Owners Manual
SILENT RUNNER INVERTER/CHARGER POWER SYSTEM
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8.2
DC Power Consumption
An inverter takes in DC power, and produces AC power to operate attached loads. In general, we can see a
direct relationship between DC input power and AC output power. This allows us to establish the following
rule:
For every 10 watts of AC output power, an inverter requires one amp of DC input power on a 12
volt input inverter.
Example:
An inverter powering a 1,000 watt AC load requires 100 amps DC.
1000 watts/10 = 100 amps
Using our rule, we can determine the requirements for an electrical system needed to power our
inverter (typically, an alternator and battery combination, or a photovoltaic panel and battery
combination).
Inverter DC Input Current Requirements
A DC to AC inverter converts DC power into AC power. For the purposes of this discussion, power (watts) is equal
to the supply voltage (volts) multiplied by the current draw (amps) from the supply for both AC and DC circuits.
For example, 2400 watts = 12 volts DC x 200 amps, and 2400 watts = 120 volts AC x 20 amps. From these two
examples of 2400 watts at 12 volts and 2400 watts at 120 volts, it is easy to see that since there is a 1 to 10 voltage
conversion (12 to 120), there is a 10 to 1 amp conversion (200 to 20). A more accurate relationship between the
input power and output power is:
(DC Input Power) x (Efficiency) = (AC Output Power)
This formal relationship has lead to the following rule of thumb for estimating the DC input amps for an inverter:
For 12 volt DC inverters: output watts ÷ 10 = DC input current
This rule of thumb can be used to estimate the minimum alternator size required for your application and is also used
later in calculating the minimum size battery required when operating from battery only. The following examples
should help to clarify the use of this rule of thumb.
Example A:
What is the DC current draw of a 12 volt DC input inverter when it is operating a vacuum cleaner with a
name plate rating of 6 amps at 120 volts AC?
The appliance rating is given in amperes, so we must first calculate the power it consumes. Then the rule of
thumb can be used to find the DC input current of the inverter.
Output power =
120 volts x 6 amps = 720 watts, and DC input current = 720 ÷ 10 = 72 amps DC.
This information on estimating the DC input current requirement for an inverter will allow you to size an
alternator or charging system to supply an inverter for continuous operation.
