How do I calculate how many batteries I need?

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  • [see Forum Thread this is taken from for more]

    How many batteries do I need?
    Simple Answer:
    • Lead:
      • Number of watts per hour /.5 x number of hours of backup / .8.
      • Example: 107W/h / .5 x 24 hrs / .8 = 6420 Watts , AH = w/v, so 535 AH @ 12V
    • LiFePO4:
      • Number of watts per hour / .9 x number of hours of backup / .8.
      • Example: 107W/h / .9 x 24 hrs / .8 = 2854 Watts, 238 AH @ 12V
    But, it's not quite that simple!

    [see Forum Thread this is taken from for more]

    The actual capacity of a lead acid battery, for example, depends on how fast you pull power out. The faster it is withdrawn the less efficient it is.

    For deep cycle batteries the standard Amp Hour rating is for 20 hours. The 20 hours is so the standard most battery labels don’t incorporate this data. The Amp Hour rating would mean, for example, that if a battery has a rating of 100AH @ 20 Hr rate, it can be discharged over 20 hours with a 5 amp load. If it has the rating of 200 AH, it can handle a 10 amp load for 20 hours.

    Deep Cycle Battery datasheets will often show more detailed characteristics such as this Trojan:
    TROJAN T-105 RE 225Ah 6V Battery
    C-Rate2-Hr Rate5-Hr Rate10-Hr Rate20-Hr Rate48-Hr Rate72-Hr Rate100-Hr Rate
    In this example you can see that when discharging the T-105 battery within 2 hours, it only stores 146 Ah and not 225 Ah.

    All batteries are affected by temperature. For example this is from the Trojan SPRE 12 225's datasheet :

    Lithium batteries are extremely sensitive to freezing temperaturs and can be damaged by charging at low temperatures. In extreme temperatures these batteries should be automatically disconnected or have a device to keep them warm.

    Finally, most energy storage devices loose power over time. From the chart below you can see the Trojan SPRE 12 225 looses about 15% power per month.

    So, which battery has more energy? You'll have to understand how you will use it and check the datasheets to know!
    Another thing to consider:

    [see Forum Thread this is taken from for more]

    How many Batteries do I need?

    To answer this, you need to know your power consumption rate, how long you run it for, and much reserve you want for rainy days.

    Let's say you look at your monthly power bill and it says you consume on average 892 kWh in 31 days. So, 892/31/24 = 1.2 kWh/hr

    Discharging from a battery has inefficiencies, lead around .88 and lithium .96 to .98. So, if you're using Lithium it's 1.2/.96=1.25 kW/hr

    With that number we can see the power consumed per day is 24 x 1.25 = 30 kWh.
    If you want enough power for 3 days, you'd need 30 x 3 = 90 kWh.

    As discussed in the post above, the power in batteries are rated at a standard temperature, the colder it is the less power they have.
    You should check the actual datasheet for your batteries, but for typical lead acid it might be:
    Temp FDe-rating Factor
    So, with batteries expected to be at 40 to supply 10 kWh, with this data you'd multiply by 1.3 to see you would need 13 kWh of batteries.

    A Tesla power wall is ~$700/kWh, so for 90 kWh it would cost $63,000.
    This illustrates why it's so easy to get frustrated with batteries. Solar is cost effective, but batteries? Not so much right now. But prices are falling and new technologies are emerging.

    The trick to minimizing your battery needs are to first reduce your power needs. For example, for emergency power you could turn your hot water tank off the breaker, they consume an average of 4 kWh/d.
    And yet another thing to consider:

    [see Forum Thread this is taken from for more]

    Designing a Battery Bank

    Batteries come in discrete sizes: 18 Ah, 100 Ah, 200 Ah and so forth. When you need more stored energy than can fit in a single battery it is common to put batteries in series in strings, and to have multiple parallel strings. This works the same way as with solar panels in regards to voltages and currents, so if that's not clear to you start with What does it mean to have solar panels in parallel and series?

    12, 24, 48, 300V?
    Conversion efficiencies are typically higher with higher voltages and higher voltages for the same overall power demands mean lower current so less expensive wiring. But many people chose to stay at low voltages for compatibly with existing equipment.

    How do I convert my Watt Power needs into a number of battery Ah?
    You need 6 kWh/day and you want 3 days autonomy: 6000 x 3 = 18,000 Wh
    You've selected lead acid batteries and you pick a conservative 40% Depth of Discharge: 18,000 / 0.4 = 45,000 Wh
    You need that 6 kWh/d day when the ambient temperature will be 60F: 45,000 X 1.11 = 49,950 Wh.
    Let use a 48V battery string. Watts = amps x volts, so amps = watts/volts: 49,950 / 48V = 1040 Ah

    How do I design my Battery Bank?
    When using lead-acid batteries it's best to minimize the number of parallel strings to 3 or less to maximize life-span. This is why you see low voltage lead acid batteries; it allows you to pack more energy storage into a single string without going over 12/24/48 volts.

    There are many configurations that could work in the example above:
    • 4x 12V batteries rated at 1040 Ah
    • 8x 12V batteries in two strings of 4 all rated at 520 Ah
    • 16x 6V batteries in two string of 8 all rated at 260 Ah