Batteries are the heart of any off-grid system and provide energy storage. This means that when the sun isn't shining or the wind isn't blowing you can still used the power generated when they were.

Just a quick summary of battery basics for anyone starting out as they should be maintained and understood. 2 Volt cells make up the required voltage. Leisure and car batteries come in 6 cells to make 12 Volts and most domestic systems are a multiple of 12V (e.g. 24V, 48V). The higher the voltage the thinner cable can be used and the more efficient for inverting to 240V.

Batteries & cells can be wired up to work in series or parallel. In series the Volts add up and the Amps stay constant, so you use series configurations to increase the Voltage. In parallel the Amps add up and the Volts stay constant, so you use parallel configurations to increase the Amps which increase the storage capacity.

An Amp hour (Ah) is literally one amp for one hour, so with 100Ah you can have a one amp load for 100 hours, or a 5A load for 20 hours, or 50A load for 2 hours, and so on.

For smaller systems in boats and caravans we usually recommend 12V leisure batteries that can fit under seats or cupboards. You need to keep the weight down and they must fit inside the vehicle for transit. However when you get to static systems it's usually possible to use larger batteries. It's always nice to have a dedicated space for off grid power systems, such as a shed or external room to the dwelling. Vented batteries give off hydrogen gas which can be dangerous in closed and hot spaces, but don't worry, it can be managed.

Side note* In our original solar truck we had limited space and so the batteries were next to the wood burner. We built a box to house the batteries with 2 air gaps, fire resistant board and metal, as well as a sloped lid with active venting.

So larger batteries will obviously cost more so for a substantial domestic system you're looking at possibly 50% of the whole system cost to be the batteries, that's if you buy brand new. But in the true light of reduce, reuse and recycle, we have always used second hand forklift truck traction cells, plus they are generally about a quarter of the price! Many of the batteries used in industry have not exhausted their worth before being replaced, usually after 5 years or often if the machinery they are powering breaks.

Traction cells are great for domestic systems as they are designed to be run down to a low percentage every day (shift) and charged back up quickly overnight on a high charge. This means they can take a battering, unlike leisure batteries which require a trickle charge. To put this into context, if there are a few days of no input (no sun or wind) the supply will not be affected, if they are sized correctly, and if there is a huge amount of resource one day (very sunny or windy) they will absorb loads of that lovely energy for you to use later!

As with all components of a renewable energy system there are lots of points to weigh up and consider and unfortunately it often boils down to budget. But you can arm yourself with knowledge and reduce the demand for new items. Lead acid batteries are recyclable BUT it is done in developing countries and quite often to the detriment of the local people and ecosystems. Compared to lithium-ion batteries where the raw material is finite and although they can be recycled it is not 'economically viable yet.

So whats the most ethical decision? We have always fantasied about water energy storage. You need a hill with a reservoir at the top. You use excess power to pump water up the hill into the reservoir and run a hydro turbine when you need the power. Extremely site specific and requires constant management by either yourself or some electronic trickery.

More info like state of charge and sizing your own battery bank can be found on our downloadable battery flyer here.

It’s not often I get our business accounts all up to date in April but due to the lockdown I’ve had a chance that catch up with the paperwork so yes I’ve got the books up to date. But this time as well as logging the dates and amounts, I also logged the litres of fuel consumed by the fire engine and the hi-lux during the event season. I’m fun like that.

So here’s some fun facts for you..
We filled the fire engine the grand total of 9 times. It used 647.5 litres of fuel. The hi-lux used a similar 670.6 litres but will have driven many more miles as we use it for errands and visiting in between events. A guesstimate of miles per gallon is 25mpg for the hi-lux and 13mpg for the fire engine.

After looking through a few carbon emission calculators and other sites I settled on the larger number of 2.68 kg of carbon dioxide emitted for every litre of diesel consumed. If anyone reading this has a better calculation of this with reference please do comment. We’re all here to learn!

So skip the maths for those who don’t like it and we have 1735 kg or 1.7 tons of CO2 emitted from the fire engine and 1797 kg/1.8 tons CO2 from the hi-lux. Now I have my own personal feelings about off setting carbon and it can be misused by heavy emitters making millions from polluting the planet. But for us, it’s something we can practically do to say sorry to the world for driving dirty diesel engines about.

I’ve lost count of the amount of times we’ve been heckled for using a diesel truck to raise awareness about sustainable living. Our response usually highlights our low impact lifestyle and the fact that the fire engine towing our home from event to event is still far less than the average household. So how much carbon would a household emit over a 5 month festival season?

From a study I just found on the internet (1) it states a family of two adults and two children (as we are) would emit 28 tons of CO2 from home energy, transport and indirect emissions per year. Divide and multiply this figure for 5 months and you get 11.7 tons compared to our 3.5 tons for both vehicles over the 5 month season, not bad eh. Now the 11.7t figure does include indirect emissions which our 3.5t does not but the difference is still clear and even with indirect emissions added we would emit less than half CO2 than the ‘average’ family our size. Modern houses are designed to consume energy but that’s a whole other blog about passive houses!

Back to the offsetting and although there are quite a few organisations who can off set carbon for you, in the true spirit of self sufficiency and off grid living I thought that us planting enough trees would suit us better. Now having spent the last seven winters living in the woods we have planted many trees but not to off set. According to a google search, and please correct me if you have credible reason to, one tree will absorb 250kg (1/4 ton) of CO2 in its lifetime.

Great, now we’re getting somewhere, so the fire engine needs just under 7 trees and the hi-lux just over, give or take a few kgs. Therefore we need to plant 14 trees to offset our summer diesel use. That doesn’t seem to hard to achieve so I will update you all on my progress with that!


(Picture is of trees saved by nana to plant).

(1) http://ftp.iza.org/dp7204.pdf