a power management system according to preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings . fig1 is a block diagram showing a power management system according to the first embodiment of the present invention . referring to fig1 , a power company 1 and power user corporation management server 3 are connected via the internet 2 . the power user corporation management server 3 is connected to a lan 4 , to which client computers 5 a to 5 e that are to undergo peak power control are connected . each of these client computers 5 a to 5 e has a battery ( rechargeable secondary battery ) which is used when is used when the computer is mobile and is not plugged into an ac source . each computer is also designed to operate by power supplied via an ac adapter . fig2 is a block diagram showing the arrangement of the power user corporation management server 3 according to the first embodiment of the present invention . as shown in fig2 , the server 3 according to the first embodiment of the present invention comprises a power load information reception unit 11 , power load information monitor unit 12 , data storage unit 13 , packet generation unit 14 , and packet transmission unit 15 . the power load information reception unit 11 receives power load information from the power company 1 . note that the power load information pertains to the load on electric power supplied from the power company . the power load information monitor unit 12 monitors the load information received by the power load information reception unit 11 . more specifically , the power load information monitor unit 12 determines whether or not the received load information exceeds a first or second threshold value 13 a or 13 b stored in the data storage unit 13 . fig6 is a graph showing the relationship between the power load and threshold values . fig7 is a table showing the relationship between the threshold values and power supplies . as shown in fig7 , when the power load is higher than the first threshold value and is equal to or lower than the second threshold value , the battery of each client computer is inhibited from being charged , and use of an external power supply ( ac adapter ) is permitted . when the power load is equal to or higher than the second threshold value , the battery of each client computer is inhibited from being charged , and use of an external power supply ( ac adapter ) is also inhibited . the packet generation unit 14 generates a packet to be transmitted to each client computer on the basis of the information monitored by the power load information monitor unit 12 . fig3 shows a packet generated by the packet generation unit 14 . the embodiment of the present invention uses a packet similar to so - called magic packet ® ( see amd , inc . publication # 20213 : “ magic packet technology ”, incorporated herein by reference ) that can implement wol ( wake - on - lan ). as shown in fig3 , this packet comprises a destination field 22 , source field 23 , misc field 24 , sync field 25 , target address field 26 , peak shift control field 27 , misc field 28 , and crc field 29 . the destination field 22 stores information indicating the destination of the packet . the source field 23 stores information indicating the source of the packet . each of the misc fields 24 and 28 stores information indicating an arbitrary pattern having an arbitrary length . the sync field 25 stores 1 - byte data “ fa ” that follows 5 bytes or more of data “ ff ”, and has a 2 - bit difference from the sync field of magic packet , which stores 6 bytes of “ ff ”. the target address field 26 stores 15 pieces of identical information each indicating the mac address of a client computer which is to undergo peak shift control . the peak shift control field 27 is unique to the power management system according to the first embodiment of the present invention , and details thereof will be explained later . the crc field 29 is a general field for error detection control of the packet . information stored in the peak shift control field 27 will be described below . fig4 shows the peak shift control field 27 according to the first embodiment of the present invention . as shown in fig4 , the peak shift control field 27 consists of 6 bytes , and fig4 shows the contents of the first byte . note that data of the first byte are repetitively set in the second to sixth bytes as needed , and the first bit of an insignificant byte is set to “ 0 ”. referring to fig4 , “ b ” indicates an undefined bit , bit “ c ” indicates information associated with charging of the battery , and bit “ b ” is information indicating a power supply destination . when “ c ”= 1 , charging of the battery is inhibited in principle ; when “ c ”= 0 , inhibition of charging of the battery is canceled . when “ b ”= 1 , electric power from the battery is preferentially used ; when “ b ”= 0 , electric power from the external power supply ( ac adapter ) is preferentially used . in this embodiment , every time the state of a client computer which is to undergo peak shift control is changed , the server transmits the above packet to the client computer . alternatively , the packet may be transmitted at predetermined time intervals . fig5 shows the arrangement of the client computers 5 a to 5 e . as shown in fig5 , each of the client computers 5 a to 5 e comprises a packet reception unit 41 , packet analysis unit 42 , power supply control unit 43 , battery 44 , and external power supply circuit 45 ( serving as an external power supply input portion ). while a battery is indicated at element 44 , it is understood that embodiments of the invention more generally include any second power source such as a fuel cell . the second power source is preferably portable and is generally located within the computer 5 a – 5 e . however , a large