Welcome to Learn Ease Guide! Your step-by-step resource for solar design, CAD drafting, and clean energy engineering. We share practical tutorials on AutoCAD, 3D modeling, and software tools to help engineers, designers, and students simplify complex technical workflows with ease.
Scale guide: Small is off-grid, cabins, RVs and backup (up to about 3 kW). Medium is homes, farms and small businesses (about 3 to 30 kW). Large is commercial, industrial and utility scale (30 kW up to MW and GWh).
Size your setup first: Not sure how many panels, how big an inverter or how much battery you need? Check the solar calculators in my previous post to size your desired setup, then use this guide to pick the equipment.
Read before buying: Ratings are typical manufacturer figures (STC for panels) and vary by variant, so confirm on the current datasheet. Country (HQ) is where the brand is headquartered; factories are often elsewhere. Availability, certification and warranty differ by country.
1. Solar Panels (PV Modules)
N-type TOPCon is now the standard for Tier 1 brands, with back-contact and HJT at the premium end.
You've installed AutoCAD and opened a blank drawing. Now you're looking at a screen full of buttons, panels, and tabs, and it can feel like a lot. Don't worry. Once you know what each area is for, it all starts to make sense.
In this post, we'll take a guided tour of the AutoCAD interface, one area at a time, so you always know where to find things. If you haven't installed AutoCAD yet, start with Part 1 and Part 2 of this series.
Ohm's Law is the foundation of electronics. Once you understand it, circuits stop looking like magic and start making sense.
What Is Ohm's Law?
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it, as long as the temperature stays constant.
V = I × R
V = Voltage (volts, V): the "push" that moves electric charge
I = Current (amperes, A): the flow of charge
R = Resistance (ohms, Ω): how much the material resists that flow
Rearranged, you can find any one value from the other two:
I = V ÷ R
R = V ÷ I
Tip: Picture water in a pipe. Voltage is the water pressure, current is the flow rate, and resistance is how narrow the pipe is.
Example 1: Finding Current
A 12 V battery is connected to a 4 Ω resistor. What is the current?
I = V ÷ R = 12 ÷ 4 = 3 A
Example 2: Finding Voltage
A current of 0.5 A flows through a 10 Ω resistor. What is the voltage across it?
V = I × R = 0.5 × 10 = 5 V
Example 3: Finding Resistance
A device runs on 9 V and draws 0.03 A. What is its resistance?
R = V ÷ I = 9 ÷ 0.03 = 300 Ω
Example 4: A Real-Life LED Circuit
You want to power an LED (2 V, 20 mA) from a 5 V supply. The resistor must drop the extra 3 V.
Voltage across resistor = 5 − 2 = 3 V
Current = 20 mA = 0.02 A
R = 3 ÷ 0.02 = 150 Ω
Without this resistor, the LED would draw too much current and burn out.
Try It Yourself: Ohm's Law Calculator
Choose what you want to find, enter the other two values, and the answer appears instantly. Try the examples above to check your understanding.
Ohm's Law Calculator
Enter two values to see the answer.
Quick Reference
Find
Formula
Voltage
V = I × R
Current
I = V ÷ R
Resistance
R = V ÷ I
Common Mistakes to Avoid
Forgetting to convert units (mA to A, kΩ to Ω)
Mixing up which value goes on top in the formula
Assuming Ohm's Law applies to every component (it doesn't work for diodes or transistors in the same simple way)
Final Thoughts
Ohm's Law is simple, but it's the key to understanding everything from phone chargers to household wiring. Practice with a few numbers of your own and it'll soon feel natural.
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In Part 1, we covered what AutoCAD is and what your computer needs to run it. Now it's time to get it installed.
In this post, I'll walk you through the three ways to get AutoCAD (including the free options), then take you step by step through downloading, installing, and opening it for the first time.
A solar panel datasheet is a one or two page spec sheet full of abbreviations. It looks complicated, but you only need a handful of numbers to size a system safely. This guide explains each important line in plain language and shows how to use it in your design.
