Solar and wind energy are excellent options for powering equipment in remote areas. We’ll show you how to size an off-grid solar system so you can generate electricity even if you’re miles away from the nearest power line in this article.
Off-grid houses, as well as a few industrial applications requiring equipment to be powered at remote work sites, can benefit from these types of systems. Lighting, telecommunications equipment, sensors, environmental monitors, security cameras, traffic signals, water pumps, cathodic protection systems, and anything else that requires consistent power in a remote location are some of the most common applications.
The following principles can help you size an off-grid solar system depending on your location, energy usage, and desired autonomy days (how long the battery bank can supply power before it needs to be recharged).
Step 1: Figure out how much energy you’ll need.
To begin, you must first determine how much energy the equipment consumes regularly. The number of watt-hours or kilowatt-hours used every day is measured in watt-hours or kilowatt-hours. Assume the equipment consumes 10 watts of power and is operational 24 hours a day:
10 Watts x 24 hours = 240-watt hours per day (0.24 kWh)
What method do you use to gather this data? Check your equipment’s data sheet or manual to see how much power it uses (in Watts), then multiply it by the number of running hours each day. If feasible, test the power consumption with a meter to get an exact real-world reading.
Remember to account for the inverter’s self-consumption and efficiency losses when utilizing an inverter to provide AC power for a load. While in operation, inverters need a little amount of power. Add the self-consumption to your daily total using the inverter specifications sheet. Depending on the inverter, self-consumption can range from under a watt to 30 watts.
Depending on the inverter and how much it is loaded, efficiency losses might range from 5% to 15%. When sizing the batteries, this will be considered. It is critical to get a high-quality, high-efficiency inverter.
Step 2: Assess the Location of the Site
Next, establish where the system will be installed to calculate the amount of solar energy available.
Estimate available PV resources using a solar insolation map (also known as a ‘sun hours map’). The system should be sized for the month with the highest power usage and/or the least amount of solar resource, which is usually December or January.
The National Renewable Energy Laboratory (NREL) includes an online tool for calculating solar radiation availability.
In January, solar insolation across most of Nigeria is relatively low. A decent estimate is 5.5 sun hours; however, it could be lower or higher depending on your location. For this purpose, we’ll assume 5.5 minimum sun hours.
Solar panels are intended for use in direct sunlight. The performance will be affected by the amount of shade. Even a small amount of shade on a single panel can make a big difference. Examine the location to ensure that your solar array will receive full sun during the day’s peak hours. Remember that the angle of the sun changes throughout the year.
There are a couple of other considerations at this point:
System voltage: Determine the power requirements of your equipment by looking at the system voltage. 12Vdc, 24Vdc, 48Vdc, or 120Vac are the most common voltages produced by off-grid PV systems.
DC power is used by solar panels and batteries, and some equipment can be connected directly to the batteries if it can tolerate real-world battery voltages. For a 12-volt system, they can range from 10-15 volts, 20-30 volts for a 24-volt system, and 40-60 volts for a 48-volt system.
Days of autonomy: The number of days the device must run on battery power with only a little amount of solar energy. Depending on the area and the expected operating performance, it can take anywhere from 5 to 20 days. You’ll need enough autonomy to keep the equipment running in cloudy conditions for long periods:
Determine the size of the battery bank
With this information, we should be able to size the battery bank. Following the sizing of the battery bank, we can calculate how much solar power is required to keep it charged.
In our 240Wh/day example, here’s how to calculate battery bank size using lead-acid batteries:
First, we must account for the inverter’s inefficiency (if you are using an inverter). 5-15 percent is generally sufficient, depending on the equipment. Check the inverter’s spec sheet to see how efficient it is. For this example, we’ll use a 10% inefficiency:
264-watt hours = 240 Wh x 1.1 efficiency adjustments
This is the amount of energy used to power the load through the inverter from the battery.
The impact of temperature on a battery’s ability to supply energy must then be considered. As the temperature drops, lead-acid batteries lose capacity, and we may use the chart below to increase battery capacity based on the projected battery temperature:
To account for a battery temperature of 20°F in the winter, we’ll multiply our battery bank size by 1.59:
419.76-watt-hourwatt-hours x 1.1 x 1.59-watt-hourwatt-hours that, factor in the efficiency loss that occurs during charging and discharging batteries. In most cases, we utilize a 20% inefficiency for lead-acid batteries and a 5% inefficiency for Lithium-ion batteries.
