How to choose the size of the products to best fit your demands?

In this article, we will cover the basic knowledge you will need for designing a stand-alone (off-grid) system. We will go through several steps to identify the requirements and size of the components.

Standalone systems are typically employed where there is no access to the electrical grid. A few examples are off-grid houses, RVs, boats and etc.

Here is the basic schematic for a common solar standalone system. As you can see in this diagram it has consisted of a Solar Inverter, Solar Charge Controller, Batteries (storage), Solar Panels, Optional Generator, connections, and AC loads.

The power inverter converts the DC electricity into AC electricity. Charge controller manages the charging and discharging of the battery bank. Charge controllers usually come with two types: PWM and MPPT. MPPT charge controller tracks the maximum power point of solar panels and keeps the panels producing their higher potential. A battery bank is responsible to store the electrical energy, and to provide the required power, for a planned duration. A backup generator can be included to ensure that the power will be available during extended periods of time, or for supporting an occasionally large load.

Here are the designing steps:

Step 1: Daily power usage

Figuring out the amount of the power you need, is the first step for designing a solar off-grid system. We need to take a list of every electrical device that we are powering and how long we need to power them. Actually, for off-grid systems, we don’t have anything called “average usage” or “high usage”, and based on every single consumer, the size of the system will differ.

Typically, every electrical equipment will have a power label on it. We have AC consumers and DC consumers, and based on the type of the consumer, we may see different values on the label.

Regardless of the type of the consumer, “voltage” measured by volts, “current” measured by amperes and “power” measured by watts, are the three essential elements, we need to know, prior to reading the labels. Multiplying volts by amps get watts. Here is the simple equation for calculating the power of an AC consumer:

If the voltage and current mentioned on the label, are DC values, the power is still calculated by multiplying volts to amps, however, this time, the consumed power is DC power.

Sometimes the voltage mentioned on the label is not a determined number, but is a range, for example, 110V to 250V, here we can use the grid voltage in a local area (or your Inverters output voltage) for calculating the power of the consumer.

Sometimes, the power is directly mentioned on the label, and this way, no calculations are needed to get the power of the consumer.

Watt-hours per day are the amount of energy consumed by a piece of electrical equipment during a day. In order to calculate watt-hours, we need to multiply the power to hours:

Sometimes, the consumed power is an average number for a year and is expressed by watt-hours. Here, we can divide the mentioned number by 365 (days of the year) to get the average consumed energy per day.

However, another method to calculate the power consumed by a piece of equipment is to use an energy meter, over a period of time, and get the precise value of the consumed energy.

After all, if we don’t have access to our consumers, for example when we are just pre-designing a system, or when we are not sure about the wattage of our appliances, this power consuming reference table can help us, find out the commonly associated power to different appliances.

Power consuming reference table

Kitchen Appliances

Lighting

Heating & Cooling

Water

Laundry

Office

Living Room / Electronics

Tools

Cleaning & Personal Care

RV appliance

Note that the power values mentioned at this table, are approximate values and to determine the exact values, the power label or power meter must be used.

Once the power consumption of each electrical appliances is determined, we can make a list of them. The load calculator helps you to do it in a simple way. Just fill out the form, and you’ll have access to your daily watt-hours.

Most of the AC appliances will require a higher power while getting started, than their nominal power. That’s called the peak (surge) power. The peak power is used to determine the inverter size, and to make sure that the inverter will support the appliances, in the case of simultaneous start-up. Another principle that plays a role in sizing an inverter, is the number of devices that are on at once. So if you have several pieces of equipment running together, make sure to check the “Running at the same time” box to run a proper size calculation for the inverter, unless your devices won’t be able to be turned on, or it will damage the inverter.

Finally, we need to take this into consideration that the DC loads, are not running through the inverter, but directly gets their needed power from the batteries. So DC loads, will not be included in sizing the inverter, and we will use this data when sizing the battery bank.

Step 2: Sizing the battery bank

Now that we know how much power we consume each day, it’s now the time to determine the battery bank size. Here are the terms which determine the battery bank size:

• Daily power consumption
• System voltage
• Days of autonomy
• Depth of discharge
• Temperature
• Inverter efficiency

The Battery calculator helps you to calculate the required battery bank size in a simple way. Just fill out the form, and you’ll have access to your daily watt-hours.