externally located secondary power source ( i . e ., battery ) may be used to service a plurality of computers . the packet reception unit 41 receives a packet 21 which is sent from the server and contains information associated with power supply control . the packet analysis unit 42 analyzes information associated with power supply control , which is contained in the packet 21 received by the packet reception unit 41 . more specifically , the unit 42 analyzes information associated with charging of the battery and information indicating the power supply destination , which are contained in the first byte of the peak shift control field 27 in the packet 21 , and informs the power supply control unit 43 of the analysis result . the power supply control unit 43 controls the battery and external power supply circuit on the basis of the analysis result of the packet analysis unit 42 . more specifically , when bit “ c ” ( in the peak shift control field 27 contained in the packet 21 )= 1 , charging of the battery is inhibited ; when “ c ”= 0 , inhibition of charging of the battery is canceled . when “ b ”= 1 , electric power from the battery 44 is preferentially used ; when “ b ”= 0 , electric power from the external power supply ( ac adapter ) 45 is preferentially used . the operation of the power management system according to the first embodiment of the present invention will be described below with reference to the flow charts in fig8 and 9 . fig8 is a flow chart for explaining the operation of the server . upon reception of the power load information from the power company ( s 1 ), the power user corporation management server checks based on the received power load information if the power load is larger than the predetermined second threshold value shown in fig6 ( s 2 ). if it is determined in s 2 that the power load is larger than the second threshold value , bits “ c ”= 1 and “ b ”= 1 in a power management field contained in a packet are determined ( s 3 ). on the other hand , if it is determined in s 2 that the power load is smaller than the second threshold value , it is then checked if the power load is larger than the first threshold value shown in fig6 ( s 4 ). if it is determined in s 4 that the power load is larger than the first threshold value , bits “ c ”= 1 and “ b ”= 0 in a power management field contained in a packet are determined ( s 5 ). on the other hand , if it is determined in s 4 that the power load is smaller than the first threshold value , bits “ c ”= 0 and “ b ”= 0 in a power management field contained in a packet are determined ( s 6 ). after that , a packet is generated ( s 7 ) in which information determined in s 3 , s 5 , or s 6 is set in predetermined bits of the power management field , and the generated packet is transmitted to the client computer ( s 8 ). fig9 is a flow chart for explaining the operation of the client computer . upon reception of the packet transmitted from the server ( s 11 ), the client computer analyzes bits “ c ” and “ b ” contained in the peak shift control field 27 in that packet and checks if bit “ c ”= 0 ( s 12 ). if it is determined in s 12 that bit “ c ”= 0 , the battery is charged . also , it is determined that bit “ b ”= 0 , and the external power supply is used ( s 13 ). the reason why “ b ”= 0 is determined is that a combination of “ c ”= 0 and “ b ”= 1 is not available . on the other hand , if it is determined in s 12 that bit “ c ”≠ 0 , it is checked if “ b ”= 0 ( s 14 ). if it is determined in s 14 that “ b ”= 0 , charging of the battery is inhibited , and the external power supply is used as a power supply ( s 15 ). if it is determined in s 14 that bit “ b ”≠ 0 , charging of the battery is inhibited , and the battery is used as a power supply ( s 16 ). therefore , according to the power management system of the first embodiment of the present invention , the server transmits a packet for peak power shift control on the basis of the power load information transmitted from the power company . the client computer switches its power supply between the battery and external power supply on the basis of the received packet . hence , adaptive peak power shift control can be implemented without limiting the functions of the client computer . a power management system according to the second embodiment of the present invention will be described below . the power management system according to the second embodiment of the present invention prepares a plurality of power supply control patterns in advance , acquires parameter information used to determine this power supply control pattern , and transmits information associated with a power supply from a server to a client computer on the basis of the power supply control pattern determined based on the parameter information . fig1 is a block diagram showing a server in the power management system according to the second embodiment of the present invention . as shown in fig1 , a server 3 according to the second embodiment of the present invention comprises a sensor 51 , rtc ( real time clock ) 52 , parameter information acquisition unit 53 , power supply control pattern determination unit 54 , power supply control pattern storage unit 55 , packet generation unit 57 , and packet transmission unit 58 . the parameter information acquisition unit 53 acquires parameter information used to determine a power supply control pattern . in this embodiment , the unit 53 acquires current ambient temperature information from the sensor 51 and date & amp ; time information from the rtc 52 . the power supply control pattern determination unit 54 determines one of a plurality of power supply control patterns stored in the power supply control pattern storage unit 55 on the basis of the parameter information acquired by the parameter information