Study - 2

• 2. Investment and value for money

Either using the UPS or solar energy, I would pay a considerable sum of money, but I need to study both cases well.

At first I have to decide upon what load I want to have powered without any downtime due to power shortage. I began with a very big fancy dream of powering all my apartment without downtime. That means that I need enough power for all the electrical appliances for at least 1.5 Hr daily. With a simple calculation, I consume an average of 250 kWh monthly and I will assume I use them in the period of 6 hours daily. Therefore my month will have 26 days (because of weekends) x 6 Hr = 156 hours of electrical usage a month. that means that I need an average net power output about 1.6 kW, and I need it for 1.5 Hr.

In Egypt the voltage reaching domestic areas for home usage is 220 V AC. That means that I need a current of  1600 Watt / 220 V AC = 7.3 Amperes. Converting the voltage to 12 V DC (which is the batteries' voltage that I assume to use either in the UPS or solar energy system), therefore the current rises to about 133 Amperes (that is calculated by dividing the required power 1600 Watt by 12 V DC, because the power is constant). This is roughly calculated because both systems have losses which will be discussed later.

Assuming that I need this power for 1.5 Hr, therefore I need a power storage of 133 x 1.5 = 200 Amperes.

UPS

A typical UPS of 2000 VA = 1200 Watt has two batteries of  12 V DC, 9 Ah. That's a total of 18 Ah.

Dividing 18 Ah by 133 Ah, the UPS battery will be completely consumed in about 8 minutes (taking into consideration that batteries should never be completely discharged, I will assume it will be discharged to 30% then the power will cutoff, I will only have about 5 minutes.

That UPS from GE (pure sine wave) will cost me 840$ = 5880EGP + 30% customs and shipping = 7644 EPG, and the surprise is that is not enough to power my full load, but that is the nearest one I could get a price for. By common sense, the sufficient UPS (from power output perspective, not from time perspective) will cost me at least 1000$ = 7000 + 30% = 9100EGP .

I certainly can get extra batteries to prolong the time for working in power outage. I will need 133 Ah plus taking into consideration that I will keep the batteries charged above 50% (to prolong their lifetime), I will need a battery bank of 266 Amperes for every hour of power outage (in my assumption of 1.5 Hr, I will need 400 Ah) This will cost me an extra of 2000 EGP per 100 Ah (batteries type are either Trojan or Bosch). The system cost will reach about 17100 EGP. (that's a big sum of money).

The second noticeable thing is that all the stored power inside batteries is charged from my grid and is not free.

Another important point to be taken into consideration that the battery charger built in the UPS has a maximum charging current, and that affects the charging time of the batteries. My example UPS  claims to have a recharge time of 6 Hrs. By calculation, my 400 Ah batteries will need (400 Ah / 18 Ah) x 6 Hr = 133 hours. In this case, I can use my UPS with the large battery bank once or twice a week.

to be continued...