100 Watts to Amps

One rating, five different currents

Divide the power by the voltage and the current falls out: A = W / V. Because the denominator changes while the numerator stays fixed at 100 watts, the same load pulls widely different currents on different systems, and that difference drives every fuse and wire choice downstream. Multiply back to check: 12 V x 8.33 A = 100 W, and 120 V x 0.83 A = 100 W.

That doubling rule is worth internalizing: halve the voltage and the amps double. A 100 watt panel on a 12 volt battery bus moves 8.33 amps of charging current, while a 100 watt bulb screws into a 120 volt socket and moves less than one amp. Same watts, a tenfold jump in amps, and only the system voltage changed.

Worked example: a 100 watt bulb on a 120 volt circuit

The classic 100 watt incandescent on a household circuit draws 100 / 120 = 0.83 A, or 833 mA. Left on 5 hours a day it uses 100 x 5 = 500 watt-hours, which is 0.5 kWh a day and 0.5 x 365 = 182.5 kWh over a year. The EIA puts average household electricity use at about 10,500 kWh a year, so one such bulb accounts for roughly 1.7 percent of the whole house, night after night.

The DOE's Energy Saver guide (2022) advises shoppers replacing a 100 watt incandescent to look for an LED producing about 1,600 lumens, and notes that ENERGY STAR certified LEDs use up to 90 percent less energy. A 12 watt LED in the same socket draws 12 / 120 = 0.1 A, one eighth of the incandescent current, and the household wiring and dimmer never notice the swap. Watts describe the bulb, amps describe what the circuit carries, and both numbers shrink together.

Worked example: a 100 watt solar panel on a 12 volt battery

Panel ratings are in watts, batteries live in amp-hours, and the link between them is the bus voltage. A 100 watt panel at full output pushing into a 12 volt battery bank delivers 100 / 12 = 8.33 A of charging current. Over five good sun hours it makes 100 x 5 = 500 watt-hours, which is 500 / 12 = 41.7 amp-hours into the bank. NREL's PVWatts calculator models a grid tied array's monthly output from rated capacity, tilt and derate factors, and the same watts to amps division sizes the charge controller at any scale.

Controller sizing is where the amps become a purchase decision. A controller rated 30 A can absorb 30 / 8.33 = 3.6 panels' worth of current, so three 100 watt panels add 3 x 8.33 = 25 A and fit with room to spare, while four panels would push 4 x 8.33 = 33.3 A and overload it. Count panels by amps first and watts second, because the controller never sees watts directly.

Worked example: a 100 watt laptop away from the wall

A laptop charger drawing 100 watts from a 120 volt outlet pulls 100 / 120 = 0.83 A. The wall cord stays thin and light because the current is under one amp.

Feed the same charger from a 12 volt bank through an inverter and the low voltage wiring must supply 100 / 12 = 8.33 A, a factor of ten above the wall figure, before the inverter's own overhead. A 100 Ah 12 volt battery stores 100 x 12 = 1,200 watt-hours, so the runtime ahead of that overhead is 1,200 / 100 = 12 hours in the ideal case.

This is why mobile workstations care about amps. Two such chargers at once double the battery side to about 16.7 A, and the cable between battery and inverter, not the wall cord, becomes the part that has to carry it. The same 100 watts is an easy load on a wall circuit and a serious load on a battery bank, and amps are how you tell the two apart.

Inverter overhead when the 100 watts come from a battery

A 100 watt AC load fed from a 12 volt battery through an inverter draws more than 8.33 A on the battery side, because the inverter loses part of the energy as heat along the way. Divide the AC watts by the inverter efficiency to get the DC watts, then divide by the battery voltage for the real current. At 85 percent efficiency the battery supplies 100 / 0.85 = 117.6 watts, which is 117.6 / 12 = 9.8 A, close to a fifth again the textbook figure.

The table explains why two inverters with the same 100 watt rating can move different currents: the wasteful one pulls harder on the same battery. Small inverters at light loads sit near the bottom of their efficiency curve, so a real draw can land between two rows. Sizing the battery cable and fuse from the nameplate 8.33 A alone can leave them undersized once the losses land on the low voltage side.

Fuses and wire for a 12 volt 100 watt load

At 12 volts the steady current is 8.33 A, so the fuse must sit above that with headroom for surges, and the equipment manual wins wherever it names a size. A fuse protects wire rather than devices: it goes near the battery so the run it guards stays short, and it must open before the conductor can overheat.

Wire sizing follows two limits. The first is heating: the conductor must carry 8.33 A continuously without cooking its insulation, which is a question for an ampacity table matched to the insulation type and installation conditions. The second is voltage drop, and ohm's law gives it directly: V = I x R. Suppose the round trip path adds 0.12 ohm of resistance. The drop is 8.33 x 0.12 = 1.0 volt, so the 12 volt battery delivers 11 volts at the load, and the missing volt turns into heat along the wire. Move the same 100 watts to a 24 volt bus and the current halves to 4.17 A: the drop falls to 4.17 x 0.12 = 0.5 volt, and the wasted heat, which grows with current squared, falls to a quarter.

That squared law is the whole argument behind higher voltage battery banks. Halving the current cuts heating losses to a quarter, which is why long wire runs at 100 watts favor 24 or 48 volts, and why 12 volt runs should stay as short as the layout allows.

What the calculator returns

The calculator mirrors the arithmetic used throughout this page. Type any wattage alongside a voltage to read the current, add a second voltage to compare a battery bus against a wall socket, and type an amp reading to recover the wattage that produced it. With the defaults of 100 watts at 12 volts against 120 volts, the readout shows 8.33 A (8,333 mA) on the low voltage side, 0.83 A on the wall side, 100.00 W on the reverse check, 200.00 amp-hours moved in a day and 2,400 watt-hours of energy.

Every figure follows from A = W / V or its inverse, so when a spec sheet quotes a fuse, a controller or an inverter in amps, divide the watts by the system voltage and confirm the two agree before buying anything. The watts to amps page covers the general conversion, and the amp-hour and watt-hour calculators pick up the battery side of the same arithmetic.

Runtime on a 100 Ah battery at different depths of discharge

A 100 Ah 12 volt battery holds 100 x 12 = 1,200 watt-hours on paper, but lead chemistry ages fast when emptied completely, so planners count only the portion they are willing to use. Lithium cells tolerate deeper cycling, and the table shows how the same battery supports a 100 watt load depending on where the floor is set.

Every row divides the same way: usable watt-hours divided by 100 watts gives the hours, and multiplying back by the bus voltage turns hours into the amp-hours actually withdrawn. At the 50 percent floor the load moves 600 / 12 = 50 amp-hours, half the nameplate rating, which is the point of the floor. Rest losses, inverter overhead and cold weather all shave the real figure lower, so treat the table as a ceiling rather than a promise.