Amps vs Volts vs Watts vs Ohms: What's the Difference?

Amps, volts, watts, and ohms are the four core units used to describe how electricity behaves in any circuit, from a single AA battery to a full home solar system. Each one measures something different: amps track how much current is flowing, volts measure the electrical pressure pushing that current along, watts describe the power produced when the two combine, and ohms measure resistance to the flow. Together, these four units explain almost everything you need to know when wiring a circuit, sizing a battery bank, or choosing solar equipment. 

Understanding how they relate, and how to convert between them, is the difference between a system that runs safely and one that overheats, underperforms, or fails outright.

Amps, Volts, Watts, and Ohms: What's the Difference?

Volts measure electrical pressure, amps measure the rate of flow, watts measure the power that results from both, and ohms measure resistance to that flow. Voltage pushes, current flows, resistance opposes, and power is the work being done.

Two formulas link all four:

• Ohm's law: V = I × R

• Power formula: P = V × I

Everything else in electrical design comes from rearranging those two equations.

What Are Amps and What Do They Measure?

An ampere (amp) measures electric current, the rate at which charge flows past a point in a circuit. One amp equals one coulomb of charge per second, or roughly 6.24 × 10¹⁸ electrons passing a point every second.

The ampere is one of the seven SI base units. Since the 2019 revision of the SI, it is defined by fixing the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs. It is named after André-Marie Ampère, the French physicist who laid the foundations of electrodynamics in the 1820s.

Current is what your wiring, fuses, and breakers are sized around. Heat in a conductor rises with the square of current, which is why amps, not volts, determine cable thickness. A clamp meter measures current without breaking the circuit; a multimeter in series does the same for smaller loads.

In battery terms, amp-hours (Ah) describe how much current a battery can supply over time. A 100 Ah battery can theoretically deliver 5 A for 20 hours.

What Are Volts and What Do They Measure?

A volt measures electric potential difference, the pressure that pushes current through a conductor. One volt is the potential difference across a conductor when one ampere dissipates one watt of power, which is the same as delivering one joule of energy per coulomb of charge.

The unit honours Alessandro Volta, who built the first chemical battery in 1800. Common voltages you'll meet in Canada:

• 1.5 V, alkaline AA and AAA cells

• 3.2 V, a single LiFePO4 cell

• 12 V, 24 V, 48 V, standard DC system voltages for RVs, cabins and off-grid homes

• 120 V / 240 V at 60 Hz, Canadian household split-phase supply

Voltage exists between two points, never at a single point. That is why a multimeter needs two probes, and why "voltage to ground" and "voltage across a load" are different readings.

What Are Watts and What Do They Measure?

A watt measures power, the rate at which energy is used or produced. One watt equals one joule per second. The unit is named after James Watt, the Scottish engineer behind improvements to the steam engine.

Watts tell you how hard a device works at a given moment. Watt-hours (Wh) and kilowatt-hours (kWh) tell you how much energy it consumes over time, and kWh is the unit your utility bills you in.

A 100 W fridge compressor running for 6 hours consumes 600 Wh, or 0.6 kWh. This distinction matters when sizing a battery bank: your inverter is rated in watts, but your battery capacity is really about watt-hours.

What Are Ohms and What Do They Measure?

An ohm measures electrical resistance, how strongly a material opposes current. One ohm is the resistance that allows one ampere to flow when one volt is applied across it. The symbol is the Greek letter omega (Ω), and the unit is named after Georg Simon Ohm, who published the relationship in 1827.

Resistance depends on four things:

1. Material. Copper conducts better than aluminum; both beat steel.

2. Length. Longer conductors have more resistance.

3. Cross-section. Thicker wire has less resistance.

4. Temperature. Copper resistance rises as the conductor heats up.

Resistance is not always unwanted. Heating elements, incandescent filaments and current-limiting resistors exist precisely to convert electrical energy into heat or to restrict flow. In wiring, though, resistance is pure loss, it shows up as voltage drop and waste heat.

You measure resistance with an ohmmeter or a multimeter set to the Ω range, always with the circuit powered down. A reading of zero or near-zero across two points that should be separate indicates a short; an infinite reading where you expect continuity means an open circuit or a broken conductor.

How Amps, Volts, Watts, and Ohms Work Together

The four units are locked together by Ohm's law and the power formula. Know any two, and you can find the rest.

Worked example: a 12 V water pump draws 8 A.

• Power: 12 × 8 = 96 W

• Resistance: 12 ÷ 8 = 1.5 Ω

• Over 3 hours: 96 × 3 = 288 Wh

That last figure is what your battery bank actually has to supply.

Amps vs Volts: Understanding the Difference

Volts describe pressure; amps describe flow. A circuit can carry high voltage with almost no current, or low voltage with very high current. A 48 V battery bank and a 12 V bank can hold identical energy, but the 12 V version moves four times the current to deliver the same power.

That difference drives real design decisions:

• Amps size the conductor. Wire gauge, fuses, breakers, and busbars are all rated in amps.

• Volts size the insulation. Voltage determines clearance, insulation rating and shock risk.

• Amps do the damage. Current passing through the body causes injury; voltage is what drives it there. Neither is safe to treat casually.

Under CEC Rule 8-104, continuous loads, those running more than an hour in every two, must generally be limited to 80% of the circuit rating on standard breakers. That is an amps rule, not a volts rule.

