How To Save On Electricity Bill - Analysis

Bill details from 5th Apr to 5th Jul for a particular establishment are shown below:

The rate per unit (KWH) varied during the said period. 

And the billing rates has two slabs: For the first 10units usage per day, the rate is 44.454 cents and beyond that it is slightly higher at 46.5 cents per unit. 

                                                          Usage       Rate

                                                          (Units)

For the 1st 10 units /day from Jul 1st  :   54.79    0.44454  

For the balance units from Jul 1st:           67.54    0.46500

For the 1st 10 units/day till Jun 30th  :   953.43    0.34241  

For the balance units till Jun 30th:        1175.24    0.36860

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Total units:                                           2251

Previous meter reading: 251

Current meter reading: 2502

As per bill total units consumed = 2502-251=2251 which matches with the value above. Values in the table above give the breakup of 2251 units because each of the 4 individual unit values is charged at a different rate.

The establishment also has a solar power unit which, during daytime, powers the establishment and also supplies any surplus, to the electricity company. There is no provision to store the surplus for usage during night. 

This is rather unfortunate because the electricity company charges the establishment about 44c/KWH while it pays the establishment 8c/KWH when the company consumes the surplus electricity (generated during daytime).

The surplus fed back to the electricity company for the 3 month period is 1055 units for which the establishment gets a credit of 1055*.08= $84.4. Had this surplus been stored in a battery and used internally then the establishment would have saved 1055*(.465-.08)=$406 which works out to $406/3=$135 monthly. 

The solar generating unit is rated 5KW. It has supplied 1055 units over a 3 month period which is about 12 KWH / day. Assuming it works about 12 hours/day that works to 1 KWH / day which is supplied to the electricity company. Is the balance about 5-1=4KW used by the establishment? Seems unlikely to me. If that is so, then what happened to the balance? It is to be noted that during daytime the establishment electricity requirement in the establishment is quite low.

Assuming the cost of a battery is about $9000 including maintenance etc, the payback period is $9000/$135= 66 months.

Does it make sense for the establishment to buy a battery now? Well aaaah ummm. Maybe if the battery costs $2000 in which case the payback period would have been 15 months or so. Is it possible to rent a battery @$50 a month? If yes, I would say go for it. One would pay $50 and save $135 as shown above.

Meanwhile if the idea is to save on electricity, the establishment has to be explored to see where it's excessive, where it's lean and where there is a sweet spot to cash in on.

The establishment has consumed 2250 units from the electricity company over a 90 day period which works out to about 25 units per day. 
Assuming that during the day there is an energy surplus owing to the solar power, the electricity usage from the company is primarily during the 12 hours after dusk. During daytime as explained earlier 1KW is "exported" during daytime and not much power is consumed during daytime. This means power consumption is 25/12=2 KW at night. 

Note:
I have assumed that daytime and night time are equal and both equal 12 hours per 24 hours.

Summary - The following questions remain nagging:

• Why only 1KW is exported by solar generator during daytime? 
• What is the breakup of 2KW power consumption at night? Which appliances consume the 2KW?