How to size an off-grid solar power system for home

Investing in a solar system is a smart solution for homeowners in the long run, particularly under current environments that energy crisis happen at lots of places. The solar panel can work more than 30 years, and also lithium batteries are getting longer life span as the technology develops.

Below are basic steps you need to go through to size an ideal solar system for your home.


Step 1: Determine the total energy consumption of your house

You need to know the total power used by your home appliances. This is measured by unit of kilowatt/hour daily or monthly. Let us say, the total equipment in your house consumes 1000 watt of power and operate 10 hours a day:

1000w * 10h = 10kwh per day.

The rated power of each home appliance could be found on the manual or their websites. To be accurate, you could ask technical personnel to measure them with professional right tools such as a meter.

There would be some power loss from your inverter, or the system is on a stand-by mode. Add extra 5% – 10% power consumption according to your budget. This would be put into consideration when you size your batteries. It is critical to buy a quality inverter. (Find out more about our strictly tested inverters)



Step 2: Site Evaluation

Now you need to have a general idea about how much sun energy you can get daily on average, so you will know how many solar panels you will need to install to meet your daily energy need.

The information of sun energy can be collected from a Sun Hour Map of your country. The mapping solar radiation resources can be found at,-4.042969,2

Now, Let us take DamascusSyria as an example.

Let us use 4 average sun hours for our example as we read from the map.

Solar panels are designed to be installed in full sun. Shade is going to impact performance. Even partial shade on one panel will have a large impact. Inspect the site to make sure your solar array will be exposed to full sun during daily peak sun hours. Keep in mind that the sun’s angle will change throughout the year.

There are a few other considerations that you need to remember. We can talk about them throughout the process.



Step 3: Calculate Battery Bank Size

By now we have basic information to size the battery array. After the battery bank is sized, we can determine how many solar panels are needed to keep it charged.

First, we check the efficiency of the solar inverters. Usually the inverters come with built-in MPPT charge controller with more than 98% efficiency. (Check our solar inverters).

But it is still reasonable to consider 5% ineffciency compensation when we do the sizing.

In our example of 10KWh/day based on lithium batteries,

10 KWh x 1.05 efficiency compensation = 10.5 KWh

This is the amount of energy drawn from the battery to run the load through the inverter.

As the ideal working temperature of lithium battery is bwtween 0 to 0~40, although its working temperature is in the range of -20~60.

Batteries lose capacity as temps go down and we can use the following chart to increase battery capacity, based on the expected battery temperature:

For our example, we’ll add a 1.59 multiplier to our battery bank size to compensate for a battery temperature of 20°F in the winter:

10.5KWhx 1.59 = 16.7KWh

Another consideration is that when charging and discharging batteries, there is energy loss, and to extend the life span of batteries, it is not encouraged to fully discharge the batteries. (Usually we maintain the DOD higher than 80% ( DOD = depth of discharge ).

So we get the minimum energy storage capacity: 16.7KWh * 1.2 = 20KWh

This is for a single day of autonomy, so we need to then multiply it by the number of days of required autonomy. For 2 days of autonomy, it would be:

20Kwh x 2 days = 40KWh of energy storage

To convert watt-hours to amp hours, divide by the system’s battery voltage. In our example:

40Kwh ÷ 24v = 1667Ah 24V battery bank

40Kwh ÷ 48v = 833 Ah 48V battery bank


When sizing a battery bank, always consider the discharge depth, or how much capacity is discharged from the battery. Sizing a lead acid battery for a maximum 50% depth of discharge will extend the battery’s life. Lithium batteries are not as affected by deep discharges, and can typically handle deeper discharges without substantially affecting battery life.

Total required minimum battery capacity: 2.52 kilowatt hours

Note that this is the minimum amount of battery capacity needed, and increasing the battery size can make the system more reliable, especially in areas prone to extended overcast weather.



Step 4: Figure Out How Many Solar Panels You Need

Now that we’ve determined battery capacity, we can size the charging system. Normally we use solar panels, but a combination of wind and solar might make sense for areas with good wind resource, or for systems requiring more autonomy. The charging system needs to produce enough to fully replace the energy drawn out of the battery while accounting for all efficiency losses.

In our example, based on 4 sun hours and 40 Wh per day energy requirement:

40KWh / 4 hours = 10 Kilo Watts Solar Panel Array Size

However, we need to other losses in our real world caused by inefficiencies, such as voltage drop, which are generally estimated to be around 10%:

10Kw÷0.9 = 11.1 KW minimum size for the PV array

Note that this is the minimum size for the PV array. A larger array will make the system more reliable, especially if no other backup source of energy, such as a generator, is available.

These calculations also assume that the solar array will receive unobstructed direct sunlight from 8 AM to 4 PM during all seasons. If all or part of the solar array is shaded during the day, an adjustment to the PV array size needs to be made.

One other consideration needs to be addressed: lead-acid batteries need to be fully charged on a regular basis. They require a minimum of around 10 amps of charge current per 100 amp hours of battery capacity for optimal battery life. If lead-acid batteries aren’t recharged regularly, they will likely fail, usually within the first year of operation.

The maximum charge current for lead acid batteries is typically around 20 amps per 100 Ah (C/5 charge rate, or battery capacity in amp hours divided by 5) and somewhere between this range is ideal (10-20 amps of charge current per 100ah).

Refer to the battery specs and user manual to confirm the minimum and maximum charging guidelines. Failure to meet these guidelines will typically void your battery warranty and risk premature battery failure.

With all of these information, you will get a list of following configuration.

Solar panel: Watt11.1KW20 pcs of 550w solar panels

25 pcs of 450w solar panels


1700AH @ 24V

900AH @ 48V


As for the inverter, it is selected based on the total power of the loads that you would need to run. In this case, 1000w home appliance, a 1.5kw solar inverter would be enough, but in real life, people need to operate more loads at the same time for different period of times daily, it is recommended to buy 3.5kw or 5.5kw solar inverters.


This information is intended to serve as a general guide and there are a lot of factors that can influence system size.


If the equipment is critical and in a remote location, it is worth to invest in a oversized system because the cost of maintenance can quickly exceed the price of a few extra solar panels or batteries. On the other hand, for certain applications, you may be able to start small and expand later depending on how it performs. System size will ultimately be determined by your energy consumption, the site location and also the expectations for performance based on days of autonomy.


If you need help with this process, feel free to contact us and we can design a system for your needs based on the location and energy requirements.



Post time: Jan-10-2022