What solar battery size do i need？

Today we're delving into the selection process of the ideal size for batteries, the type of batteries to settle for and much more. The focal point of our discussion will be determining the adequate battery pack size you require.

Starting with the basics, we're looking at integrating batteries into solar power systems. Basically, a battery pack is a group of batteries that work in conjunction, providing power when there's no sunlight. The right battery technology and proper size of the battery pack can offer enough power for your electrical needs during non-sunny periods. Among the wide range of battery technologies available, lead-acid batteries, including liquid lead-acid types, are commonly used.Diving straight into differences - in energy storage, lithium-ion batteries can store four times as much power as lead-acid batteries. In terms of energy density, it's still a win for lithium-ion batteries, holding 150 watt-hours per kilogram compared to 40 watt-hours for lead-acid batteries. They also have a longer lifespan, with a cycle life that's three times as much. But, when it comes to usable energy, lithium-ion batteries are recommended for about 85% usage while lead-acid batteries for 50%.

When discussing the battery pack in relation to an off-grid solar panel system, we're essentially referring to an off-grid system which depends on battery storage to supply electricity during periods devoid of sunlight. As such, it's imperative that our calculations are accurate avoiding potential power shortages. The initial step is accurately determining your daily power consumption requirements. For this calculation, it's necessary to be as detailed as possible. While you can utilize existing historical data, it is also crucial to consider the duration for which you utilize each individual load on a daily basis.

Let's take a unique situation as an example, say you've decided to move to an isolated location and live in a quaint wooden cabin, orthogonally distant from the bustling hubs of civilization. Under such conditions, certain utilities like a freezer become a necessity. If I were to employ a conventional 30-watt freezer for this purpose, then it would lead to a daily power consumption of 720 watts. The next piece of the puzzle is the second load - a mini-fridge. Drawing upon real-life scenarios, I've calculated the kilowatt-hours it tends to consume within a year, converted it into an hourly electricity consumption rate and finally multiplied it by the 24 hours in a day. The result obtained is a daily consumption of 856 watt-hours. Furthermore, we'd likely want to use various forms of entertainment and electronic devices. Here, we have a typical laptop, consuming approximately 80 watts when operational, and given that we use it for say three hours a day, the energy consumed amounts to 240 watt-hours.

These figures might seem diminutive, but when you consider necessities like a mobile phone, which needs to be charged every night, that's another 12 watt-hours per day. Assuming that you also desire to watch television in the evenings, you'd need an LCD or an LED TV, or any other variant. Typically, smaller televisions hover around a range of 100-120 watts. Suppose we consider a 120-watt television that's in operation for two hours a day, we are looking at a consumption of 240-watt hours. Therefore, upon summing up all these individual power utilization figures, we’d have a total daily consumption of 2068 watt-hours.

My personal advice for determining the number of reserve days would be to aim for a minimum of two days worth of backup power, although there might be circumstances warranting a larger buffer. For instance, if you reside in places that endure extensive cloudy cover such as Seattle, you might want to consider having a surplus of two to three days. Now that we have ascertained the amount of power we need to reserve, we multiply this figure by the daily power consumption we've previously computed. So, three days multiplied by 2068 watt hours gives us 6204 watt hours.

We will now apply Ohm's law to convert the watt hours we just calculated into amp hours. To achieve this, we divide 6204 watt hours by our battery voltage; in this case, we will use 48 volts which yields us a total of 129.25 ampere hours- the cumulative capacity of our battery pack. But here's the caveat, I may have oversimplified the explanation thus far. Indeed, the number we computed is the power we need daily, and while the calculations are certainly spot-on, owing to the internal working mechanisms of batteries, particularly lead-acid batteries, if you select this route, they cannot be drained over 50% without causing impact on the lifespan of the battery.

As such, we need to factor in this 50% discharge level, an aspect that heavily leans into the longevity of your battery storage system. Should you decide to routinely deplete your batteries to 50%, their longevity is unlikely to be impressive. However, with appropriate battery pack dimensions, relegating your batteries to a 25% discharge should subsequently enhance their lifespan.