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Since the start of the coronavirus pandemic, we have received a phenomenal amount of calls from people asking us if being energy self-sufficient with a solar powered battery system is realistic. Although this desire for energy autonomy is completely understandable, it should not be forgotten that it is not accessible to everyone. Indeed, the sacrifices to be made are not always easy and require a lot of energy management, without forgetting the fact that they come with a fairly high financial cost.

This article answers the following questions:

  • What is the average capacity of a battery?
  • How many batteries do I need?
  • How much does a battery cost?
  • What is the average lifespan of a battery?
  • What are the sacrifices to be made?
  • Battery vs generator: what to choose?
  • Do I really need a backup?
  • Can I start by installing a standard solar energy system and add batteries afterwards?

What is the average capacity of a battery?

The capacity of a battery is the amount of energy it can store. It is expressed in ampere-hours (Ah) and depends on the intrinsic characteristics of the materials it contains. It ranges from 5 to 1500 Ah per battery, although the most common models have a capacity of 50 to 150 Ah.

The higher the battery capacity, the greater the energy autonomy it provides. The acquisition of larger capacity batteries is therefore suggested, although their purchase cost is higher than that of lower capacity batteries.

How many batteries do I need?

According to Hydro-Québec, a 2,000 square foot single-family home that does not have a pool or spa consumes an average of 25,000 kWh of electricity per year (including heating). This consumption is theoretically equivalent to 70 kWh/day. However, given that our average winter electricity consumption is two to three times greater than our average summer electricity consumption, dividing this 25,000 kWh by the number of days in the year would not be representative of average daily electricity consumption. Suppose therefore that in winter your electrical demand is 120 kWh / day and that it oscillates around 45 kWh/day for the rest of the year. Excluding heating, which generally accounts for 60% of our electricity consumption, your energy demand is 48 kWh/day in winter and 18 kWh/day for the rest of the year.

To have 12 hours of autonomy in the worst possible scenario, you would have to have 24 kWh of stored energy. This 24 kWh is equivalent to 1000 Ah assuming that your battery bank is at 24V (24,000 Wh ÷ 24V = 1000 Ah). Knowing that the batteries of 100 Ah capacity and 12V voltage are the most popular, reaching these 1000 Ah of autonomy at 24V voltage requires the connection of two batteries in series (24V ÷ 12V/battery = 2 batteries) and ten batteries in parallel (1000 Ah ÷ 100 Ah/battery = 10 batteries). The total number of batteries required is therefore 20 (number of batteries in series * number of batteries in parallel = total number of batteries).

With 20 batteries of 100 Ah/12V, you are theoretically able to have 12 hours of autonomy depending on the average level of Quebec consumption. Obviously, this scenario is theoretical and may vary if your consumption level is higher or lower than 25,000 kWh/year.

How much does a battery cost?

People are often shocked to learn the true cost of a battery, and for many, the conversation begins and ends there. Indeed, seeing the price of a single battery (knowing that you can need up to 20 depending on the theoretical scenario above), few are those who continue their efforts.

In this regard, the table below shows the average price of a single battery according to its capacity. It should be noted that the price varies enormously depending on the technology chosen (lithium-ion, lead-acid, AGM or Gel) and that it is subject to variation depending on the supplier. Prices are before taxes and are in Canadian dollars. When a box is empty, it means that battery technology is not available for the given capacity.

The table above shows that lead-acid batteries have very low capacities compared to the other three battery technologies. Although much more affordable, lead-acid batteries are generally used for basic energy utilities rather than for a backup system since too many batteries would be required to achieve a satisfactory autonomy.

Regarding the prices shown above, the table below shows the cost of the 20 batteries required according to the 12-hour autonomy scenario previously presented. For lead-acid batteries, the 10Ah/12V model was used while for the other three technologies, the 100Ah/12V model was used. The choice of these models is justified by their popularity and their availability in several retail businesses.

The table shows that in the best possible scenario, an energy autonomy of 12 hours would cost you $ 7,500 (before taxes) while in the worst case it would cost $ 30,000. It should be remembered that this cost includes only the batteries and excludes the cost of the solar panels, the solar panel anchor system, the inverters, the wiring and the labor.

What is the average lifespan of a battery?

Not only are batteries expensive, but they do not last long. Their lifespan varies between 2 and 5 years depending on the storage conditions, the intensity of the calls for power as well as the depths of average discharges.

  • Storage conditions refers to the location (interior or exterior, interior is strongly suggested) as well as at room temperature (20 ° C is ideal; the greater the difference, as much negative as positive, the less the battery life).
  • The power demand corresponds to the intensity of the discharge current generated using one or more devices. The higher the intensity, the shorter the battery life in the long run.
  • The depth of average discharges corresponds to the amount of energy drawn from the battery before recharging. The greater the depth of the discharges, the shorter the lifespan of the battery in the long term.

That said, it is important to mention that the batteries do not stop working after 5 years, but rather have a lower energy density. Its reduction in capacity depends on all the factors explained above and is difficult to predict given its non-linearity.

Thus, if a battery has a capacity of 100 Ah when purchased, it can have 70 Ah after 5 years, or even 50 Ah. It still works but stores less energy. This means that the older your batteries are, the less energy they will give you.

