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I know that amperes are volts x watts. If I draw 5V at 3 amperes, is it almost the same as drawing 12V at 3 amperes from a 12V system? I expect the 5V would draw more as there is inefficiency when dropping the voltage.

What I'm looking to do is hook a Google Wifi up to an SLA battery and solar panel and I'm attempting to calculate the run time for the draw at 5V 3amperes.

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    \$\begingroup\$ Your math is a bit strange. 3 A at 5 V are 15 W, 3 A at 12 V are 36 W, more than twice times the power. \$\endgroup\$
    – Uwe
    Apr 17, 2022 at 14:36
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    \$\begingroup\$ Your power formula is wrong - it should be watts = volts times amps (P = EI). \$\endgroup\$ Apr 17, 2022 at 16:10

3 Answers 3

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If you want to hook a Google Wifi up to an SLA battery, you need a DC to DC converter to adapt the voltage. The Wifi needs 5 V but the battery delivers 12 V.

If you use a linear voltage regulator to produce the 5V, the current at 12 V and 5 V is the same, input power to the regulator is 3 A at 12 V = 36 W and output power is 3 A at 5 V = 15 W. The difference between input and output power is wasted, so 36 W - 15 W = 21 W are lost.

If you use a switching DC to DC converter, the input current of this converter is lower than the output current. An ideal converter with 100 % efficiency would take 15 W from the battery to produce the 15 W for the wifi. So the current at 12 V is only 15 W / 12 V = 1.25 A.

If the battery has 60 Ah, (ampere hours), the run time with the linear regulator is 60 Ah / 3 A = 20 h. If the switching DC to DC converter is used the run time is much longer, 60 Ah / 1.25 A = 48 h.

Real converters need a little power for themselve so we should assume the run time to be about 10 % less, so 18 hours or 43.2 hours.

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I know that amperes are volts x watts

That’s incorrect.

$$ \begin{aligned} P &= I \times V \ \ \ \ /V \\ \frac{P}{V} &= I, \end{aligned} $$

where P is power, I is current, V is voltage.

Amperes x Volts = Watts

3A x 5V = 15W

3A x 12V = 36W

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For DC circuits (e.g. a battery supplying power to a circuit) power (watts) is amps times volts. So 5V at 3A is 15 watts; 12V at 3A is 36 watts.

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