Updated June 2017
Knowing the best batteries for stand alone solar is confused by over-promotion of whatever makes the most profit. This Solar Success guide explains all.
Knowing the best batteries for stand alone solar system saves huge sums. Lithium-ion is increasingly promoted as producing more power. Indeed it does. But power and energy are very different. In almost all stand-alone systems it is energy, not power, that matters.
Energy and power explained
Energy is that property that enables work to be done: e.g. electrically. Power relates to how fast energy is used. Starting a car engine is a good example. It needs high power. The energy needed, however, is tiny! It’s about that needed to run a five watt LED for an hour.
Another, for example is lifting (say) 100 one kg cans to a shelf two metres high. This a certain amount of energy but a 10 year old can readily do it. Were that 100 kg to be a barbell however, hoisting it two metres needs exactly the same amount on energy, but a lot more power.
Knowing that difference ensures you buy batteries you need. Costly ones may well have greater power. Unless you truly need it, however, buying them is like paying a weightlifter to stack those cans!
Knowing the best batteries for stand alone solar – lead acid
Lead acid batteries are totally known, relatively cheap, simple and affordable. Furthermore, countless stand alone solar systems use them. Their main downside is bulk and weight. The still-used wet cell versions need regular topping up with distilled water. Moreover, for longevity they need using only from 100% to 80% of full charge.
Knowing the best batteries for stand alone solar – AGM
These too are bulky and heavy, but are maintenance-free and electrically rugged. They hold their charge for a year or more. They are undamaged if left at low charge and favoured for stand-by use in critical applications. Virtually unused ones can often be bought at very low prices second-hand.
Knowing the best batteries for stand alone solar – lithium-ion
The costly LiFePO4 versions are strongly promoted for home systems. They do much of that claimed (although lifespan is yet unproven). Their ability to provide ultra-high peak power, however, is needed only if you do commercial-scale arc welding etc. But for that you need a great deal of energy as well. Here, a diesel generator makes more sense. No normal loads in a typical home or property system need anything like lithium-ion’s power capability.
They are about 60% lighter, smaller but that is of little relevance for most homes and properties. It is claimed that more capacity can be used without overly shortening their life, but yet to be proven.
Conventional sealed lead acid, gel cell and AGM (Absorbed Glass Matt) battery banks are cheaper and simpler to install. They are also far more readily available.
Knowing the best batteries for stand alone solar – further issues
Solar capacity has fallen in price by 75%-80% since 2010. Battery storage cost has risen by over 50%. Solar produces at least some output in all but (rare) totally overcast days. It is thus increasingly pays to have more cheap solar and less costly battery capacity.
People living on solar tend to use far less energy than those on grid power. That for a typical electrically-efficient off-grid solar home is likely to average 10-12 kWh/day. Those already optimised for energy efficiency use far less. How to do this is explained in detail in Solar Success.
Knowing the best batteries for stand alone solar – capacity required
The amount of battery back-up required depends on the extent of generator back-up. It makes no sense to scale the system for zero back-up. It can be done but solar and battery capacity needed is huge. Excepting where there is less than two Peak Sun Hours a day at times (see * below) etc, using solar and battery capacity for about 90% of the time is cost effective. Designing for 95% almost doubles system price!
Assuming 12 kWh/day, of which overnight draw is 4.8 kWh (40%), depleting the battery by 10% requires about 48 kWh of conventional battery capacity. This also provides for two full day and nights with (rare zero solar input) that depletes the batteries by an occasional 50%.
Knowing the best batteries for stand alone solar – cost involved
Most off-grid home systems use battery banks such as that below. This, of 16 by 235 amp hour (12 volt) gel cell lead acid batteries, stored 45 kWh. It would cost (in 2017) A$10,500 to A$15,000.
Author’s bank of sixteen Exide 12 volt gel cell batteries. Each is of 235 amp hour – about 45 kWh total. They are connected in series-parallel to provide 48 volts. Pic: successfulsolarbooks.com
Lithium-ion (LiFePO4) batteries can be safely discharged to a level far below that of lead acid batteries. This enables a smaller capacity bank. Assuming only 25 kWh, the cost, however, would still be around A$25,000. The probable (claimed) life span (of 10 years) is similar.
Even were LiFePO4 to provide an improbable longer lifespan, the loss of interest on that extra $10,000 (at 5% per annum) is $500 each year! Lithium-ion prices may fall, but the product is in such huge demand, and sources limited, this is not likely.
LiFePO4 batteries are being promoted as superior in many ways. In some ways they. But not necessarily of benefit for basic stand alone systems. Traditional batteries (particularly AGMs) are still seriously worth considering. They are far cheaper, readily obtainable and much simpler to understand, install and maintain.
Lead acid batteries are excellent for storing energy
Lead acid batteries are excellent for storing energy. They have ample power for domestic and property needs. Lithium ion batteries can supply far higher power, but in typical home and property solar systems, having them is like a Ferrari engined VW Kombi.
By and large a 48 volt lead acid, gel cell or AGM bank of about 45 kWh is ample for most three-bedroom homes. Given an adequate inverter, this enables several major appliances to be used simultaneously. Their life span is related to ambient temperature. They prefer 50 – 250 C. If typically discharged by 20% they should 10-14 years.
Most battery makers see ‘lifetime as that when the battery delivers 80% of original capacity. If discharged less, they last longer. Some still work (if not that well) are 20 years old.
The author’s self-designed and built solar system powers the self-built home on a 10 acre property north of Broome ( far north west of Australia.) The Indian ocean is in the background. Pic: successfulsolarbooks.com
* Peak Sun Hours
A Peak Sun Hour (PSH) is a solar industry term. It is an equivalent solar insolation of 1000 watts per (horizontal) square metre for one hour. Were that to be 500 watts for two hours that’s 1000 watt hours – (1 PSH). Solar for home and property systems is feasible in areas that have a yearly average exceeding 2 PSH. To establish PSH: Google your locality plus Peak Sun Hour.
Solar marketing and promotion overlays stand-alone system reality. I am free of all that: apart from my books, I have no financial interests in the solar industry. My background is in electrical and mechanical engineering and technical writing – with careers in each. I have personally designed and installed my own major stand-alone system and assisted many others do the same. This enables me to tell it as it really is.
Not as the industry would necessarily prefer you to believe.
My (Solar Success) tells all you need to know to design and install your home or property system. Tens of thousands use it worldwide. It is totally revised between prints runs. Now, (June 2017) it is in its Third Edition. The (4th edition) Solar That Really Works! does likewise for cabins and RVs. My other books are Caravan & Motorhome Electrics, and the Caravan & Motorhome Book. All are technically sound and written in plain down to earth English. For more about the author please click on Bio.
Please also see our major RV related site: caravanandmotorhomebooks.com