HOW DO I SELECT AND SIZE MY BATTERY?

We have talked about the different types of batteries and their pros and cons. Just a few pointers below on battery selection from the previous post:

i. Cost- For lower upfront cots the lead-acid batteries are cheaper per Ah or Wh on initial cost…. However since their lifespan is typically 4 to 5 years, you replace them twice before replacing a typical Lithium Iron Battery. This usually makes the Lithium Iron battery cheaper on the long-term. So if your cost considerations are short-term upfront costs then your best bet should be the lead acid options like Gel and AGM, but if you are looking at longer term holistic life-cycle cost then Lithium Iron is your best option

ii. Compatibility- Lithium Iron batteries are not compatible with all types of inverters and systems because of the circuit protection… this has to be part of your selection process

iii. Durability and Warranty- this one is closely related to the advice we always give on all other products…ensure that you get a reputable, certified and tested battery brand. Certified and tested batteries come with warranty, and guarantee durability of your product. Warranty allows you to return the battery for repair or replacement, provided it is within the warranty terms. If the manufacturer/seller is not giving you warranty, this might be a lack of confidence in their own product!!

Ok now that you have made your choice on the type and Brand, how do you size your battery system or battery “bank”??

The starting point is the equipment that you intend to use on back-up power. This is usually technically referred to as ”essential loads”… get the power rating of each appliance that will be on essential load. Please see typical sizing below

Please note that the Wattage column for each appliance incorporates efficiency of the inverter

The size of your battery bank will depend on the wattage of your appliance and the total number of hours that you intend to use during back-up mode on your batteries… From the total energy calculated from your consumption profile, we then get the Ahs (amp-hours) or kWhs (kilo-watt hours) that your system will use. This of-course will be done, after factoring in losses and Depth of Discharge (D.o.D). So in simple terms: the more items you put on essential load, and the longer you want those appliances to have back-up, the bigger the battery system… and vice versa

Battery Voltage: the battery voltage of your battery bank (battery layout), will depend on the allowable voltage of your inverter, and also cable current considerations. Voltage is added in series, and the same in parallel. Most batteries are 12V batteries, so for example if your inverter is a 24V inverter then you will need 2 batteries in series 12V +12V = 24V

Batteries are usually the most expensive component of your solar system, and we advise that care should be taken in selection and sizing. It is best that you get an expert to help you design & size and install your batteries and whole solar system. Please kindly note that we offer FREE battery and system sizing and selection guidance at Rentech…

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About Author:

Lovewell Chitiyo is a Mechanical Engineer with 15 years experience in the building services Engineering consultancy, renewable energy systems and sustainable design fields. He is currently studying Masters in Renewable Energy at Stellenbosch University in South Africa

Lovewell is an Accredited Professional with the Green Building Council of South Africa for Existing Buildings Performance and New Buildings; Interiors, holding several qualifications and certification in Renewable Energy and Sustainability.

Lovewell also has vast experience in design, supervision, installation and commissioning of several projects locally and regionally in the last 15 years.