1. Total average amp hours per day from the System Loads Worksheet,
line 10. 

2. Maximum number of continuous cloudy days expected in your area. 

3. Multiply line 1 by line 2. 

4. Divide line 3 by (maximum) 0.8 to maintain a 20% reserve after deep
discharge period.
To prevent less than a maximum 80% discharge divide by a lesser
number in #4 above. 

If no special conditions below apply, skip lines 5 through 9 and
proceed to line 10.
Special Conditon #1: Heavy electrical load
5. Maximum amperage that will be drawn by the loads for 10 minutes
or more. 

6. Discharge rate of battery. If unknown, check with battery supplier. 

7. Multiply line 5 by line 6. 



Special condition #2: High Charge Current
8. Maximum output amperage of PV array or other battery charger. 

9. Multiply line 8 by 10.0 hours. 

10. Amp hours from line 4, 7, or 9, whichever is largest. 

11. If you are using a leadacid battery, select the multiplier
below which corresponds to the battery's wintertime average ambient temperature:
Battery Temperature Multiplier 
80°F/26.7°C 
1.00 
70°F/21.2°C 
1.04 
60°F/15.6°C 
1.11 
50°F/10.0°C 
1.19 
40°F/4.4°C 
1.30 
30°F/1.1°C 
1.40 
20°F/6.7°C 
1.59 


12. Multiply line 11 by line 10. This is your optimum battery size
in amphours. 

13. Amphours of battery chosen. (L16=360, T105=225 etc.) 

14. Divide line 12 by line 13. This is the total number of batteries
in parallel required. 

15. Round off to the next highest whole number. 


