Example only, not actual product.

General Information

As a matter of principal, it is not recommended at this stage to quote a viable off-grid system without a back up generator of some kind because running it totally on batteries would make it prohibitively expensive. The fuel for the generator we recommend in this case is LPG because of its lower price, easy availability, cleaner emissions and importantly can be also be used for cooking or running a small gas refrigerator providing clients more options. However other fuels can also be used (i.e. petrol, ethanol, diesel, etc) each with a different price and efficiency.

All systems will consist of -

  • ·         a PV array
  • ·         a DC/AC Inverter
  • ·         a battery bank
  • ·         a generator
  • ·         a controller

It is not possible to provide a “One Price Fits All” design since every user will have very different energy consumption habits so we have developed a choice of scenarios, which will cover a fairly wide range of possible energy uses. We have developed and provided three graphs below from which we can choose the most appropriate fit. In order to understand what the graphs mean here is a technical overview, which I hope is not too complicated to understand. I will also provide a diagram outlining the design of the system. Which may help you understand the technical explanation.   

Technical overview

The inverter and the generator will feed into the common busbar, which will support the loads. The controller will log all system behavior and will start/stop the generator automatically as needed. The system will monitor itself and will suggest modifications over time in order to maximize efficiency. An explanation on how efficiency is defined follows:

For any given amount of kWh required on a 24h basis, certain assumptions have to be made on the usage profile.

Firstly, this energy amount will determine the size of the PV array.

Secondly, the percentage of this energy to be used during the night will determine the size of the battery bank.

Thirdly, the percentage of the required energy that will not be produced by the PV array but will come from the generator, will decrease the size of the PV array and will determine the generator’s size.

 

For residential applications (up to 100 kwh/24h), a generator of 6 kW is adequate and this parameter will be fixed at this level. The other 2 parameters, “night %” and “gen %” will have to be determined by the system itself after running for some time. An average value of 30% is a recommended starting point for both of these assumptions. So, each system will be sized according to the daily kWh/24h figure, the 2 percentages at 30% and will include a 6 kW LPG generator. Indicative prices and annual fuel costs for various situations are given in the tables below:

Table 1Systems with 30 % night, 30 % gen

kWh/24h à

5

10

15

PV array kW

0.9

1.8

2.6

Bat kWh usable

1.5

3

4.5

System Price $

6599

9522

12339

Fuel Cost $

192

383

575

 

To see the effect the other 2 parameters can have, the same energy needs require different systems under different assumptions. Also, costs differ. The same table follows based on 10 % night, 30 % gen:

Table 2 Systems with 10 % night, 30 % gen

kWh/24h à

5

10

15

PV array kW

0.9

1.8

2.6

Bat kWh usable

0.5

1

1.5

System Price $

5483

7230

8975

Fuel Cost $

192

383

575

 

If we keep the night % at 30 %, but decrease the gen % to 10%, the table will be:

Table 3 Systems with 30 % night, 10 % gen

kWh/24h à

5

10

15

PV array kW

1.1

2.3

3.4

Bat kWh usable

1.5

3

4.5

System Price $

6944

10150

13354

Fuel Cost $

64

128

192

 

The effect of the usage profile is obvious. This is the reason that the system has to be designed for each specific client after enough information is gathered. The tables above can be used to market the off – grid solutions and the most appropriate one can be recommended and installed by completing a simple energy survey with the customer, for example:

 

Appliance

Power kW

Hours of usage

Energy kWh

Fridge

1.2

3

3.6

TV

0.4

3

1.2

Lights

0.2

6

1.2

PC

0.5

2

1

Oven or AC

2

1.5

3

Total

10

 

In the case of this completed example above, the customer could choose any of the systems in the second column of the tables depending on their requirement for night consumption. If they are used to going to bed early then table 2 could be the most appropriate because it is lowest cost and little storage for evening. If they are “night owls” and stay up late then table 3 might be best for their needs and table 1 is a good “all round” option.

Hopefully this information is useful, clear enough and not too complicated. If you need to discuss it further, please feel free to contact us.

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