acquisition unit 53 . the power supply control pattern storage unit 55 stores a spring power supply control pattern 56 a , summer power supply control pattern 56 b , autumn power supply control pattern 56 c , and winter power supply control pattern 56 d . note that the power supply control patterns for respective seasons are stored in this embodiment . however , the present invention is not limited to such specific patterns . for example , various other patterns such as patterns for respective days of the week , those for respective temperatures , and the like may be used . it is also possible to use a combination of one or more of ( 1 ) time / date information ( 2 ) day of week information ; and ( 3 ) temperature information to store pattern data . as an example , the seasonal data for summer , fall , winter and spring may be overridden by an ambient temperature range so that if a fall day is particularly hot , the current ambient temperature parameter will modify the normal fall setting to compensate for the unusually hot weather . however , the time interval for such an power saving adjustment may be less than the time used for a normal hot weather pattern that would be appropriate for a summer setting . of course , weekend days are treated in consideration of a reduced power demand than normal workdays . in this embodiment , the power supply control pattern determination unit 54 determines a power supply control pattern on the basis of the date & amp ; time information from the rtc 52 acquired by the parameter information acquisition unit 53 since the power supply control patterns stored in the power supply control pattern storage unit 55 are determined in correspondence with respective seasons . the power supply control pattern determination unit 54 acquires information associated with power supply control of a day from the determined power supply control pattern . more specifically , the unit 54 acquires information indicating the times at which the first and second threshold values described in the first embodiment are expected to be reached , and information indicating charging of the battery and the power supply destination from the determined power supply control pattern . fig1 shows a peak shift control field 27 in a packet used in the power management system according to the second embodiment of the present invention . as shown in fig1 , the peak shift control field 27 consists of 6 bytes , and fig1 shows the contents of the first byte . note that data of the first byte are repetitively set in the second to sixth bytes as needed , and the first bit of an insignificant byte is set to “ 0 ”. in fig1 , the first bit “ 1 ” indicates that this byte is a significant byte . bit “ t ” stores information indicating time information , and the control shifts to a state defined by bits “ c ” and “ b ” at the time indicated by bit “ t ”. bit “ c ” indicates information associated with charging of a battery , and bit “ b ” is information indicating a power supply destination . when “ c ”= 1 , charging of the battery is inhibited ; when “ c ”= 0 , inhibition of charging of the battery is canceled . when “ b ”= 1 , electric power from the battery is preferentially used ; when “ b ”= 0 , electric power from the external power supply ( ac adapter ) is preferentially used . in this embodiment , every time the state of a client computer which is to undergo peak shift control is changed , the server transmits the above packet to the client computer . alternatively , the packet may be transmitted at predetermined time intervals , or information for a day may be sent at the same time . the packet generation unit 57 generates a packet to be transmitted to a client computer on the basis of the information obtained by the power supply control pattern determination unit 54 . the packet transmission unit 58 transmits the packet generated by the packet generation unit 57 to a client . fig1 shows the arrangement of client computers 5 a to 5 e . as shown in fig1 , each of the client computers 5 a to 5 e comprises a packet reception unit 41 , packet analysis unit 42 , power supply control unit 43 , battery 44 , external power supply circuit 45 , and timer 71 . the packet reception unit 41 receives a packet 21 which is sent from the server and contains information associated with power supply control . the packet analysis unit 42 analyzes information associated with power supply control , which is contained in the packet 21 received by the packet reception unit 41 . more specifically , the unit 42 analyzes information associated with charging of the battery , information indicating the power supply destination , and information indicating the time at which control associated with charging of the battery and supply of the power supply is to be started , which are contained in the first byte of the peak shift control field 27 in the packet 21 , and informs the power supply control unit 43 of the analysis result . the power supply control unit 43 controls the battery and external power supply circuit at the predetermined time on the basis of the analysis result of the packet analysis unit 42 . more specifically , when bit “ c ” ( in the peak shift control field 27 contained in the packet 21 )= 1 , charging of the battery is inhibited at the time indicated by the time information ; when “ c ”= 0 , inhibition of charging of the battery is canceled at the time indicated by the time information . when “ b ”= 1 , electric power from the battery 44 is preferentially used at the time indicated by the time information ; when “ b ”= 0 , electric power from the external power supply circuit ( ac adapter ) 45 is preferentially used at the time indicated by the time information . whether or not the time is reached is determined by the power supply control unit 43 with reference to