Watts vs Volts: Why They Are Not the Same

Volts measure pressure. Watts measure the power that pressure delivers once current flows. Voltage alone tells you nothing about power output, you need current as well.

This is why appliance labels list both. A 1,500 W kettle on a 120 V circuit draws 12.5 A. The same 1,500 W on a 240 V circuit draws only 6.25 A. Identical power, half the current, and a much easier wiring job.

Higher voltage delivering the same wattage means:

• Thinner, cheaper cable

• Smaller fuses and breakers

• Less resistive loss and less heat at terminals

This is the whole argument for building larger off-grid systems at 48 V rather than 12 V.

Ohms and Resistance: How They Affect Current

Resistance limits current. From Ohm's law, if voltage stays constant and resistance doubles, current halves. In practical wiring, unwanted resistance shows up as voltage drop, energy lost as heat before it reaches the load.

Three places resistance costs you:

• Undersized cable. Thin wire over a long run drops voltage badly, especially in 12 V DC circuits where you have little margin to spare.

• Loose or corroded terminals. A poor connection is a small resistor generating heat exactly where you don't want it.

• Long runs. Resistance scales with distance, so array-to-controller and battery-to-inverter distances matter.

Under CEC Rule 8-102, voltage drop shall not exceed 3% in a branch circuit or feeder, or 5% overall from the supply point to the point of use. Unlike the equivalent NEC guidance, the Canadian limits are written as a requirement rather than a recommendation.

How These Electrical Terms Apply to Batteries and Solar Systems

Every component in a solar system is rated in some combination of these four units. Reading them correctly is how you avoid mismatched hardware.

Two conversions worth memorising:

• Ah to Wh: amp-hours × nominal voltage. A 12.8 V 100 Ah LiFePO4 battery holds 1,280 Wh.

• Watts to amps: watts ÷ volts. A 2,000 W inverter at full output pulls roughly 42 A from a 48 V bank, but about 167 A from a 12 V bank.

That second figure explains why large systems move to 48 V. Pulling 167 A continuously demands very heavy cable, large fuses, and careful terminal work.

Why Understanding Electrical Ratings Matters When Choosing Equipment

Getting these units right protects your budget and your safety. Before buying anything, work through this order:

1. List your loads in watts and estimate daily watt-hours.

2. Note the largest surge load, well pumps, compressors and power tools draw several times their running wattage at startup.

3. Pick your system voltage. Roughly: under 1,500 W use 12 V; 1,500–3,000 W use 24 V; above 3,000 W use 48 V.

4. Convert watts to amps at that voltage to size cable, fuses, and busbars.

5. Size the battery bank in watt-hours, then check the maximum continuous discharge current the BMS allows.

6. Check the controller's PV voltage ceiling against your array's cold-weather open-circuit voltage.

Skip step four, and you end up with an inverter that trips under load or a cable that runs hot. Skip step six and a cold January morning can push array voltage past the controller's limit.

One more thing worth building into your numbers: usable capacity is not the same as rated capacity. Lead-acid banks are normally sized so you draw no more than half the rated amp-hours, while LiFePO4 tolerates much deeper cycling. A 100 Ah lead-acid battery and a 100 Ah lithium battery carry the same label but do not deliver the same usable watt-hours.

Final Thoughts

Amps, volts, watts, and ohms describe four different properties of the same circuit: flow, pressure, power, and opposition. Ohm's law and the power formula tie them together, and almost every sizing decision in a solar or off-grid build comes from rearranging those two equations. Get comfortable converting watts to amps at your chosen system voltage, and component selection stops being guesswork. If you're unsure where to start, list your loads in watts first, everything else follows from that number.

FAQs

What Is The Difference Between Amps, Volts And Watts? 

Volts measure electrical pressure, amps measure the rate of current flow, and watts measure power. Watts = volts × amps.

What Does An Ohm Measure? 

An ohm measures resistance, how strongly a material opposes current. One ohm allows one ampere to flow when one volt is applied across it.

Is It Amps Or Volts That Are Dangerous? 

Current passing through the body causes injury, but voltage is what drives that current. Both need to be treated with respect; neither is safe on its own terms.

How Do I Convert Watts To Amps? 

Divide watts by volts. A 1,200 W load draws 100 A at 12 V, 50 A at 24 V, or 25 A at 48 V.

How Do I Convert Amp-Hours To Watt-Hours? 

Multiply amp-hours by the battery's nominal voltage. A 12.8 V 100 Ah battery holds 1,280 Wh, or 1.28 kWh.

What Is Ohm's Law? 

Ohm's law states that voltage equals current multiplied by resistance (V = I × R). Rearranged, it gives I = V ÷ R and R = V ÷ I.

Are Watts And Watt-Hours The Same? 

No. Watts measure power at a moment in time. Watt-hours measure energy used over a period. A 100 W device running 5 hours uses 500 Wh.

Why Does Higher Voltage Mean Lower Current? 

Because power is fixed by the load. If watts stay the same and volts go up, amps must come down. This is why 48 V systems use thinner cable than 12 V systems of equal wattage.

What Units Are Used To Size Wiring And Breakers? 

Amps. Conductor ampacity, fuse ratings, and breaker ratings are all current ratings. Voltage determines insulation and clearance requirements instead.

Which Rating Matters Most When Choosing An Inverter? 

Continuous watts and surge watts, plus the DC input voltage. The surge rating must cover the startup draw of motors and compressors, which can be several times their running wattage.