First, understand STC
Most numbers on a datasheet are measured under STC (Standard Test Conditions): sunlight of 1,000 watts per square metre, a cell temperature of 25 C, and a standard light spectrum. A real roof is rarely that perfect, so actual output is usually lower. Datasheets may also list NOCT (or NMOT) values, which are measured in more realistic conditions: 800 W/m2, 20 C air temperature and a light breeze. NOCT figures are lower than STC figures.
The key electrical specs
Here is a sample for a typical 400 W panel:
Spec
Example
What it means
Pmax (maximum power)
400 W
The most power the panel makes under STC. This is the "size" of the panel.
Vmp (voltage at max power)
37.0 V
The voltage when the panel is working at its best point.
Imp (current at max power)
10.81 A
The current at that best point.
Voc (open-circuit voltage)
45.2 V
The highest voltage, when nothing is drawing current. Used for safety limits.
Isc (short-circuit current)
11.4 A
The highest current, when the panel is shorted. Used for fuse and wire sizing.
Efficiency
20.5%
How much sunlight turns into electricity.
A quick check: Pmax is roughly Vmp x Imp. Here 37.0 x 10.81 = about 400 W, so the numbers agree.
What efficiency really tells you
Efficiency = Pmax / (panel area x 1,000 W/m2). For a panel measuring 1,722 x 1,134 mm (about 1.95 m2): 400 / 1,953 = about 20.5%. Higher efficiency means more watts from the same roof space. If space is tight, it matters. If you have lots of room, price per watt matters more.
Temperature coefficients: why hot panels make less
Solar panels lose power as they heat up, and their voltage changes with temperature. Datasheets list percentage changes per degree C:
Pmax coefficient (about -0.30%/C): power drops as the cell gets hotter than 25 C. Example: a cell at 65 C is 40 C above STC, so power falls by about 12%. A 400 W panel gives roughly 352 W.
Voc coefficient (about -0.27%/C): voltage rises when it is cold. At -5 C the factor is 1 + (0.0027 x 30) = 1.081, so Voc becomes 45.2 x 1.081 = about 48.9 V per panel. You must use this cold Voc when checking string length.
Isc coefficient (small, positive): current rises very slightly with heat.
A smaller (closer to zero) Pmax coefficient is better, especially in hot climates.
Limits you must not exceed
Maximum system voltage (often 1,000 V or 1,500 V DC): the total voltage of a string, using the cold Voc, must stay below this and below your inverter's limit.
Maximum series fuse rating (often 20 A to 25 A): the largest fuse or breaker allowed to protect the panel. It sets the upper limit when sizing string fuses.
Operating temperature range: commonly -40 C to +85 C.
Mechanical load: for example about 5,400 Pa for snow and 2,400 Pa for wind. Check this for heavy snow or storm areas.
Physical and build details
Dimensions and weight: needed to plan roof space, racking and handling.
Cell type: monocrystalline, PERC, TOPCon or half-cut cells. These affect efficiency and shade tolerance.
Junction box and connectors: look for an IP67 or IP68 rating and standard MC4-type connectors.
Frame and glass: anodized aluminum frame and tempered glass are standard.
Bifacial: bifacial panels also collect light on the back, which can add output on bright ground or ground mounts.
Warranty and degradation
Product warranty: covers defects, commonly 10 to 15 years.
Performance warranty: guarantees a minimum output, often about 80% to 90% after 25 to 30 years.
Degradation: panels slowly lose output, typically a small amount in the first year and about 0.4% to 0.55% per year after.
Certifications to look for
Look for independent safety and quality tests such as IEC 61215 and IEC 61730, and UL 61730 where required (for example in the USA). Fire class and power tolerance are also listed. A tolerance of "0 to +5 W" is good because the panel will not deliver below its rating. Avoid unclear tolerances like "+/-5%".
