Every spec sheet, breaker size, wire gauge, and inverter rating in this business comes down to four numbers: volts, amps, watts, and ohms. Learn what each one measures and solar stops being mysterious. It becomes arithmetic.
The water analogy, used correctly
Picture a garden hose.
Voltage is the water pressure. It is how hard the electricity is being pushed. A panel sitting in the sun builds electrical pressure whether or not anything is connected, the same way a closed hose still has pressure behind the nozzle. That is why a panel datasheet lists a Voc, the open-circuit voltage: the pressure with the nozzle shut.
Current, measured in amps, is how much water is actually flowing. Shut nozzle: pressure, but zero flow. Open it and water moves. Current only exists when there is a complete path, a circuit, for it to move through.
Resistance, in ohms, is the diameter of the hose. A skinny hose fights the flow. A skinny wire does the same thing, and the fight shows up as heat. That heat is why undersized wire is a fire risk, and why wire sizing gets its own guide later.
Power, in watts, is the useful work at the end: pressure times flow. A pressure washer works because it has high pressure and decent flow at the same time. Watts equal volts times amps. That one equation carries half of solar.
One place the analogy breaks, worth knowing so it never misleads you: water can spill out the end of a hose. Electricity has to loop back to its source. There is always a round trip.
The two equations
Ohm's law: volts equal amps times ohms. Push equals flow times resistance. Rearrange it any way you need.
Power: watts equal volts times amps. You will rearrange this one constantly. Amps equal watts divided by volts is the version you will use most, because appliances are labeled in watts and breakers are labeled in amps.
The same math on real gear
Start with a panel we stock, the Canadian Solar 395W. Its datasheet says it runs 12.91A at its maximum power point. So what voltage is it producing at that moment? Volts equal watts divided by amps: 395 divided by 12.91 is about 30.6V, and the datasheet's Vmp line says exactly that. Notice it is lower than the 36.6V open-circuit number. Under load, working pressure always sits below shut-nozzle pressure.
Now a battery. An EG4 wallmount holds 14.3kWh. Watts measure work happening this moment; a watt-hour measures a quantity of work stored or used. A house drawing a steady 1,200W empties 1.2kWh every hour, so that battery carries it for about twelve hours.
And one from the kitchen. A 1,500W space heater on a 120V outlet draws 1,500 divided by 120, which is 12.5A. That is most of a 15A household circuit right there, and it is why two space heaters on one bedroom circuit trips the breaker. Same math, kitchen scale.
Why solar runs at high voltage
The same power can travel as high voltage with low current, or low voltage with high current. The watts do not care: 400V at 10A and 40V at 100A are both 4,000W. The wire cares enormously, because heat loss in a wire follows the current, and it follows it squared. Ten times the current means a hundred times the heat in the same wire.
So we stack panels in series to raise the voltage and keep the current low, and we get away with reasonable wire sizes on long runs. That is the entire reason string design exists, and it gets its own guide.
Try this at home
Next time you wonder whether a battery will run your fridge overnight, do this math out loud. Find the fridge's nameplate watts (or measure it with a plug-in meter), multiply by the hours, and compare against the battery's kWh. Say the fridge averages 150W (a made-up round number; yours is on its nameplate or your meter). Ten hours at 150W is 1.5kWh, a small dent in a 14.3kWh battery. That is the whole conversation, and now you can have it yourself.
Sources
- Canadian Solar HiKu6 All-Black datasheet, models CS6R-380 to 405MS-HL, v1.1C25, Aug 2022: Voc 36.6V, Vmp 30.6V, Imp 12.91A for the 395W class, from the STC electrical table. Pull the current sheet before designing anything; specs belong to the datasheet, never to memory.
- EG4 WallMount Indoor battery, 14.3kWh nameplate, per the EG4 datasheet. Check the current sheet for the exact model before sizing.
- Household figures (120V outlets, 15A circuits, a 1,500W heater) are US residential norms used for arithmetic, never equipment specs.
- Ohm's law and the power equation are physics, no document needed.