Electricity basics for solar and batteries come down to one formula, watts = volts × amps, and a handful of ideas built on it, according to the U.S. Department of Energy’s “How Does Electricity Work?” primer and the beginner explainer from DIY Solar Power with Will Prowse embedded below. This guide covers each idea in plain terms, with worked examples, and links to the deeper guides in the series.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
Electricity Basics: The Units
- Volts (V) are electrical pressure. A car battery is about 12V, and a US wall outlet is about 120V.
- Amps (A) are the rate of current flow.
- Watts (W) are power: volts × amps. A 120V appliance drawing 10A uses about 1,200W.
- Watt-hours (Wh) and kilowatt-hours (kWh) are energy, meaning power over time. Your utility bills you in kWh.
- Amp-hours (Ah) are a battery’s charge capacity. Multiply by voltage to get Wh: 100Ah × 12.8V ≈ 1,280Wh.
Ohm’s Law in One Line
Voltage = current × resistance (V = I × R). That’s why thin, long wires waste power and get hot: more resistance means more voltage lost as heat.
AC vs DC
Batteries and solar panels produce direct current (DC), which flows one way. Homes use alternating current (AC), which reverses direction 60 times per second in the US. An inverter turns DC into AC, and a charger or converter turns AC into DC. US homes get split-phase service: two 120V legs that give 240V across them.
Wiring and Protection
| Topic | The rule of thumb |
|---|---|
| Wire size | Pick wire rated for the current (ampacity), then check voltage drop |
| Voltage drop | Many designers aim for about 3% or less on important circuits |
| Fuses and breakers | They protect the wire, so size them to the wire’s ampacity |
| Grounding | Connects metal parts to earth and gives fault current a path back to trip the breaker |
| GFCI and AFCI | GFCIs protect people from shock, and AFCIs help prevent arc fires (NEC 210.8 and 210.12) |
Inverters and Loads
Size an inverter for both running watts and the short surge a motor or compressor pulls at startup. Pure sine wave inverters match grid power. Modified sine units are cheaper, but some motors, chargers and medical devices run hot or misbehave on them.
Batteries and System Voltage
LiFePO4 (lithium iron phosphate) is now the usual choice for solar storage because of its long cycle life and thermal stability. Higher system voltage (24V or 48V) cuts the current for the same power. Lower current means thinner cable and less loss, which is why bigger systems move to 48V.
Example Math: A Small Cabin Load
Example only, using typical wattages, not measurements: a 60W fridge average, 20W of LED lights for 5 hours, and a 50W laptop for 4 hours use about 1,440 + 100 + 200 = 1,740Wh per day. A 12.8V 100Ah LiFePO4 battery stores about 1,280Wh, so this load needs roughly two of them, plus solar to refill them each day.
Start Here: The Series Map
This page is the hub for the Electricity Basics series. Read the guides roughly in this order:
| Step | What you learn | Guide |
|---|---|---|
| 1 | The units and the water analogy | Volts, amps and watts explained |
| 2 | How resistance links volts and amps | Ohm’s law explained |
| 3 | Converting between units | Watts to amps at 12V, 120V and 240V |
| 4 | Battery capacity the right way | Amp-hours vs watt-hours |
| 5 | Why homes are AC and batteries are DC | AC vs DC explained |
| 6 | Wire, fuses and voltage drop | Wire gauge chart, voltage drop, what size fuse |
| 7 | Combining batteries and panels | Series vs parallel |
| 8 | Sizing an inverter | What size inverter do I need, surge vs running watts |
| 9 | Choosing a system voltage | 12V vs 24V vs 48V |
| 10 | Home wiring | 120V vs 240V split phase, neutral vs ground, GFCI vs AFCI |
| 11 | Staying safe | Electrical safety for DIY solar |
New terms are defined in the electrical terms glossary.
Volts, Amps and Watts: The Water Hose Picture
In the Will Prowse video, voltage is compared to the water pressure in a garden hose. The pressure is there even when the tap is shut, which is why he treats voltage as a compatibility check: a 12V appliance goes on a 12V battery, and a 120V appliance goes in a 120V outlet. Amps are how much is flowing, like the size of the hose. More amps need a thicker wire, and a wire that’s too small turns the extra current into heat.
Watts combine the two. His example: a panel making 10V at 1A and one making 1V at 10A both produce 10W, but the 10A panel needs much heavier wire. That trade-off between voltage and current is behind nearly every design choice in this series.
| Quantity | Symbol | Unit | Plain meaning | Everyday example |
|---|---|---|---|---|
| Voltage | V | volt | Electrical pressure | 12V battery, 120V outlet |
| Current | I | amp (A) | Rate of flow | 15A household circuit |
| Resistance | R | ohm (Ω) | Opposition to flow | Long, thin wire |
| Power | P | watt (W) | Rate of energy use | 60W fridge average |
| Energy | E | watt-hour (Wh) | Power × time | 1 kWh on your bill |
| Charge capacity | Ah | amp-hour | Current × time at a given voltage | 100Ah battery |
The Four Formulas You’ll Actually Use
- Watts = volts × amps. 120V × 10A = 1,200W.
- Amps = watts ÷ volts. A 1,200W load on a 12V inverter pulls about 100A from the battery before losses.
- Watt-hours = watts × hours. 100W for 10 hours = 1,000Wh, or 1 kWh.