How to use these numbers in your design
Datasheet value
Use it for
Pmax
Counting panels to reach your array size
Voc (cold corrected)
Maximum panels per string, to stay under the inverter's voltage limit
Vmp (hot)
Making sure the string stays above the MPPT minimum voltage
Imp and Isc
Choosing parallel strings, fuses and wire sizes
Max series fuse rating
The upper limit on string fuses
Temperature coefficients
Estimating real-world output in hot or cold conditions
Pmax, Vmp, Imp, Voc and Isc are all listed, and Pmax is close to Vmp x Imp.
The power tolerance is positive.
The temperature coefficients are listed and reasonable.
Maximum system voltage and series fuse rating fit your inverter and design.
Certifications are listed and checked by independent labs.
The warranty terms are clear and come from a company you can reach.
The size and weight fit your roof or mount.
Try it yourself: Enter your panel's Pmax, Vmp, Imp, Voc, Isc and temperature coefficient into our free solar system calculator to size your array and check voltage limits.
Disclaimer: This article is for general information and education only. Always use the exact values from your panel's own datasheet, and have your final system design reviewed by a qualified electrician or solar installer. Example numbers are illustrative, not a recommendation of any product.
If you've ever wondered how architects draw building plans, how engineers design machine parts, or how solar installers lay out panels on a roof, there's a good chance the answer is AutoCAD.
This is the first post in our beginner series. By the end, you'll know what AutoCAD is, what it's used for, which version to choose, and whether your computer can run it.
Wire and fuse sizes are the safety backbone of a solar system. A wire that is too thin overheats and wastes power, and a fuse that is too big will not protect it. This guide gives you simple rules and worked examples so you can understand how installers choose them.
Safety first. Solar DC circuits can cause fires and serious shock. Use this guide to learn and to check your numbers, then have the final design and installation done or reviewed by a qualified electrician, following your local electrical code.
The two jobs: protect the wire, then limit the loss
Every circuit needs two checks:
Current capacity (ampacity): the wire must safely carry the current without overheating, and the fuse must protect that wire.
Voltage drop: the wire must be thick enough that little power is lost along its length.
Choose the size that passes both checks. Voltage drop usually decides long runs, and ampacity decides short, high-current runs such as battery to inverter.
Step 1: Find the current
Circuit
Current to use
Panel or string to charge controller
Isc (short-circuit current) of the panel, times the number of parallel strings
Charge controller to battery
Controller's maximum output amps
Battery to inverter
Inverter watts / (battery volts x inverter efficiency)
Step 2: Size the fuse
Solar panel strings. A common rule is to take the panel's Isc and multiply by 1.56. That comes from two 125% safety margins (1.25 x 1.25). Then round up to the next standard fuse size, without going above the panel's maximum series fuse rating printed on its label.
Battery and inverter circuits. Multiply the maximum continuous current by 1.25, then round up to the next standard fuse size.
Standard sizes include 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175 and 200 A.
Step 3: Choose a wire that the fuse protects
The wire's ampacity must be equal to or higher than the fuse rating. This is the key rule: the fuse protects the wire. The chart below shows typical copper wire ampacities (75 C column). Real values drop with high heat, bundled cables and long conduit runs, so ask an electrician when in doubt.
AWG size
Approx. mm2
Typical ampacity
14
2.1
20 A
12
3.3
25 A
10
5.3
35 A
8
8.4
50 A
6
13.3
65 A
4
21.2
85 A
2
33.6
115 A
1/0
53.5
150 A
2/0
67.4
175 A
Values are general guides based on common copper-wire tables. Always check the cable's own datasheet and your local code.
Step 4: Check the voltage drop
For copper wire, the minimum cable size in mm2 is:
mm2 = (2 x length x current x 0.0172) / allowed voltage drop in volts
Length is one-way distance in metres. The 2 covers the round trip.
Allowed voltage drop: a common target is 3% or less for panel wiring, and 1% to 2% for battery and inverter cables.
Worked example 1: panels to charge controller
Three 400 W panels in series. Imp 10.81 A, Isc 11.4 A, array Vmp 111 V. The cable run is 20 m one way.