Batteries are one of the most important parts of every standalone solar system. Flooded Lead Acid Batteries, Sealed Lead Acid Batteries, Lithium Batteries are the commonly used batteries in the market. Systems application, budget, and expectations for back-up time are those factors that play a role in picking the right battery type. The type of batteries used in solar PV systems is a deep cycle or solar batteries which are able to produce consistent power output while being discharged to a greater extent than a typical battery. Lithium batteries, also have a longer lifespan and get charged faster. Deeper discharge rate means lower capacities in battery sizing.

A battery is recognized with its voltage (V) and capacity measured by amp-hours (AH). To provide desired system voltage, one can wire the batteries in series and parallel. Wiring strings in series, increased the voltage, while wiring in parallel, adds up the total battery bank capacity. However, it’s not recommended to have more than two parallel strings on a system, since a little voltage mismatch in strings, will lead to a shorter lifespan of the batteries. A battery banks voltage should be coordinated with the solar array voltage, to ensure proper charging. However, an MPPT charge controller will step the input voltage of the solar arrays, down and up to adjust it with the battery voltage. Inverter power is one of the factors that play a role in determining the size of the system. Since the inverters usually support a single input voltage (12V, 24V, 36V, and 48V) then the type and size of the inverter, is an intersecting parameter while choosing the system voltage. The higher the inverter size is, usually it comes with higher DC input values. Additionally, increasing the voltage of the system, will reduce the power loss in wiring elements, and therefore will lead to the higher performance of the total system.

The capacity of a battery bank rated by amp-hours indicates how much energy it can store and therefore how much back up time, the stand-alone system will provide for loads. Once we know how much power we need a day, it also needed to determine how many days, we want to run our equipment off the battery bank, when there is no sun to recharge the batteries. This is called days of autonomy. The bigger the days of autonomy is, the longer the backup time and the higher the number of batteries will come with the system. This is a matter of cost, therefore using an external generator is a great way, to reduce the number of days of autonomy. This means that for example, if we are planning for more than 3 days, then we can use a generator to charge the batteries in the last day and reduce the battery bank costs.

The depth of discharge (DOD) is also used for designing the battery bank. Besides the voltage and capacity, the other primary design decision is how much to let the batteries discharge.

As you can see in this figure, the more a battery is allowed to discharge, the shorter its lifespan. While allowing batteries to discharge more seems to be cost-effective, but this way the batteries won’t last long. Deep cycle batteries are designed to discharge 80% of their capacity, but it recommended to choose a value of around 50% as a good trade-off between longevity, cost, and the hassle of replacing batteries. If the days of autonomy is more than one day, the actual DOD on a sunny day will be less than 20% resulting in a higher lifespan.

Temperature is another term that should be taken into consideration while calculating battery bank size.

Battery capacity is usually rated in standard temperature 25^o C (77^o F). As shown in the figure, when the temperate varies, the capacity also varies. In higher temperatures, batteries have higher capacities and in lower temperatures, it’s vice versa. The colder the battery is, the larger the capacity needs to be. So it’s better to consider the coldest days of the year while calculating the system. Temperature coefficient can be extracted from this figure, which will be used in battery bank calculations.

Inverter efficiency is another term which effects the battery size. There are no ideal inverters and there is a power loss, converting DC to AC. So this loss should be taken into consideration while calculating the power bank size.

The equations are as below:

Here is a simple example of a battery bank calculation. We have a daily consumption of 2550Watts AC load and 120W DC load. We need 2 days of autonomy. We will use the battery in 50% depth of discharge. The coldest temperature that batteries will experience is this example, is -10^o C. System voltage is 48V and the inverter efficiency is 95%. So we have:

Depending on the battery voltage and capacity, the total number of batteries will be determined. The number of strings determines the minimum amp-hours capacity of each battery, and system voltage determines the number of the batteries needs to be in series.

So in this example, if we consider two parallel strings, we have:

So for example, a 12V 170Ah battery looks like suitable for this purpose.

The final step is to determine the number of batteries in each string. This is calculated by dividing the system voltage, to battery voltage:

So in our example:

Therefore, in each parallel string, there should be 4 pieces of 12V batteries.

The total number of required batteries is:

And for this example we have:

Battery calculator