the value of the timer 71 . the operation of the power management system according to the second embodiment of the present invention will be described below with reference to the flow charts in fig1 and 14 . initially , parameter information used to determine a power supply control pattern is acquired in the server 3 ( s 31 ). in this embodiment , since the power supply control pattern storage unit 55 stores the spring , summer , autumn , and winter power supply control patterns 56 a , 56 b , 56 c , and 56 d , date & amp ; time information required to determine one of these patterns is acquired from the rtc 52 as a parameter . when power supply control patterns are set in correspondence with temperatures , temperature information from the sensor 51 is acquired as a parameter . a power supply control pattern is determined on the basis of the parameter information acquired in s 31 ( s 32 ). for example , if the date & amp ; time information as the parameter information indicates “◯:×, aug . 15 , 2002 ”, the summer power supply control pattern 56 b is selected ; if the date & amp ; time information as the parameter information indicates “◯:×, dec . 15 , 2002 ”, the winter power supply control pattern 56 d is selected . after that , information associated with the control time and use of a power supply is acquired from the power supply control pattern determined in s 32 ( s 33 ). more specifically , information indicating the times at which the first and second threshold values described in the first embodiment are assumed to be reached , and information indicating charging of the battery and the power supply destination are acquired from the determined power supply control pattern . a packet is generated by setting the information acquired in s 33 in the peak shift control field 27 shown in fig1 ( s 34 ), and the generated packet is transmitted to the client computer ( s 35 ). fig1 is a flow chart for explaining the operation of the client computer . the operation of the client computer according to the second embodiment of the present invention is basically the same as that in the flow chart shown in fig9 , except that the step ( s 21 ) of determining whether or not the control time is reached is executed between s 11 and s 12 . that is , in this embodiment , since the peak shift control field 27 in the packet sent from the server contains information associated with the execution time of control of the power supply and charging , the client computer refers to the information associated with the time , and starts execution of the power supply control when the time indicated by that information associated with the time is reached . therefore , according to the second embodiment of the present invention , parameter information is acquired , a power supply control pattern is determined based on the acquired parameter information , and charging of the battery and a power supply to be used of the client computer are controlled in accordance with the determined power supply control pattern . therefore , adaptive peak power shift control can be implemented without limiting the functions of the client computer . since the packet contains time information , information associated with peak shift control for a day can be simultaneously transmitted to the client computer . a power management system according to the third embodiment of the present invention will be described below . the power management system according to the third embodiment of the present invention controls power supplies for respective client computer groups in a peak power shift time band , thereby leveling peak power . fig1 is a block diagram showing the arrangement of a server in the power management system according to the third embodiment of the present invention . as shown in fig1 , the server according to the third embodiment of the present invention comprises a peak power shift time band determination unit 81 , grouping unit 82 , client group management table 83 , time band determination unit 84 , packet generation unit 85 , and packet transmission unit 86 . the power shift time band determination unit 81 determines a time band in which peak power shift control is to be made . this peak power shift time band can be predicted on the basis of power load information sent from the power company in the first embodiment , or can be obtained from the power supply control pattern in the second embodiment . in this embodiment , a method of obtaining the peak power shift time band is not particularly limited . the grouping unit 82 determines client groups which are to undergo peak power shift control on the basis of the client group management table 83 . the client group management table 83 manages clients to be controlled and their addresses for respective groups . in this embodiment , as an example of grouping , the client group management table 83 is referred to . however , various other grouping methods may be used . for example , the server reads the hardware serial numbers of client computers , and executes peak power shift control in the former or latter half of the peak shift time band depending on whether the hardware serial number is an even or odd number . since the method of reading the hardware serial number of a client computer is the state - of - the - art technique , and is known as dmi ( desk top management interface ), a description thereof will be omitted . note that this method of reading the hardware serial number of a client computer is preferably implemented as an application ( utility ) software for peak power shift control . of course , this method can be implemented as firmware of a bios ( basic input output system ) or ec ( embedded controller ). the time band determination unit 84 assigns the peak power shift time periods determined by the peak power shift