Solar Pump: How Many Batteries Do I Need?

Usually, this question is better phrased as “How long do you want your Solar pump to RUN?”, then specific calculations can be made to determine the proper battery bank size.

Remember “RUN” is an important word. Calculate battery size for constant “RUN” time and for normal use, since the Solar pump will turn on/off depending on the desired period and the external ambient temperature.

Time for some math!

STEP 1
Amps Calculations
Watts = Volts x Amps
Battery capacity is expressed by how many Amps for how many hours a battery will last. Amp-Hour (A.H.) capacity

Estimate the total Watts (or Amps) the Solar pump uses while operating, and how long the pump needs to operate. The electrical loads are not constant, so estimations must be made.
​Suppose you were to run an Solar pump for one hour, which draws 500 Watts from a Solar Panel (for simple calculations).

500 Watts from a 12-volt battery requires the battery to deliver approximately 42 Amps.
(500 Watts ÷ 12 Volts = 42 Amps)
500 Watts from a 24-volt battery requires the battery to deliver approximately 21 Amps.
(500 Watts ÷ 24 Volts = 21 Amps)
500 Watts from a 48-volt battery requires the battery to deliver approximately 10 Amps.
(500 Watts ÷ 48 Volts = 10 Amps)

Step 2
Amp-Hour Calculations
If the hypothetical Solar pump is to operate for 3 hours:
​Taking 500 Watts from a 12-volt battery requires the battery to deliver approximately 84 Amps.
(500 Watts ÷ 12 Volts = 42 Amps) x 3 = 126 Amps
Taking 1000 Watts from a 24-volt battery requires the battery to deliver approximately 41 Amps.
(500 Watts ÷ 24 Volts = 21 Amps) x 3= 63 Amps
​Taking 1000 Watts from a 48-volt battery requires the battery to deliver approximately 21 Amps.
(500 Watts ÷ 48 Volts = 10 Amps) x 3 = 30 Amps
​
For a 12-Volt battery: 84 Amps DC x 3 hours = 126 A.H.
For a 24-volt battery: 41 Amps DC x 3 hours = 63 A.H.
For a 48-volt battery: 21 Amps DC x 3 hours = 30 A.H.

Example 1
A swimming pool Solar pump running at 500-Watt, running 1/3 of the time would be estimated at 166.66 Watts-per-hour. (500/3 = 166.66 watts )
Example 2
A hypothetical solar swimming pool pump draws 13.8 Amps at 12 Volts DC. By multiplying 14 Amps x 12 Volts, you find out the pump uses 166 Watts. The batteries will need to deliver 13.8 Amps to run the pump (166 Watts/12 Volts = 13.8 Amps).
Example 3
Let’s say you have a 100A.H battery for your system that is 12 volts (push) and 100 amp hours (storage capacity).

You could find out approximately how much energy this battery will store/provide by calculating the watt-hours. To do this, just multiply the volts (V) x the amp hours (AH) and divide by 500 (the hypothetical solar swimming pool pump).
Volts x Amp Hours / 500 = Watt Hours
12V x 100AH = 1200 / 500 = 2.4 Watt Hours
What this means is that you can power a 500-watt solar swimming pool pump for 2.4 hours on a fully charged battery.

STEP 3
Battery Usage
You do not want to use 100% of the battery’s capacity for fear of ruining the battery. When calculating battery size remember to also calculate a safe usable percentage of the batteries.
​(battery capacity)mAh ÷ (solar swimming pool pump consumption)mA * 0.7 = estimated hours
(battery capacity)mAh ÷ (solar swimming pool pump consumption)mA * 0.6 = estimated hours
(battery capacity)mAh ÷ (solar swimming pool pump consumption)mA * 0.5 = estimated hours
(battery capacity)mAh ÷ (solar swimming pool pump consumption)mA * 0.4 = estimated hours

​Battery Usage Example
Suppose you operate a swimming pool Solar pump for one hour, which draws 500 Watts (for simple calculations).
​Taking 500 Watts from a 12-volt battery requires the battery to deliver approximately 42 Amps.
(500 Watts ÷ 12 Volts = 42 Amps)
​(100)mAh ÷ (42)mA * 0.7 = 1.67 estimated hours
(100)mAh ÷ (42)mA * 0.6 = 1.43 estimated hours
(100)mAh ÷ (42)mA * 0.5 = 1.19 estimated hours
(100)mAh ÷ (42)mA * 0.4 = 0.95 estimated hours

Let’s put this all together.
​Step 1:
​Determine swimming pool Solar pump Watts.
Watts = Volts x Amps
Step 2:
Determine pump Amp Hours.
Amps = Volts x Watts
​Step 3:
Determine Battery Amp Hours.
Batteries x Amp Hours
​Step 4
Determine total battery system capability based on percentages.
mah÷mA*(percentage of charge)= total estimated run time