What are the sacrifices to be made?

Living with batteries requires making sacrifices if we do not want to discharge them too quickly and shorten their lifespan hastily (which is already short). The sacrifices are in relation to the number of devices that can be used at the same time as well as the number of devices that can be used in a day.

The quantity of devices that can be used at the same time refers to the concept of power (or power demand), which corresponds to the intensity of the battery discharge current induced using a given electrical appliance. The problem is that the capacity of a battery exponentially decreases the more the intensity of the discharge current increases, the latter increasing the higher the power of an electrical appliance. Thus, the capacity of a battery is closely linked to the power of the devices it powers.

This means that if you do not want to drain the energy in your batteries too quickly, you have to be careful with the power of the devices that you decide to operate at the same time. The power of a device is measured in watts and is often indicated on the device’s data sheet. A 1000 watt device is considered to have high power. In case of doubt, this website gives you a good idea of ​​the average power of the devices of our daily life (https://www.wholesalesolar.com/solar-information/power-table).

The figure below shows the discharge speed of a lead-acid battery (x-axis) in the face of an increase in the intensity of its discharge current (y-axis). The light blue color curve is for a low discharge intensity while the dark blue color curve is for a high discharge intensity. The light blue curve battery discharges in just under 9 hours while the dark blue curve battery discharges in just 1 hour. Unfortunately, this phenomenon is applicable for any type of battery and is inevitable.

The number of devices that can be used in a single day refers to the concept of energy. The more devices you use simultaneously, the more energy it will take to operate them. In the same logic as before, if you do not want to drain the energy contained in your batteries too quickly, you have to be careful about the number of devices you will use in the same day.

Battery vs generator: what to choose?

Batteries are not the only option for backup power. Indeed, you could get a gas generator that would cost less, provide more energy, and have a longer lifespan.

However, batteries have the advantage of being more durable and environmentally friendly, with the additional possibility of recharging from solar, which generators cannot do.

In addition, the start-up time of a generator is a few minutes (compared to a few milliseconds for a battery), which means that during a breakdown, the current does not come back instantly when you have a generator. Thus, if certain devices must remain permanently on, the generator start time causes problems.

Do I really need a backup?

For most people, power outages are nothing more than a minor inconvenience given their short duration. If you experience one or two power failures per year, it is quite easy to wait for the power to return. In these situations, a backup system is not worth the investment, even in the case of a gas generator.

There are however certain scenarios where the backup power can be invaluable:

  • When you live in an area where there are frequent natural disasters.
  • When you live in an area with frequent power outages due to an unreliable power grid.
  • When you absolutely must operate essential electronic equipment or devices that must always remain on.

It is up to you to decide if you really need a backup. Keep in mind most people end up opting for no backup at all.

Can I start by installing a standard solar energy system and add batteries afterwards?

Many people start by installing a battery-less solar power system, possibly adding batteries when their budget allows. It is doable, but it is important to plan your system with a view to future expansion. Indeed, some inverters are configured to manage solar panels while others only operate with batteries.

There are three types of inverters: grid-tied inverters, storage-ready inverters and hybrid inverters. Grid-tied inverters have the function of transforming the direct current produced by the solar panels into alternating current while the storage-ready inverter have the function of drawing the direct current from the batteries and transforming it into alternating current. A grid-tied inverter cannot do the job of a storage-ready inverter and a storage ready inverter cannot do the work of a grid-tied inverter. The two types of inverters therefore have separate functions and do not interchange.

Hybrid inverters are intelligent inverters capable of performing the work of grid-tied inverters and storage-ready inverters. They are obviously more expensive than the other two types of inverters, but the fact that they fulfill two functions partially offsets the additional cost. That said, rather than systematically storing energy in batteries (with efficiency losses of at least 20% according to current technology), hybrid inverters only store energy in batteries when necessary. In case there is more production than consumption, the hybrid inverter recharges the batteries while when there is more consumption than production, the electricity produced by the solar panels is sent directly to your electrical devices. Hybrid inverters therefore limit conversion losses, which indirectly gives you more energy.

That said, if you want to start by installing a solar energy system without a battery, to possibly add batteries, it is strongly suggested to opt for the installation of hybrid inverters since they decrease the amount of inverters to buy.

If you already have solar power and want to add backup power, you will probably need to add a battery inverter. You may also need to modify your electrical service panel to add a dedicated backup panel, which requires a fair amount of electrical work. This is why we suggest opting directly for a hybrid inverter.

Conclusion

To conclude, in the current economic and energy context, energy autonomy using batteries is still very inaccessible for most of the population. Being very expensive and having a fairly short lifespan, the acquisition of batteries is not recommended for those who experience infrequent and short-lived power outages. However, in the case of off-grid installations (such as a chalet), the acquisition of batteries is still interesting due to the comfort they offer you. However, keep in mind that you will not be able to have the same lifestyle as when you are connected to the Hydro-Québec network.

Please do not hesitate to contact us for more information, we will be happy to assist you. Our affordable, reliable and turnkey solar energy systems will fully satisfy you. Give us the chance to impress you and share our solar expertise with you.