Solar Battery Capacity, Size and Installation Explained
Two solar batteries, are needed to power the average house in the United States. Each solar battery will be a daily-cycle battery of 7KWhcapacity. This article discusses solar battery capacity, and how many solar batteries are needed to power a house.
The outline is as follows;
-How Many Solar Batteries are Needed to Power a House: Factors to Consider
How Many Solar Batteries are Needed to Power a House: Factors to Consider
There are some factors to consider when estimating how many solar batteries are needed to power a house;
1. Energy Demand/Load
2. Solar Battery Capacity
3. Size of Solar Panel Array
4. Purpose of Installation
5. Budget
In this section, each of these factors is extensively discussed, as follows;
1). Energy Demand of the Building or Facility
As at 2019, the average electricity consumption of a residential building in the United States was about 11,000KWh per annum [3]. By 2020; this value was about.10,715KWh [4].
Between the years 2011 and 2021, the average electricity consumption for residential buildings in the United States is about 10,080KWh per annum
This value is equivalent to 840KWh per month, and about 28KWh per day.
The derivation of the above values is as follows;
10,080KWh per annum/12 months per annum= 840KWh per month,
840KWh per month/average 30 days per month= average 28KWh per day
We can assume that under ideal conditions, solar energy will be available for half of each day; or 12 hours. For grid-connected buildings, the utility grid may also provide electricity.
Within this period, the solar batteries will not be used, but will rather be storing excess power from the solar panels.
Given a daily power consumption rate of 28KWh, the amount of power which will be consumed for the first half of the day (while the solar batteries are charging) will ideally be about 14KWh.
This value may be less for some residential buildings, especially because the peak hours of electricity consumption mostly occur during the day.
However, under ideal conditions of battery efficiency and power consumption, the backup needed from the solar batteries will amount to 14KWh per day. This will require the installation of two batteries; each solar battery having a capacity of 7KWh.
2). Solar Battery Capacity
The capacity of a solar battery is the amount of power which it can store.
Solar batteries may range in capacity from as low as 3KWh to as high as 20KWh (although most batteries fall within the 5-15KWh range).
The capacity of each solar battery in an array will determine how many of these batteries are needed to power a building or facility.
For example, considering the 28KWh average daily consumption for residential buildings, five (5) solar batteries each having 3KWh capacity will be needed to back-up a building in times of power outage.
The estimate is derived as follows;
Electricity demand for half day (when solar energy us not readily available) = 28KWh/12 hours= 14KW
If 14KWh is required per day, five (5) solar batteries with 3KWh capacity each will produce= 5×3KWh= 15KWh.
This will be sufficient to back-up the building during the 12 power-outage hours.
For solar batteries which have capacity of 10KWh each, two batteries will need to be installed. These batteries will be equivalent to 20KWh when combined.
It is important to note that the above estimations are for an ideal scenario. This means that significant variations will occur in real life.
For example, other estimates place the average daily power consumption of American households at 30KWh per day [1].
It is also possible for the performance of a solar battery to differ from that of another solar battery having equal storage capacity, due to factors such as battery efficiency and depth of discharge.
3). Size of Solar Panel Array
The size of the solar panel(s) installed in a building, directly affects the number of solar batteries that are needed.
It is important to note that ‘size’ of the solar panels is equivalent to the power-production capacity of these panels.
With regards to the link between solar panel size and solar battery number; a simple rule applies. This is the 1:1 rule, which suggests that the ratio of solar panel production-capacity to solar battery storage-capacity, should be 1:1 [2]. This rule is aimed at preventing overload of the inverter and charger of the solar system.
In essence, the capacity of the solar battery array should ideally be equal to the capacity of the solar panel array.
For solar panels, the average production capacity is about 265W per hour. A solar panel may generate between 220 and 400 Watts of electricity in one hour, depending on its working conditions, brand, composition, type, performance and efficiency.
If a solar panel produces 265W of power per hour, and solar energy is available for 12 hours in a day, then ideally, the panel will be expected to generate 265 Watts×12 hours= 3,180Wh or 3.2KWh (per day).
Given an average electricity consumption of 28KWh per day, at least 9 solar panels will be needed to make up the solar panel array which will supply power to the building. This is estimated as follows;
3.2KWh per day × 9 solar panels= 28.8KWh per day
For the 9-solar panel array described above, the number of solar batteries which will be installed should ideally amount to a combined capacity that is be equal to (or approximately equal to) the capacity of the solar panel array, which is 28.8KWh per day.
In spite of the 1:1 rule, in practical scenarios, the storage capacity of the solar batteries can be equal to half of the production capacity of the solar panel array.
This is simply because it is expected that part of the power produced by the solar panels will be consumed directly, while the remainder (excess power) will be stored by the batteries.
For the 1:1 scenario, where the production capacity of the solar panels is 28.8KWh, four solar batteries each having 7KWh can be used. Three 10KWh solar batteries; or ten 3KWh solar batteries may also serve the sane purpose.
