16i in-line gas water heater draws 68W, when running, which is easily manageable for short periods. This LPG heater will provide instant showers and hot water with twice the power of many combi boilers. If fed by pre-warmed water the unit dials down the gas consumption to produce the same output temperature. This enables any solar heated water to cut down gas use – a project for the future.

The supply of power is looking very generous for the summer but will hopefully still be close to requirements in the dark winter when the input from the panels will just about halve.

Even this starter kit gives a feeling of independence and empowerment and, of course, freedom from price rises for years to come. To run washing machines etc you’d need a 3kW kit which comes to about £5,000. A full 6kW mains equivalent kit would be nearer to £10,000.

There is a section in my book ‘Dream House – Down to the details’ on Amazon – here – which discusses the idea of using systems like these as large uninterruptible power supplies, in daily use but keeping the mains as more of a back up. There are charge controllers (e.g. Victron) designed to do just this so the UPS concept is really quite simple to implement.

£1,000 heat pumps

The Bimble web site is a delight to browse through. I was particularly taken with the circa £1,000 heat pumps at a fifth of the usual prices and looking good value with Toshiba compressors and modern r410a fluid. A heat pump is just like a large fridge, and usually just as reliable, so one of these might be worth a punt at this price but you might need advice on what to connect them to.

In all probability these are Chinese but they often come with quality European components so they should be mechanically sound.

Have a look here at a system especially suited to integrating heat pumps.

FYI the UK Renewable Heat Incentive pays 7.63p/kW.hr on air source heat pumps.

Solar Immersion Controller SOLiC 200 – free(ish) hot water from your excess solar

Another find on the Bimble site.  If you don’t get much for exporting your PV power to the grid this little box can divert it to your immersion heater. The original kit that could do this disappeared off the market so it looks as though the SOLiC 200 has stepped in to fill the gap. The £200 price will be repaid pretty quickly if you pay a high price for electricity, however if you are defraying gas use in the UK then maybe not. Gas is still really cheap.

The Rinnai 16i in-line gas water heater – a solution for guest houses?

This useful boiler heats water just as you use it but without the losses incurred from storing the hot water in a tank. The water is delivered at the set temperature which can be quite low compared to stored water. It is powerful too – like having a couple of combi boilers together. Being much simpler than a combi though, the Rinnai only costs £500 and for larger properties it makes sense have more than one with the added reliability when the outputs are cross linked.

The Rinnai is marketed as a base for a tankless system but its modulating feature makes it even more effective when used in conjunction with a tank. For example if you were running a small guesthouse, needing an unlimited hot water supply, you could use wood and solar to heat the tank and only use the Rinnai to top up the hot water coming from the tank when necessary. The boiler only uses enough gas to raise the water temperature to the set point so when pre-warmed water arrives the gas throttles back. With cheap energy sources pre-heating  the water, gas consumption will plummet but constant supply will be maintained automatically. The Xcel heat banks I supply have been used like this for years so if you want to discuss such a system let me know.

Free air conditioning?

If you were in Italy this summer you might have experienced temperatures of well over 40 degrees. As there is almost a whole year to go before that all starts again perhaps a cool look at some air conditioning might be in order. You’ll know those portable units where water is evaporated and exhausted to the outside through a tube. It takes a lot of heat to evaporate that water. Imagine trying to boil a large saucepan of water dry on your stove – that’s how much heat is needed and how much heat would be removed from the air. So if a lot of water can be evaporated then things will tend to cool down, a lot. The cheapest way to employ this principle is to mop your floors in the early morning and open up the doors and windows to let the moist air out before you have to close up again. Chilly floors will get you comfortably through the rest of the day.

My school in Kenya had a cool room made with wetted charcoal walls; a great example of the power of water evaporation. If you fancy making an evaporative aircon unit I have a design using charcoal, irrigation hose and corrugated pipe …. happy to share.

Nights can be unpleasantly hot though so what about them? Well the outside temperature falls off a good 10 degrees from the highs of the day and if it wasn’t for intruders and mosquitos it would be great to open up the windows again, especially the downstairs ones where natural convection will move the hot air up through the house. Mosquito nets over barred windows make a good start but it takes a lot of air to make much difference so a fan on the window sill will help to pull in that lovely cool air. The air it displaces will need to get out so other netted windows will be needed upstairs.

Liquid pool cover – HeatSavr Ecosavr Fish

Evaporation is the last thing you want on your pool, it not only lowers the temperature, it uses significant amounts of precious water too. A liquid pool cover is the answer. One or two £20 fish shaped sachets of liquid in your strainer slowly dose the pool with a molecularly thin coating that floats on the top and prevents evaporation for 5-6 weeks. Well done Martin Daykin for trialing this product and telling us about it. It works.

Heat bank tweaks

The heat bank specification just got better. No doubt you know this is a stainless steel tank with a complete plumbing solution – pumps, valves, wires, solar coil etc – all fitted to it and ready for a speedy installation. Two new mods are available. To prevent the heating pumps depleting the hot water too much there is a new cut off thermostat; unless extra heat is being supplied to the tank the pumps will cut off to leave a block of hot water at the top of the tank.

Home automation enthusiasts will welcome a couple of extra sensor pockets for temperature monitoring. One use might be to turn on the domestic hot water pump to deliberatly destratify the tank, when it becomes very hot, before the overheat stat turns on the heating. Now the hottest water is sent down to the cooler bottom of the tank and as the whole tank becomes uniformly hot the effective capacity is increased.

There is still time to get your heating organised before winter so do get in touch if you want to discuss your strategy.