Fuse or protection: 11.4 x 1.56 = 17.8 A, so a 20 A device (if the panel label allows it). With a single string you often need no fuse, but a disconnect is still recommended.
Voltage drop at 3%: allowed drop = 111 x 0.03 = 3.33 V. Cable size = (2 x 20 x 10.81 x 0.0172) / 3.33 = about 2.2 mm2.
Ampacity: needs at least 17.8 A, which 12 AWG (25 A) satisfies.
Choice: 10 AWG (about 5.3 mm2) PV cable gives a comfortable margin for heat and future changes.
Worked example 2: battery to inverter
A 3,000 W inverter on a 48 V battery bank, 93% efficient. The cable run is 3 m one way.
Current: 3,000 / (48 x 0.93) = about 67 A.
Fuse: 67 x 1.25 = 84 A, so use a 100 A DC fuse (the next standard size above 90 A gives more room for surges).
Wire: the cable must carry at least 100 A, so 2 AWG (115 A).
Voltage drop check: 2 AWG is about 0.5 milliohm per metre. Round trip 6 m gives 3 milliohm. Drop = 67 x 0.003 = about 0.2 V, which is 0.4% of 48 V. That is excellent.
The same 3,000 W on a 12 V battery would pull about 270 A, which needs very thick cable. This is why larger inverters use 24 V or 48 V.
Try the wire and fuse calculator
Choose your circuit, enter your numbers, and the calculator gives the fuse size, the minimum cable by ampacity and by voltage drop, and a PASS or FAIL check. You can export the result as a PDF.
Wire and Fuse Size Calculator
Fuse size, minimum cable and voltage drop for solar circuits
Circuit
Your numbers
Result
Checks
Fuse: Isc x 1.56 for panel strings, 1.25 x continuous current for battery and inverter circuits, rounded up to a standard size. Wire sizes use typical copper 75 C ampacities. Heat, bundling and conduit lower real ampacity. Estimates only. Have your design reviewed by a qualified electrician and follow your local code.
Where to put fuses
Battery positive: place a fuse as close to the battery terminal as practical, so the whole cable is protected.
Parallel panel strings: when you have three or more strings in parallel, each string usually needs its own fuse or breaker, typically in a combiner box.
Disconnects: add a switch or breaker so you can safely isolate the array, the battery and the inverter.
Choosing the right parts
Use DC-rated fuses and breakers. AC parts are not safe for DC, because DC arcs are harder to extinguish.
Voltage rating: the fuse's DC voltage rating must be higher than your highest system voltage, including the cold-weather Voc of your strings.
PV cable outdoors: use sunlight-resistant solar PV cable, and matching connectors from the same maker.
Battery cable: use flexible, fine-stranded copper cable with crimped lugs, not just clamped wire ends.
Colors: red for positive and black for negative, and keep connections tight to avoid heat.
Ground and bond: follow your local code for earthing the frames and the system.
Common mistakes
Choosing a fuse by guesswork. A fuse larger than the wire's ampacity will not protect the wire.
Ignoring voltage drop on long panel runs, which wastes harvested power.
Using AC breakers on DC circuits.
Skipping a fuse at the battery. Batteries can deliver huge short-circuit currents.
Forgetting that hot roofs and enclosed conduit reduce a cable's safe capacity.
Try it yourself: Use our free solar system calculator to get your array current, inverter DC current and suggested fuse sizes.
Disclaimer: This article is for general information and education only. Wire and fuse sizes depend on temperature, installation method, equipment ratings and local codes (such as the NEC, the Philippine Electrical Code or IEC-based standards). The numbers here are examples and not a certified design. Have your system designed or reviewed by a qualified electrician or solar installer before installing.
You have a battery, or a portable power station, and you want to know how long it will keep your fridge, TV or lights running. The answer takes one short formula. This guide shows how to work it out, with a runtime chart for a common 12 V 100 Ah battery.
