## 4 Simulation Results
### Comparision of Electricity Bill  
To understand the electricity bill reduction benefit of the C-_u_grid for home users and compare different electricity bill information, two Taiwan Power Company electricity rates, non time-of-use (Non-TOU) rates and residential-type simple time-of-use (RSTOU) rates [21], are discussed in this work. Electricity simulation results in the above-mentioned cases are used separately to calculate the overall C-_u_grid electricity bill under the two different rates. Non-TOU rates are commonly used for most residential electric customers in Taiwan, and use a meter rate for lighting service. TOU rates include two seasonal periods, summer (1 June to 30 September) and non-summer (all other days of the year), and each period uses different rates, as shown in Table 3. For the calculation of electricity bill using non-TOU rates, different rates among different electricity use intervals are covered to calculate the total used electricity for one electricity bill. In the example of an electricity bill that gives 150 kWh of used electricity, according to Table 3, a rate of 1.63 NTS/kWh is used to calculate first 120 kWh of electricity, and a rate of 2.10 NTS/kWh is then used to calculate the remaining 30 kWh of electricity. Table 3 also shows the results for whole C-_u_grid electricity bill calculated by Non-TOU rates. In this calculation, one electricity bill includes two months (62 days) of electricity use, and the electricity rate for the non-summer period is considered here since the used load electricity profile (i.e., Figure 13) in simulations is collected in non-summer months. In case 1, it was found that five home users in the C-_u_grid used in total 30.75 kWh of electricity for one day and 1906.5 kwh in two months. Here it is supposed that the daily used electricity for each home user is the same; and thus the total electricity bill calculated using the equation in Table 3 is NTS6424. Using the same assumption and calculation, a total electricity bill of about NTS12,477 is obtained for case 2. Also, for the case of home users, required electricity is all from the utility grid and without any C-_u_grid support. The simulated electricity response may identical to the original electricity profile in Figure 13, and in this condition total electricity bill is about NTS21,807  
for all day electricity; and a rate of 0.91 NT$/kWh is required for the part of total used electricity exceeding 2000 kWh. Finally, a base cost of 75 NT$/month must be included for one electricity bill. Table 4 also shows the electricity bill calculated by RSTOU rates; meanwhile, one electricity bill still includes two months of 62 days electricity use, and it supposes 42 days for weekdays and 16 days for weekend days. It is found that one-day peak time electricity use for the case 1 sunny day scenario is quite low, the use is only 5.17 kWh since there is sufficient solar insolation to drive the PV system to supply much more electricity to home users, and the total electricity bill is about NT$3857. For the cloudy day in case 2, peak time electricity use increases to 25.35 kWh, and the total electricity bill is NT$8270. For home user electricity coming only from the utility grid, the total electricity bill reaches about NT$15,925. Compared to the results in these two tables, the electricity bill savings benefit for the residential home users with C-_u_grid power supply is more significant than that without C-_u_grid operations. Moreover, it is found that the residential home users in this study who choose RSTOU rates can obtain more savings in electricity bills. However, it is thought this may be possible for the residential home users with a high degree of electricity use, i.e., the used case in this study; other cases should be considered so home users can appropriately determine used electricity rates. Investigations and discussions of above-mentioned results are mainly focussed on single C-_u_grid configurations.