Off-grid PV systems are designed to balance solar input and generator input depending on several factors: irradiation potential (available space/sunlight), practicality (of getting and refueling diesel), upfront cost vs. running cost, and battery lifespan.
A larger solar array will cost more and take up more space. However it will reduce running costs and should improve battery lifespan by decreasing the amount of deep cycles. More sunlight means less panels are required for the same output. In areas with less seasonal variation and more sun, say Africa, solar array output is relatively constant making it easy to size the array. In the UK, seasonal variation is greater, with winter solar output 4 times less than summer. If you were to size the array for winter you would have a huge surplus in the summer, therefore many off-grid systems in the UK rely on wind or a diesel generator during the winter period. This is where you weigh up the upfront cost of more panels vs. the running costs of more diesel during winter and the practicality of accessing diesel and refueling once or twice a week.
When you size the battery bank, you must establish how much electricity you need to store. Consider upfront costs, necessity, and practicality. It will be the most expensive component when you first buy the system. However, choosing to go with a smaller battery bank may mean you don't have many days of backup to cover periods of overcast weather. This isn't too much of a problem as you should (must) have a generator to provide this backup and recharge the bank if necessary, but if you were to have several days of sunshine, you may not be able to use all the power if your bank is too small. A smaller bank also means more frequent deep discharges resulting in shorter lifespan and a faster rate of discharge (C rate) which causes more energy to be lost as heat. On the other hand, oversizing your bank will mean the array is never able to recharge it, relying on the generator to do so. This method of charging is often limited by the charging current of the inverter/charger, requiring the generator to run for many hours. Ask yourself whether you want a small bank that has to be recharged every other day during poor sunlight, or a large bank that takes 12 hours to recharge but you only have to do it once every 4/5 days.
We deal with these trade-offs every day, if you are interested in purchasing an off-grid system please call us on +44 (0) 333 444 7655 and we will help establish your needs.
The introductory questions below will help get the ball rolling. Battery configuration is very important.
Failure to adequately understand: a) the influences on your batteries; and b) the requirements of your batteries, can end up being an expensive mistake. When planning an off-grid solar system, the battery configuration must take centre stage. There are many factors to consider.
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