The formula
Runtime (hours) = Battery Wh x Usable share x Inverter efficiency / Load watts
Battery Wh: battery voltage x amp-hours. A 12 V 100 Ah battery holds 12 x 100 = 1,200 Wh.
Usable share: the part of the battery you can safely use. About 0.9 for LiFePO4 and about 0.5 for lead-acid.
Inverter efficiency: usually 0.85 to 0.95 when you power AC appliances. Use 1 for devices that run directly on DC.
Load watts: the combined watts of everything running at the same time.
A 12 V 100 Ah LiFePO4 battery powers a 100 W TV through an inverter that is 90% efficient.
Battery energy: 12 x 100 = 1,200 Wh.
Usable: 1,200 x 0.9 = 1,080 Wh.
After inverter loss: 1,080 x 0.9 = 972 Wh.
Runtime: 972 / 100 = about 9.7 hours.
Runtime chart
For a 12 V 100 Ah battery with a 90% inverter:
Appliance
Average watts
LiFePO4 (972 Wh usable)
Lead-acid (540 Wh usable)
LED light
10 W
about 97 hours
about 54 hours
Router and modem
15 W
about 65 hours
about 36 hours
Laptop
60 W
about 16 hours
about 9 hours
TV
100 W
about 9.7 hours
about 5.4 hours
Fridge (150 W, runs about 35% of the time)
about 52 W average
about 18 hours
about 10 hours
TV + laptop + 1 light together
170 W
about 5.7 hours
about 3.2 hours
Microwave
1,200 W
about 0.8 hours
about 0.45 hours
The lead-acid column assumes a 50% usable share. Treat these as estimates. Real results depend on battery age, temperature and the actual draw of your device.
Try the battery runtime calculator
Pick your battery type, enter its voltage and capacity, and add the appliances you want to run. The calculator shows your runtime, the battery size you would need for a target runtime, and lets you export the result as a PDF.
Battery Runtime Calculator
How long will your battery run your appliances?
Battery
Extras (optional)
Appliances running at the same time
Result
Checks
Cycle = share of time the appliance draws full power (1 for steady loads, about 0.35 for a fridge). Usable share is about 0.9 for LiFePO4 and 0.5 for lead-acid. For a power station, enter its Wh. Estimates only. Check your battery and inverter datasheets and have wiring reviewed by a qualified electrician.
A quick shortcut
For a fast estimate with an inverter, use: Hours = Volts x Ah x 0.8 / Watts. The 0.8 covers both usable capacity and inverter loss for lithium. For lead-acid, use about 0.45 instead.
Portable power stations
Power stations list capacity in Wh, so skip the voltage step. A 1,000 Wh unit running a 100 W load lasts roughly 1,000 x 0.85 / 100 = about 8.5 hours. The 0.85 allows for conversion losses, and many units reach slightly more or less than that.
Why real runtime can be shorter
Inverter standby draw: an inverter uses power even with nothing connected, often 10 to 50 W for larger models.
High loads: lead-acid batteries deliver less capacity at high current, so heavy loads drain them faster than the formula says.
Cold weather: batteries give less energy at low temperatures.
Battery age: capacity falls over the years.
Start-up surge: motors and compressors briefly pull more power than their running watts.
Limits on output: your battery and inverter must be able to supply the watts at once. A small battery may shut down on a heavy load even if the formula shows some runtime.
Turn the formula around
To find the battery you need for a target runtime:
Battery Wh = Load watts x Hours / (Usable share x Inverter efficiency)
To keep a 52 W average fridge running for 24 hours on LiFePO4, you need about 52 x 24 / (0.9 x 0.9) = roughly 1,540 Wh. See our guide to sizing a solar battery bank for larger systems.
Planning a full solar setup? Use our free solar system calculator to size panels, battery and inverter in one place.
Disclaimer: This article is for general information and education only. Numbers are estimates and examples, not professional advice. Check your battery and inverter datasheets, and have any wiring reviewed by a qualified electrician.
