Installing solar and battery backup systems (A guide)
This guide has been written at the request of the Sudanese Engineers Association Qatar (SEAQ) after a webinar they hosted titled (Solar PV Energy Projects, Funding, Investment, and Challenges).
Before you begin buying your equipment or looking for an installer there are some things you need to know as a user:
Terminologies:
On-grid system
An on-grid solar system has two main components: solar panels and a PV inverter. It has the following characteristics:
- It is hooked directly to the utility grid.
- It doesn’t provide any backup in the event power outage.
- Usually, feed-in back to the utility grid if there is a surplus of energy produced.
Feed-in
The act of exporting energy back to the utility grid.
Feed-in Tariff
The compensation policy between you and the utility grid.
Off-grid system
An off-grid system as the name implies is not connected to the utility grid, it can serve as a backup in the event of power outages. There are many configurations of an off-grid system depending on the equipment purchased, nevertheless, it can have the following components:
- Solar panels.
- A charge controller or a PV inverter.
- A battery inverter (DC to AC).
- A battery bank.
PV Array
A photovoltaic array is multiple solar panels electrically wired together to form a much larger PV installation
PV Inverter
A device that converts the Direct Current (DC) power coming from the solar panels to 110–240V Alternating Current (AC) which can be used by electric appliances or to charge the batteries.
Charge Controller
A device that utilizes the solar energy coming from the sun to charge the batteries.
Battery Inverter
A device that converts the Direct Current (DC) power coming from the batteries to 110–240V Alternating Current (AC) in some cases can serve as a charger for the batteries.
Battery Bank
A group of batteries connected together to form larger capacity storage for energy. It is your electric energy fuel tank.
Depth of Discharge
Batteries don’t like to be drained to 0%, you will shorten their lifespan if you do so. The depth of discharge is the amount you are allowed to drain the batteries to (Usually it is recommended by the manufacturer or type of battery). A Lithium-Ion battery can be drained by 90% while Lead-Acid batteries only 50%, A battery designed to work with Solar System (Deep Cycle) can be a little higher than that may be 70%.
Watt
Unit of power,
- How much power can your PV array generate?
- What is the maximum load that your inverters (PV or Battery) can handle?
kW/h
A kilowatt-hour is a unit of energy, a kilowatt = 1000 watts
- How much energy does your PV array produce during the day?
- How much energy does your battery bank hold?
Amp
Unit of electrical current.
- How much energy in kilowatt-hours can a 200A 12V battery hold?
Watt = Amps x Volts
200A × 12V = 2,400 w/h (2.4 kW/h) - How much energy in kilowatt-hours can (12) 200A batteries in a 48V system hold?
The battery configuration would be as follows:
Every 4 batteries must be connected in series to produce the required 48V, Then 3 strings are connected in parallel (3P4S)
200 × 3 = 600 Amps (Capacity)
600A × 48V = 28,800 w/h (28.8 kW/h)
Sizing Your System:
Before purchasing the equipment you will need to measure the loads and the energy needed for a specific period of time (How much kilowatt-hours you will need?).
Loads
How much power all of your home appliances draw at any given moment. If you live in Sudan you can use your energy meter to do the measurements. On my energy meter, I can press #1# to show my instantaneous load.
Turn on all of your home appliances that you will need to work in your off-grid system, and measure the load, according to the number you will get you will be able to decide the size of your battery inverter. For example, if your home is getting a single phase from the grid and all of your appliances draw about 2kw, I would say you will need a 2.5–3.0 Kw battery inverter (Leave room for margin).
Energy
How big should your battery be? To answer this question you will need to decide how many hours you need your system to provide energy. Does your system serve as backup only or do you want to go entirely off-grid?
Let’s assume that your system is only a backup, and you decided that you need the system to work for 10 hours only. According to your load's calculations, your home draws 2kw continuously, which will base our energy calculation on this figure, but in reality, your home can draw less than that because usually, your home appliances sit idle most of the time. However, we will base our energy calculation as if your home draws 2kw at any given moment just for simplicity's sake.
10 hrs x 2 kW = 20 kW/h So our battery should hold at least 20 kW/h of energy. I say at least because we need to consider the depth of discharge. We will increase the size according to the battery type (Deep-Cycle Gel batteries with 70% recommended Depth of Discharge). 20kW/h (1 + (1–0.7)) = 26kW/h
Common battery sizes of this type can be 100, 125, 150, 200, or 210A/h
The best fit for a 48V system would be (12) 200A/h batteries with 2.8 kW/h to spare. Here are the calculations:
200 × 3 = 600 Amps (Capacity)
600A × 48V = 28,800 w/h (28.8 kW/h)
PV Array and Charge controller
Another component you need to size correctly is your PV array. How many watts will you need to charge your batteries during the day and be able to use the system at the same time?
An 8,000w PV array installed in Khartoum can produce 45 to 51 kW/h on a sunny cool day. But here are some things you need to know:
- High temperature can affect solar panels' power generation adversely. (High temps = high resistance)
- The tilt of the solar panels is a major factor in power generation. Every season has its optimum tilt but most of the time the PV array is fixed therefore the optimum tilt in Khartoum is 20° (Average for all seasons)
- Consider cloudy days, although Khartoum sky is mostly clear.
My power generation factor is 0.007149 (Based on my location and my equipment’s efficiency)
To charge a 28.8kW/h battery bank we will need 28.8kW/h ÷ 0.007149 = 4,029w PV array given that we need more power to run the appliances during the day in addition to charging the batteries 2kw × 8hrs = 16kW/h thus 16kW/h ÷ 0.007149 = 2,238w
We will need a 6,200w PV array in total.
Solar panels are measured and usually sold per watt. Let’s assume that you can purchase 250w solar panels from a local vendor that means you will need:
6,200 ÷ 250 = 24.8 (24 Solar Panels).
Make sure your Charge Controller or PV Inverter can handle the current produced by the solar panels and follow their manufacturers' instructions on how to configure and connect your panels.
Installing your system:
Here are some important issues to consider:
- This goes without saying but before you purchase your solar panels make sure there is enough space for them on your rooftop.
- Mount your equipment in a well-ventilated area. Provide cooling if necessary, Temperature can affect the efficiency of your equipment
- Put your batteries in a well-ventilated area, as some batteries may produce toxic fumes during the charging periods (don’t put them in your living area).
- There are hidden costs, consider the cost of mounting hardware, cables, electrical supplies such fuses and switches, etc.
Installing a solar system is a great feat that involves a lot of research and calculations, it is not for the faint-hearted but the outcome is extremely rewarding.
Disclosure: All of the above figures and methods used to calculate them are based on my experience, your mileage may vary!
