The Summer Cottage WorkSheet

 

Appliance Type
AC
DC
Rated Wattage/Hr
Hours on
Hrs used per day AC
Hrs used per day DC
6 low watt bulbs
yes
13
4
312W
stereo
yes
 
50
2
100W
brew coffee maker
yes
1200
10/60min
200w
toaster
yes
1200
1 cycle
150W
Ceiling Fan
yes
75
3
225W

20" colour tv

yes
 
100W
2hrs
200W
Satellite dish/receiver
yes
25
2
50W
cordless phone
yes
 
3.6
24
86W
Mac Laptop
yes
 
85w
3
255w
microwave
yes
 
900w
1
900W
Refrigerator
yes
 
150w/cycle
24
1200W
CD/DVD
yes
 
25W
3
75W
 
 
Total
3753W/day

 

Calculating Your PV size:

We will assume that you live in Canada in Southern Ontario and that you wish to setup in Cottage country in the Muskoka area. If you check the map of peak sun hours you will find that it is 4.4hrs/day during the spring summer season. For your specific area, please check the USA Natural resources sites or if you are in Canada please click on the "map of peak sun hours" link.

To calculate the PV size in the above example.

3753 Watt-hours/day 4.4 sun hours/day = 853 W PV Panels

We must derate the manufacturers ratings (by assuming a 20% derating factor):

853 Watt real power rating ÷ 0.77 reality factor = 1108 Watts/Hrs/day Manufacture rating

There are two ways of looking at this power rating:

1) If you are a weekend warrior, that means all week long you can have your PV panels charge your batteries to be ready for use the 2 or 3 days while you are at your cottage, then this means you can use fewer panels to charge your batteries.

Qty of 4 - 65 Watt PV modules = 260 Watts/day x 5 days charging = 1300 Watts

2) If you spend the entire summer season at the cottage, you should install enough panels to cover the full power rating calculated. We can provide you with 85W or even 165W panels. The choice is yours as to whichever you decide to use.

1108 watts/day ÷ 85 watt modules = 13 PV modules or

1108 watts/day ÷ 165 Watt modules = 7 PV modules

Batteries are an essential part of this PV system on non-grid connected systems. If you are Grid-tied or Grid-interconnected, batteries are an option and not essential unless you want battery backup power for when there is a power interruption.

For this summer cottage, the power generated needs to be stored in batteries for later use. Here is how to determine your battery backup:

To determine the voltage you will need in batteries; use the following guideline.

A seasonal cottage with four occupants, refrigeration and limited water pumping. A 500 watt peak PV array should do fine. This would be a 12V system wired both for DC and AC depending on the load.

A Full-time cottage or residence, including low power television, stereo, highly efficient EnergyStar compliant clothes washer and refrigerator, will require a 800 to 1000Watt PV array, wired for 24 Volts. During the Winter "dark months" an backup generator would be required. You can counter that with more PV panels. Below 7000Watts per day, a 24Volt system would work fine. This would suit a family of four.

A daily energy consumption above 7000 Watts per day, a 48 Volt PV system will be required. A full-time residence using a dishwasher, clothes dryer (gas powered), central vacuum, washing machine and electric refrigeration would require a PV array of 2000Watts or higher depending on the load. Probably a backup generator would be needed for the dark winter months.

Estimating the Required Battery Capacity (amp/hours)

 

Nominal Voltage of battery: (choose 12, 24, 48) _______ VDC

Decide the number of days of battery storage you need. (Rule of thumb is two days for a hybrid system and three days for an non-grid connected system)

_______ days

 

Battery capacity in (amp hours):

 

Total daily Load (Whrs/d) X (days of Storage) ÷ {Battery Voltage (VDC) X 0.42}

=1108Wh/d x 3 ÷ (24VDC x 0.42)

=3324 ÷ 10.08

=330 Ah

The factor 0.42 assumes an 85 percent Battery effeciency and a 50 percent maximum depth of charge. For colder climates a higher battery capacity in Ah will be required. Increase the battery capacity by 30% for temperatures of (-20 C or -4 F) 429 Ah for colder climates.