time band determination unit 81 to respective groups . fig1 shows an example of grouping . fig1 exemplifies a case wherein the peak power shift time band is divided for groups a to d . in the example of fig1 , peak power shift time periods are equally assigned to the respective groups during the peak power shift time band . for example , “ 13 : 00 to 14 : 30 ” for group a , “ 14 : 30 to 16 : 00 ” for group b , and so forth are set . fig1 shows another example of grouping . in this example , the peak power shift time band is divided into given time periods ( e . g ., 1 hour ), and the divided peak power shift time periods are assigned to the respective groups . the packet generation unit 85 generates a packet by embedding information associated with a power supply and information associated with the peak power shift time band determined by the time band determination unit 84 in the peak power shift control field 27 in the packet shown in fig3 for each group . note that the information associated with a power supply means information associated with charging of a battery and use of a power supply described in the first and second embodiments . the packet transmission unit 86 transmits the packet generated by the packet generation unit 85 to the client computers of the corresponding group . fig1 is a flow chart for explaining the operation of a power user corporation management server in the power management system according to the third embodiment of the present invention . initially , the server determines a peak power shift time band ( s 51 ), and determines groups of client computers ( s 52 ). the determination of peak power may be made with reference to a power load signal supplied from the power company ( as in the first described embodiment ) or may be made by reference to a stored pattern based on time of day / year and / or temperature data ( as in the second described embodiment ). then , the server determines peak power shift time periods of respective groups determined in s 52 in the peak power shift time band determined in s 51 ( s 53 ). after that , the server generates a packet set with information indicating the peak power shift time period determined in s 53 and information associated with a power supply for each group ( s 54 ). the packet generated in s 54 is transmitted to the corresponding group ( s 55 ). therefore , according to the power management system of the third embodiment of the present invention , since the peak power shift time band is divisionally assigned to a plurality of groups , an abrupt change in peak power can be avoided . as a result , safety of the power system can be assured . in the above description , client computers included in a given group simultaneously undergo power supply control . this modification will exemplify a case wherein client computers included in the group do not simultaneously undergo power supply control . in such case , each of the client computers has a unique delay time period , and starts power supply control from a time obtained by adding the unique delay time period to the power supply control time contained in the packet sent from the server . in this way , the client computers even in a single group can be prevented from simultaneously undergoing power supply control . fig1 is a flow chart for explaining the process of such clients . this flow chart is basically the same as those shown in fig9 and 13 , except that a process in s 61 is added after the process in s 11 . that is , in s 61 , a time obtained by adding the unique delay time period to a time indicated by information associated with the execution time of control of a power supply and charging , which is contained in the packet , is determined as a control time . in this way , client computers in a given group can start power supply control at individual times , thus preventing an abrupt change in power load . fig2 is a chart for explaining the execution timings of power supply control of client computers included in a single group . fig1 shows a case under the condition that client a has a delay time period of 0 min ; client b , 10 min ; client c , 20 min ; and client d , 30 min . in the modification of the embodiment , client computers included in a given group undergo peak power shift control . however , each individual client server executes peak power shift control without grouping . in this case , for example , when the peak power shift time period of respective client computers is 2 hours , if the start times are uniformly distributed and assigned from noon to 2 p . m ., the peak shift effect gradually increases from noon to 2 p . m ., and gradually decreases after 2 p . m . at 4 p . m ., the peak shift effect becomes zero . in this case , if each client computer cancels inhibition of battery charging 2 hours after the end time of its peak power shift control , a power demand for changing gradually increases after 4 p . m . in the first to third embodiments , when the control time is reached , the power supply control is executed . thus , there is a central management of the power load as controlled by the server . however , some users may not want to switch power supply control . in such case , step 71 of confirming if power supply control is to be switched is inserted in the client computer process , as shown in fig2 , and only when permission is granted , the subsequent process may be executed . fig2 shows an example of a user interface window 91 used to confirm such switching of power supply control . therefore , in the embodiment of the present invention , upon executing the peak power shift control , since power shift control is distributed and executed for respective groups or client computers , an abrupt change in peak power load can be prevented . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .