Of a budget Solar Inverter System - Energy Audit

One of the requests and questions I often get from people when they find out about my penchant for renewable electricity is; How can they build a system? how much does it cost etc.  Next month would make it exactly 7 years since I verged into the rabbit hole of using sunlight for generating electricity. I am in the process of building a solar inverter system for a close Family.

I have decided to use the opportunity to document every aspect of the build including design and thought process.  This is the first in a series of posts documenting the build with a budget limit of 1.2 million Naira or about $1600.

The hope is that this build can inspire others, the use case would be pretty basic and generic enough for most people to adopt and use for their own.


This post should not be considered a replacement for professional electrical advice. I am not an electrical engineer or any engineer for that matter.
Working with electricity can be very dangerous, doing the wrong thing can lead to loss of life and property. Your best bet remains working with a qualified, certified solar electrical engineer. I do not take responsibility if following this post kills you or your cat.

First thing first, Energy Audit.

This is the first step in any renewable energy conversation. The whole idea of off-grid solar is to generate electricity from the sun for use.  For things to work well, it is important to start by identifying the amount of electricity that is needed. This is what an energy audit is about.

About Watts and Watt Hours, Volts and Amp - Click me - Explanation for Beginners

To carry out an energy audit, you need to understand "watts"

It is about the watts, every electrical appliance uses "watts" and the electricity generated by solar panels is also measured in watts.

The goal here is to make sure the watts used by electrical appliances in the house are less than or at least equal to what is being generated by the solar panels. i.e. You don't want to use more than your solar panel can generate.
To do this is quite simple, every electrical appliance is rated in watts. Take a 10w light bulb. When the bulb is turned on, it consumes 10w of electrical power.

Another very important measurement to note is electrical power relative to time. If I turn on a 10w light bulb for 1 hour, this means the light bulb has consumed 10 watt hours of electrical energy. Same way if a solar panel generates 10 watts of electricity for 1 hour, it would have generated 10 watt hours of electrical energy.

If I turn a 10w light bulb for 30 minutes, it would have used 5 watt hours of energy. etc.

Watt hours relate to the duration in time an electrical power was utilized or generated.

Electrical Volts and Amp.

Watts is made up of Volts and Amp hence Watt = Volts x Amp.
Volts can be described in layman's terms as units of electrical flow, how "fast" electricity flows from one point to the other.

Amp can in layman's terms be described as the actual electricity it can also be called current when it flows within a wire. Both Amp and Voltage make up Watts.

Our light bulb which uses 10w of electricity:

Example 1: The light bulb can consume 10w by drawing 0.45 amp (A) at 220 volts (v) i.e. 0.045A x 220v =10w.


Example 2: The light bulb can consume 10w by drawing 0.45A at 12v i.e. 0.83A x 12v = 10w.

The first example is an electrical device that uses what is called high voltage electricity which is often what we get from (NEPA) the grid,

the second example is called low voltage electricity which is what we get with batteries.

Project Scope.

The scope of this project is to power essential electrical needs. The most important need for electricity by most homes in Nigeria.  Lighting, entertainment, climate control. Thus for this build the following appliance would be covered

  • Indoor light bulbs
  • Fan
  • Television
  • Basic mobile devices - Phones and laptop
  • Chest Freezer.

Basic Assumptions and Energy usage calculation

  • The House to be powered is a 2-bedroom apartment
  • The Number of lighting points to be powered is 9, with each having no more than 6w light bulb. This covers 3 living area, 1 corridor, 1 kitchen, 2 bathrooms and 2 balcony outdoor lights.
  • The fan in the house are rechargeable dc fans
  • The TV is a 40 inch LED TV
  • Chest freezer is not bigger than 200L

Energy usage Calculation and appliance run time.

9 x 6w light bulb. The light bulb turn on for 8 hours Mostly at night

  • 9 x 6 = 54w x 8 hours = 432 watt hours (wh)

3 x 20w dc fan. The fan turns on for 15 hours a day.

  • 3 x 20w = 60w x 15 hours = 900wh

1 x 100w TV. TV runs for 10 hours a day.

  • 100w x 10 hours = 1000wh

Laptops and Mobile Phones, runs for 8 Hours

  • 50w x 8 hours = 400wh

1 x 200w Chest freezer. Chest freezer runs for 6 hours a day
200w x 6 hours = 1200wh

In Total.

Total Power Use
First, we calculate the total concurrent power usage of all appliances if they were all turned on at the same time. This would be important in sizing the capacity of the inverter required.
54+60+100+50w+200 = 464w.

Total energy Usage
The energy usage of all the appliances when used for their turned on duration.
434 + 900 + 1000 + 1200 + 400 = 3934 watts hours.

This figure would be important in determining the size of solar panels and battery to be used for the system.

In conclusion

For now our audit has given us two important numbers. We know that based on usage the total energy requirement to power the essential needs of this house is an estimated 4000 watt hours.

The total number of electrical appliances when they are turn on is about 500w.

This audit would help us to proceed to the next stage of the project. We can build and design the a balacned system from here.

The next blog post would focus on design and the component that would be required to meet the energy needs we have just established.

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