How to Determine the Size of Solar Electricity System for Your Needs

Components that make up a Solar Electric System

There are 3 major components of a solar electric generating system;

  1. Solar Panels: which produce electrical energy from the sun
  2. Battery: to store electrical energy produced by the solar panels
  3. Inverter: converts direct current produced from the battery or solar panels to alternating current required to power your electrical devices.

Note that there are battery-less solar systems too. If you only need electricity only when the sun is shining then you only need a solar panel and inverter. And if your electrical appliance runs on direct current only, then you only need the appropriate amount of solar panel or panels with a direct current voltage regulator installed.

In advanced countries with 24 hour supply of grid electricity, batteryless solar systems are connected to the grid. The overall goal here is to reduce the cost of grid electricity being paid for by the owner of the solar system. Once the solar panels generate enough electric power for the day, the excess is sold off to the grid. A sophisticated device that calculates the energy produced by the solar system and the excess that is sold off is installed with the system.

Solar Panel Sizing 

Solar panels are produced in different power output ratings. There are 10 watts solar panels, 20 watts, 30 watts and so on. A standard solar panel is rated at a power output of around 300 watts. In determining the size and number of solar panels you will need, you will first need to find out your total power consumption requirements.

To know the power consumption of your electrical appliances, check under or back of the appliance.

Once you know the total power requirements of your electrical appliances you can then correctly determine the size and number of solar panels required.

Let’s say the total power consumption of all your electrical appliances is say 3000 watts, then this means you will need ten 300 watts solar panels. Assuming not all your electrical appliances are working at the same time then the panel sizing just calculated above will be adequate. Otherwise you may need to add say one more 300 watts. This is because the solar panels are not 100% efficient.

Suitable Battery and Sizing

Big sized batteries suitable for use with a solar electric system produce a direct current of 12V.

Battery capacity is the key factor here. It is the maximum amount of power the battery can store.

Batteries suitable for the solar system are deep cycle. Deep cycle batteries are designed to discharge to almost it’s full capacity without damage. Say 80 to 90% of total capacity.They look like car batteries. Note that most car batteries are not deep cycle.

There are two major types of battery in the market;

  1. Lead acid battery
  2. Lithium battery

Lead acid batteries  are very common and cheap. Even though lead acid batteries can be designed for deep cycle use, it is advisable you don’t discharge more than 50% of its capacity to extend battery life. It is also advisable to use flooded lead acid batteries rather than seal lead acid batteries. They last longer when properly maintained. Lifespan of lead acid battery is between 1.5 to 2 years

Lithium batteries are true deep cycle but are expensive. They can support discharges of over 90% of their capacities without damage. They can also support recharge cycles of between 1000 – 3000 times lasting 3 to 5 years if properly used.

Battery Sizing 

Battery capacities are usually expressed as Ah (Ampere hours). To express as Wh ( Watt hours) it should be multiplied by the supplying voltage of the battery which is usually 12V ( V stands for volts). Also note that 1kWh ( kWh stands for Kilowatts hour) = 1000Wh.

For instance, a battery rated 300Ah, and multiplied by 12V gives;

300 × 12= 3600Wh or 3.6kWh.

This means that the fully charged battery can deliver a power of 3.6kW consistently for 1 hour.

Referring to the example on solar panel sizing, recall that we sized for 3000W. Since it is reasonable that all electrical appliances can not all be put on at the same time for 24 hours, we can conclude that on the average, a power of 1500W  or 1.5kWh is being utilized every hour. Recall that our calculated battery capacity is 3.6kWh. A little consideration will show that if 3.6kWh is divided by 1.5kW ( which is the average power consumption per hour, it means that a single fully charged 3.6kWh battery will power your electrical appliances for ; (3.6÷1.5= 2.4 hours) 2.4 hours or 2.4h, (h stands for hours). So if you want a 24hour supply you divide 24 hours by 2.4 hours which equals 10. This means you will need ten 3.6kWh capacity batteries.

In truth if you are using lead acid batteries you will need much more.

Since it is not safe to discharge  above  50% the rated capacity in order not to damage the battery, then ten batteries will run for 12 hours. That is you need 2 × 10 lead acid  batteries to run for 24 hours. But if it’s lithium batteries which can support almost full discharge without damage, then near 24 hours supply is possible with the calculated 10 batteries. 

In summary of this section, if it’s lead acid battery, then you will have to double the battery size / number to run for the number of hours it has been designed for. If its lithium battery then increase battery size / number by 10-20 %.

Inverter Sizing

It is better to get a much bigger inverter than what is required. For a 3000W power consumption you can go for a 5000W rated inverter. This will give the inverter much headroom to operate safely. Inverters are prone to overheating when loaded near it’s full capacity.

Backup Generator

Your solar electric system will not be complete without a backup generator. When the solar power and batteries run out. The back up generator can restore power and at the same time charge the batteries for another cycle of use.

Generator Sizing

For the generator to last, it should run at 50% its rated capacity. So let’s say all your household electrical appliances are rated 3000W, then get a backup generator of 6000W.

Leave a Reply