- Watt-hours = amp-hours × volts. 100Ah × 12.8V ≈ 1,280Wh.
The second formula is why low-voltage systems need thick cable. The same 1,200W is 10A at 120V but about 100A at 12V, ten times the current for the same power.
Compare Batteries in Watt-Hours, Not Amp-Hours
Will Prowse makes this point directly: amp-hours only mean something at a given voltage. His 24V battery rated 250Ah would read 500Ah if it were built as a 12V battery, and the stored energy is the same either way. Wiring two batteries in series doubles the voltage but keeps the amp-hours the same. Wiring them in parallel keeps the voltage and adds the amp-hours. In both cases, the total watt-hours double.
| Two 12.8V 100Ah batteries | Voltage | Amp-hours | Watt-hours |
|---|---|---|---|
| One battery | 12.8V | 100Ah | 1,280Wh |
| Two in parallel | 12.8V | 200Ah | 2,560Wh |
| Two in series | 25.6V | 100Ah | 2,560Wh |
Runtime Math in One Line
Runtime (hours) ≈ usable watt-hours ÷ load watts. The video’s example: a 1,000Wh battery runs a 100W load for about 10 hours. For AC loads through an inverter, also divide by the inverter’s efficiency and add its idle draw. Illustrative: a 1,280Wh LiFePO4 battery running a 90W TV through a 90%-efficient inverter lasts about 1,280 × 0.9 ÷ 90 ≈ 12.8 hours, before idle draw. See how long will a 100Ah battery last and inverter efficiency and idle draw.
Reading the Numbers on Real Equipment
| Where you see it | What the number means | What to do with it |
|---|---|---|
| Appliance label: “120V 1.5A” | Maximum input current | 120 × 1.5 = 180W peak; real average is often lower |
| Battery: “12.8V 100Ah” | Nominal voltage and capacity | ≈ 1,280Wh stored |
| Solar panel: “Voc 22.5V, Isc 5.8A” | Open-circuit voltage and short-circuit current | Use for controller voltage limits and fusing |
| Charge controller: “100/ | 100V max PV input, 30A max charge current | Keep cold-weather Voc under 100V |
| Inverter: “3000VA / 2400W” | Apparent vs real power rating | Size for both; see VA vs watts |
| Electric bill: “850 kWh” | Energy used that month | Starting point for home solar sizing |
How Energy Flows Through a Solar Battery System
- Solar panels make DC power from sunlight.
- A charge controller (usually MPPT) converts panel voltage to battery charging voltage. See MPPT vs PWM.
- The battery stores energy as DC.
- Fuses or breakers on every positive cable protect the wire.
- An inverter turns DC into 120V or 120/240V AC for household loads.
- A battery monitor (shunt) counts amps in and out to show state of charge.
Worked Example: Sizing a Small Off-Grid Cabin
Illustrative inputs, not measurements: the cabin load from the example above uses about 1,740Wh a day.
| Step | Math | Result |
|---|---|---|
| Daily load | 1,440 + 100 + 200 | 1,740Wh |
| Add 10% inverter loss | 1,740 ÷ 0.9 | about 1,930Wh |
| Battery for one day at 100% usable LiFePO4 | 1,930 ÷ 1,280 | about 1.5, so two 100Ah batteries |
| Solar with 4 peak sun hours (illustrative) | 1,930 ÷ 4 ÷ 0.8 | about 600W of panels |
| Charge current at 12.8V | 600 ÷ 12.8 | about 47A, so a 50A MPPT |
The 0.8 factor is an illustrative allowance for heat, dust and wiring losses. For real sun hours at your address, NREL’s free PVWatts calculator estimates monthly production. Then pick wire for the current, fuse it for the wire, and choose a system voltage. At around 2,000W of inverter or more, many designers move to 24V or 48V to cut current.
Battery Monitors and AGM
A viewer asked under the Will Prowse video whether a battery monitor is worth adding with an AGM battery. Lead-acid voltage does track state of charge at rest, but a shunt-based monitor counts amp-hours in and out, so it shows charge while loads are running, when voltage readings are skewed. That makes it useful with any chemistry, and close to essential with LiFePO4, whose voltage barely changes across most of its range.
FAQ
What is the most important electricity basic for solar?
Watts = volts × amps. Almost every sizing question starts there.
Is 12V DC safe to touch?
The voltage is low enough that shock risk is low, but a battery’s short-circuit current can melt tools and start fires. Fuse every positive cable.
Do I need an electrician for solar?
For anything tied to your home panel or the grid, usually yes. Your local AHJ decides what needs a permit.
What are volts, amps and watts in simple terms?
Volts are pressure, amps are flow, and watts are the total power, volts × amps.
Why is 12V wiring so thick?
Low voltage means high current for the same power, and current sets the wire size.
How do I compare two batteries?
Convert both to watt-hours (Ah × volts), then compare usable capacity.
Sources: U.S. Department of Energy, “How Does Electricity Work?” and “Estimating Appliance and Home Electronic Energy Use” (energy.gov); NFPA 70, National Electrical Code 2023 (Articles 210, 240, 250, 310, 690); DIY Solar Power with Will Prowse, “Electricity Explained” (YouTube); NREL PVWatts calculator. Accessed Oct 2026.
Related: Proster Smart Multimeter Review, LiTime 12V 50Ah LiFePO4 Battery Review, Allwei 1500W Pure Sine Wave Inverter Review
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