Where the used electricity which is not stored by the batteries; is considered, the combined battery capacity which will be needed for a 28.8KWh/day solar panel will be equal to 14.4KWh (approximately 14KWh) or twelve-hours’ worth of power. This can be provided by two 7KWh solar batteries.
4). Purpose of Solar Battery Installation
Various purposes exist for the installation of solar batteries.
A solar battery (and in fact the entire solar system) may be installed in order to provide self-sufficiency, to save money, or as a support system.
Each of these purposes affect the number of solar batteries needed to power a house, because they determine the amount of power that is needed (and the required battery capacity).
When the purpose of installation is self-sufficiency, it means that there would be no dependence on, and no power supply from; the utility grid.
This type of installation may also be described as ‘off-grid’, with the primary aim being self-sufficiency.
However, in order for a building which uses a solar system (including solar batteries) to be fully self-sufficient, a relatively high-power production and storage capacity is usually required.
This means that the solar system which is installed, in such a scenario, must be able to meet the overall power needs of the building.
To achieve this, there are some quantities which must be estimated. One of these is the overall power consumption rate of the building (per day, per month, per year).
Based on this rate of consumption, the size and production capacity of the solar panels may be determined, as well as the combined capacity of the solar batteries.
Depending on the individual battery capacities, the number of batteries to be installed can be estimated.
When the purpose of installation is to save money, the combined capacity and number of solar batteries will depend on how much money needs to be saved, according to the analysis of energy cost.
In order to achieve maximum cost-savings using solar batteries, it is usually recommendable to use self-sufficient, off-grid installation, which will ensure that all electricity is supplied by the solar system.
Such an arrangement can help reduce the cost of electricity, especially when the solar system is capable of meeting the overall power-need of the building or facility.
We can assess the potential of solar batteries and solar panels to save electricity cost, by considering that the average cost of electricity from the utility grid in the United States is about $10/KWh, while the cost of solar power can be as low as $0.06-$0.08/KWh.
When the purpose of installation is to provide a support system, the primary relevance of the solar panels and solar batteries will be to supply power to the most important circuits and electrical components of the building or facility.
Another term which is used to describe this scheme of solar system installation is resiliency. It is aimed at ensuring that the building does not lack power supply where and when it is most needed.
To determine the number of solar batteries to install in order to achieve resiliency or support, it is usually necessary to estimate a quantity known as the Critical Load (CL). This is simply the total energy demand of the most essential electric components of the building.
In a resiliency-aimed solar connection, the building is backed-up such that the essential power needs are always met. It is a cheaper approach than a complete solar backup, since the critical load is usually less than the overall energy demand of the building.
5). Installation Budget for the Solar Batteries
Ideally, when budgeting for a solar battery installation project, a cost range of $8,000 to $20,000 should be considered.
There are various factors based on which this value may differ. These include the mode of installation (grid-connected; off-grid; solar panel/inverter/battery; solar battery only), the capacity of the solar battery array, the size of the solar panel(s), and the overall complexity of the solar system configuration.
In order for the installation to be economically feasible, it must be ensured that the budget for the project is not exceeded.
Conclusion
Various factors may be considered when deciding on the number of solar batteries that will be needed to power a house.
One of these factors is the energy demand of the building.
It is important to estimate the total energy demand of a building before installing a solar system. In the United States, it is estimated that the average electricity consumption rate, for a residential building is about 28-30KWh per day.
Another factor that determines the number of solar batteries to install, is the storage capacity of each solar battery.
For a 28KWh energy demand, four solar batteries will be installed if they each have a capacity of 7KWh. Three solar batteries each having 10KWh capacity may serve the same purpose, and so on.
Because the size of the solar panel array determines how much solar power will be produced, this factor also affects the number of solar batteries which will be needed.
A widely-accepted rule in this area is the 1:1 rule, which suggests that the number of solar batteries should be such that the combined storage capacity of these batteries will be equal to the production capacity of the solar panels.
There are various purposes for which a solar battery may be installed. These include resiliency, self-sufficiency and cost-savings. Each of these purposes directly affect the storage capacity that will be required, and therefore the number of solar batteries.
Lastly, the budget for the installation project determines how much, in terms of fiscal resources, will be available to purchase and install the solar batteries. This is another major factor that will affect the number of batteries which will be installed.
References
1). Bruce, J. (2021). “How Many Solar Panels Do I Need For 1000 kWh Per Month?”Available at: https://diysolarshack.com/how-many-solar-panels-do-i-need-for-1000-kwh-per-month/. (Accessed 10 March 2022).
2). Chandra, N. (2021). “How to Choose Between a 12V and 24V Solar Panel?”Available at: https://www.loomsolar.com/blogs/collections/difference-between-12v-24v-solar-panels. (Accessed 10 March 2022)
3). EIA (2019). “Use of energy explained: Energy use in homes.” Available at: https://www.eia.gov/energyexplained/use-of-energy/electricity-use-in-homes.php. (Accessed 9 March 2022).
4). EIA (2021). “Frequently Asked Questions (FAQs): How much electricity does an American home use?”Available at: https://www.eia.gov/tools/faqs/faq.php?id=97&t=3. (Accessed 9 March 2022).