Minimum energy storage requirement: 240 Wh x 1.1 x 1.59 x 1.2 = 503.71 watt-hours
Because this is for a single day of autonomy, we must increase it by the number of days of autonomy required. It would be:
Energy storage: 504-watt hours x 5 days = 2,520-watt hours
As you can see, the battery bank size is rapidly expanding because of factors such as temperature and necessary autonomy days. All these factors have a substantial impact on the size of your battery bank and should be carefully evaluated.
Rather than watt-hours, lead-acid batteries are generally measured in amp-hours (Ah) (Wh). Divide watt-hours by the system’s battery voltage to get amp-hours. As an illustration, consider the following:
2,625 Wh / 12v = 220 Ah 12V battery bank
2,625 Wh / 24v = 110 Ah 24V battery bank
2,625 Wh / 48v = 55 Ah 48V battery bank
Always keep the discharge depth in mind while sizing a battery bank. A lead-acid battery’s life can be extended by sizing it for a maximum depth of discharge of 50 percent. Deep discharges have less of an impact on lithium batteries, and they can usually withstand them without sacrificing battery life.
2.52-kilowatt-hours as a total minimum battery capacity.
It’s worth noting that this is the very minimum battery capacity required; increasing the battery size can improve the system’s reliability, especially in locations where the weather is frequently gloomy.
Step 4: Determine the number of solar panels you will require.
We can size the charging system now that we’ve calculated battery capacity. We usually use solar panels, but in locations where there is a lot of wind, or for systems that require more autonomy, a combination of wind and solar might be a good idea. The charging system must generate enough energy to entirely replace the energy removed from the battery while considering all efficiency losses.
Based on 5.5 peak solar hours and a daily energy need of 240 Wh, consider the following scenario:
PV array size: 240 Wh / 5.5 hours = 44 W
However, real-world losses due to inefficiencies, module soiling, aging, and voltage drop must be factored in, which are estimated to be roughly 15%:
The PV array must be at least 44 array watts /.85 = 51.76 W in size.
This is the smallest size for the PV array. A larger array will improve the system’s reliability, especially if there is no other backup energy source, such as a generator.
During all seasons, the solar array will receive unimpeded direct sunshine from 8 a.m. to 4 p.m., according to these calculations. If the solar array is shaded during the day, the PV array size must be adjusted.
Another factor to consider is that lead-acid batteries must be completely charged on regular battery life, they require a charge current of roughly 10 amps per 100-amp hours of battery capacity. Lead-acid batteries that aren’t recharged regularly fail, usually within the first year of use.
For lead-acid batteries, the maximum charge current is normally approximately 20 amps per 100 Ah (C/5 charge rate, or battery capacity in amp-hours divided by 5), and anywhere in the middle is optimum (10-20 amps of charge current per 100ah).
Confirm the minimum and maximum charging parameters by consulting the battery specifications and user manual. If you don’t follow these rules, your battery guarantee will be voided, and you’ll risk early battery failure.
Here are some common PV array and battery bank configurations. This table can be used to compare the battery capacity estimated in the previous stage to find a suitable system size:
Array Size: PV Watts (STC) Battery Bank Size: Watt-Hours (@ C20 rate) Battery Bank Ah Capacity
100-175 600 50Ah @ 12Vdc
200-350 1,200 100Ah @ 12Vdc
50Ah @ 24Vdc
400-700 2,400 200Ah @12Vdc
100Ah @ 24Vdc
800-1,400 4,800 400Ah @ 12Vdc
200Ah @ 24Vdc
100Ah @ 48Vdc
2,000-3,000 9,600 800Ah @ 12Vdc
400Ah @ 24Vdc
200Ah @ 48Vdc
4,000-6,000 19,200 800Ah @ 24Vdc
400Ah @ 48Vdc
8,000-12,000 38,400 800Ah @ 48Vdc
This information is meant to be used as a guide only, as numerous factors might affect system size. There are also other methods for reducing the minimum battery required, such as adding a backup gas generator or wind generator(s).
If the equipment is vital and located in a remote place, it is a good idea to oversize it because the expense of maintenance can quickly outweigh the cost of a few more solar panels. On the other hand, depending on how well your application works, you may be able to start small and scale up later. The size of your system will ultimately be determined by your energy consumption, location, and performance expectations based on days of autonomy.