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  1. #1
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    Inverter Sizing and Battery Capacity

    I need someone with more electrical knowledge than I possess to check my math.

    Given:
    Television 45 Watts
    DVD 12 Watts
    A/C Fan Only 3.4 Amps

    A/C Fan Watts = 3.4 Amps X 120 VAC = 408 W

    Total wattage = 465 watts x 1.2 (factor of safety) = 558 watts.

    So a 600 Watt inverter would be the minimum if I want to run the TV, DVD, and fan together, right?

    Now for the battery endurance. I have a 80A-Hour battery. To calculate the load on the battery I take total actual wattage and divide by 12 VDC, right? That's be 465W/12VDC = 38.75 Amps meaning that I could only run all three items for just under 2 hours. Am I doing the math right for the 12 volt system?

    Thanks.
    Last edited by MJMCCAND; 04-21-2021 at 08:02 AM.
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  2. #2
    Paid my dues 😁 FT4NOW's Avatar
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    Sounds correct (just skimmed it over) but you also need to take into account non-inverted DC loads. Lighting, fridge, safety alarms, radio memory, etc. Also consider, not sure what kind of battery you have, but a lead acid battery for example, only has 50% usable AHs for what its rated for compared to lithium which is fully usable based on its rating.

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  3. #3
    Site Sponsor EsquireRV's Avatar
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    Quote Originally Posted by MJMCCAND View Post
    I need someone with more electrical knowledge than I possess to check my math.

    Given:
    Television 45 Watts
    DVD 12 Watts
    A/C Fan Only 3.4 Amps

    A/C Fan Watts = 3.4 Amps X 120 VAC = 408 W

    Total wattage = 465 watts x 1.2 (factor of safety) = 558 watts.

    So a 600 Watt inverter would be the minimum if I want to run the TV, DVD, and fan together, right?

    Now for the battery endurance. I have a 80A-Hour battery. To calculate the load on the battery I take total actual wattage and divide by 12 VDC, right? That's be 465W/12VDC = 38.75 Amps meaning that I could only run all three items for just under 2 hours. Am I doing the math right for the 12 volt system?

    Thanks.
    The type of battery will also determine available discharge capacity and therefore the actual expected runtime of any connected loads.
    Battery types include:
    Lead Acid (LA)
    Absorbed Glass Mat (AGM),
    Lithium iron phosphate (LiFePO4)

    If your battery type is Lead Acid, you should only discharge to 50% capacity before recharging, giving you only just under 1hr of runtime. Discharging a LA battery beyond 50% capacity shortens its life dramatically.
    Discharging a liFePO4 down to 10% doesn’t affect it at all.

    For more info, try out the inverter run time calculator on SamlexAmerica’s website:
    https://samlexamerica.com/run-time-calculator/

    To approximate the A/C load, I added both the Cordless Tool Charger and Impact Driver as loads adding up to 435 run time watts.

    Your biggest constraint seems to be the available Amp hour (Ah) in your battery.
    Last edited by EsquireRV; 04-21-2021 at 08:35 AM.
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