There are many specifications you need to know when considering a purchase of a solar battery. The most important ones are the battery’s capacity & power ratings, depth of discharge, efficiency rating, battery life & warranty, and manufacturer.

  • Capacity (or Energy) is the total amount of electricity that a solar battery can store (in kWh or Ah).
  • Power Rating is the amount of power a battery can deliver at one time (in kW).
  • Depth of Discharge (DoD) refers to the amount of a battery’s capacity that can be used before recharging. The higher the DoD the greater the amount of the battery’s capacity can be used.
  • Efficiency (Beff) rating is the amount of energy the battery takes to store it to the amount of energy that can be used from it as a percentage. Usually, a higher efficiency rating of a battery means better economic value.
  • A Warranty is a manufacturer’s assurance that guarantees a certain number of charging cycles and/or years of useful life for a battery.
  • Manufacturer depends on your priorities – evaluating the warranties associated with each product can give additional guidance as you make your decision.
  • Life of a Battery, depends on the brand of battery you buy and and how much capacity it will lose over time.

12V Battery Setup Diagrams:

Solar Batteries Calculator
Number of batteries required
Total kWh/day needed:
System volts (12,24,48)volts:
Battery volts (2,6,12)volts:
Battery capacity (Ah):
Battery efficiency (Beff)%:
Depth of Discharge (DoD)%:
Inverter efficiency (Ieff)%:
Days before Charging (DbC):

DoD = Depth of Discharge – typically (Lithium-ion) 80%
DbC = Days1 before Charging
Ieff = Inverter efficiency – typically 85%
Beff = Battery efficiency – typically (Lithium-ion) 95%
Ah = (Wh / Volts) x 100/DoD x DbC x 100/Ieff x 100/Beff
1The number of days to run the battery before a recharge is needed.

Example Calculations:

Example 1:
If not a constant load, e.g., fridge, draws 20amps for 1 sec then 0.1amps for the rest of the hour, what amount of energy does the fridge use?

C = a[amps] x t[secs]/3600(secs in an hr) + (a'[amps] x t'[secs])/3600(secs in an hr)
C = 20 x 1/3600 + 0.1 x 3599/3600 = 0.1055amps
Therefore, the amount of energy the fridge uses is 105.5mAh.

Example 2.
What size 12v battery (in Ah) is required to run a 250W machine for 5hrs?

Let Inverter efficiency (Ieff)= 85%, DoD = 80%, and Beff = 95%

Find initial Wh of the machine:
Wh’ = watts x hrs = 250 x 5 =1250Wh or 1.25kWh
Actual Wh = Wh’ x 100/Ieff = 1250 x 100/85 = 1470Wh or 1.5kWh

Now calculate the actual size (or capacity) of the 12v battery needed:
C’ = Wh/V = 1470/12 = 122.5Ah
Actual C = C’ x 100/DoD x 100/Beff = 122.5 x 100/85 x 100/95 = 151.7Ah
Therefore, we need a 200Ah battery to run the 250W machine for 5hrs.

Example 3:
A home requires 9.6kWh/12hrs (overnight) to run all the necessary appliances. How many 12v, 200Ah batteries are needed?

Initial Capacity, C’ = kWh x 100/Ieff = 9.6 x 100/85 = 11.29kWh

Number of 12v batteries needed:
Actual Capacity, C = (11.29kWh x 100/80%DoD x 100/95%Beff) / 12v = (14.86kWh x 1000) / 12v = 1238Ah
So the number of batteries required = (Ah needed)/(Ah rating of the batteries) = 1238/200 = 7 of 200Ah batteries connected in parallel.



The Franklin Home Power unit with a backup gateway, aGate:

It is my preferred battery system due to the gateway (aGate) attachment, which allows total home circuit isolation from the grid during a power outage. The Franklin Home Power with the aGate offers full use of the solar panels and batteries while not requiring a separate backup load circuit to the house during a power outage (refer to the Franklin Home Power Solution diagram below). This battery system also has the option that allows the connection to a backup generator unit.