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ANSI/HI 9.6.1 (NPSH margins), ANSI/HI 14.6 (tests), API 610
US units (gpm, ft, psi)
British
BS EN ISO 9906, BS EN 806, CIBSE Guide C
SI (m³/h, m, kPa)
European
EN ISO 9906, EN ISO 5199, EN 733
SI
The physics is identical; the standards differ in units, test grades and recommended margins.
2. Flow rate (Q)
Establish the design flow from demand, then add a margin (typically 5–10%) and keep pipe velocity within 1–3 m/s (3–10 ft/s).
Q in m³/s; 1 m³/h = 4.403 US gpm.
3. Total Dynamic Head (TDH)
Hs static head, hf friction loss, hm minor (fitting) losses.
Friction loss – Darcy–Weisbach (ISO/European/British practice; also accepted in the US):
f from Colebrook–White or Moody chart.
Hazen–Williams (common in US water/fire systems, SI form):
Minor losses:
Figure 2: Pump curve, system curve and duty point
4. NPSH available and required
Pa absolute suction-surface pressure, Pv vapour pressure, Zs static suction head (+ flooded, − lift), hf,s suction line losses.
Acceptance criterion:
M ≈ 1.1–1.3 (or +0.6 m minimum). ANSI/HI 9.6.1 gives higher margins for high-energy or large pumps; NPSHr is taken at 3% head drop per EN ISO 9906 / ANSI/HI 14.6.
5. Power
US form: BHP = Q(gpm)·H(ft)·SG / (3960·η).
6. Try it: interactive calculator (SI)
Pick the unit for each value from its drop-down. Values convert automatically when you change a unit.
Show results in:
Optional: manufacturer pump curve One "flow, head, efficiency" line per point (5-6 points). Efficiency is optional: use % (e.g. 72) or a fraction (0.72). Use the flow unit chosen above and the Head unit chosen in "Show results in".
Darcy–Weisbach with Swamee–Jain friction factor; single pipe size; water properties from temperature. Indicative only.
7. Worked example (SI)
Clean water at 20 °C (ρ = 998 kg/m³, Pv = 2.34 kPa), Q = 50 m³/h, 100 mm pipe, 120 m long, ε = 0.045 mm, ΣK = 6, flooded suction. These are the calculator's default values, so you can reproduce every line below above.
Item
Value
Static head Hs
18.0 m
Friction loss (v = 1.77 m/s, Re ≈ 176,000, f = 0.0189)
3.61 m
Minor losses (ΣK = 6)
0.96 m
TDH (≈ 74 ft)
22.6 m
NPSHa = 10.11 + 2.0 − 0.8
11.3 m
NPSHr (datasheet) ×1.3
3.5 × 1.3 = 4.55 m → OK
Shaft power at η = 70%
≈ 4.4 kW (select next motor size, e.g. 5.5 kW). Flow ≈ 220 gpm.
8. Common mistakes
Using gauge instead of absolute pressure for NPSHa.
Ignoring viscosity, temperature or altitude corrections.
Selecting far from the best efficiency point (aim for 70–120% of BEP flow).
Skipping the NPSH margin on hot or hydrocarbon services.
Disclaimer: for educational use. Verify with the applicable standard and manufacturer data before design.
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Add your appliances, enter your rate per unit, and see what your household will pay this month, and which devices are driving the total.
Calculate your bill
Results update as you type. Nothing leaves your device.
ApplianceWattsHours/dayQty
Estimated bill
$0.00
Estimates assume appliances run at their rated wattage. Real use varies, especially for fridges, ACs and heaters that cycle on and off.
How an electricity bill is calculated
Your supplier charges for energy in kilowatt-hours (kWh), often called "units". One kWh is the energy used by a 1,000 watt device running for one hour.
kWh per day = (watts × hours per day × quantity) ÷ 1,000 Bill = (kWh per day × days × rate) + fixed charge + tax
For example, a 1,500 W space heater running 3 hours a day uses 4.5 kWh daily. Over 30 days that is 135 kWh, or about $21.60 at $0.16 per kWh.
Typical appliance consumption
Appliance
Typical watts
Typical use
LED bulb
9 W
5 h/day
Ceiling fan
60 W
8 h/day
Refrigerator (average draw)
150 W
24 h/day
Television
100 W
4 h/day
Air conditioner
1,500 W
6 h/day
Washing machine
500 W
1 h/day
Electric water heater
3,000 W
1 h/day
Wattage is printed on the appliance label, usually on the back or underneath.
Seven ways to lower your bill
Start with the biggest bars in your calculator results. Heating, cooling and water heating usually dominate.
1. Raise your AC thermostat by 1 to 2 degrees. 2. Swap remaining bulbs for LEDs. 3. Unplug or switch off devices on standby. 4. Wash clothes in cold water and air dry when you can. 5. Clean fridge coils and AC filters. 6. Shift heavy use to off-peak hours if your plan has time-of-use pricing. 7. Choose appliances with a high efficiency rating when replacing old ones.
Frequently asked questions
Where do I find my rate per kWh?
Check your latest bill or your supplier's tariff page. If your rate has tiers, enter a blended average: total energy charge divided by total kWh.
Why is my real bill higher than the estimate?
Common reasons are devices you did not list, standby power, older appliances drawing more than their label, and delivery or demand charges. Add a fixed charge and tax percentage to get closer.
How do I use the meter reading mode?
Subtract your previous reading from your current reading to get units used, then multiply by your rate. The calculator does this for you.
AWG Wire Gauge Chart and Calculator | learneaseguide.com
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AWG Wire Gauge Chart: Which Size Should You Use?
The wrong wire size gives you dim lights and sluggish motors at best, and a hot wire inside a wall at worst. Here is what AWG means, a quick chart, and a calculator to pick your size.
What is AWG?
AWG (American Wire Gauge) measures round, solid, non-ferrous wire. The counterintuitive part: the smaller the number, the thicker the wire. Every 6 gauge steps roughly halves the diameter; every 3 steps roughly halves the resistance.
d (mm) = 0.127 × 92^((36 − n) / 39)
For 12 AWG: 0.127 × 92^(24/39) ≈ 2.05 mm.
Quick-reference chart
AWG
Dia. (mm)
Ω / 1,000 ft
Max amps*
Common use
22
0.644
16.14
3
Signals, electronics
18
1.024
6.385
7
Lamp cords, low-voltage lighting
16
1.291
4.016
10
Extension cords, speakers
14
1.628
2.525
15
Household lighting
12
2.053
1.588
20
Outlets, garage circuits
10
2.588
0.999
30
Dryers, water heaters
8
3.264
0.628
40
Ranges, subpanels
6
4.115
0.395
55
Large appliances, feeders
4
5.189
0.249
70
EV chargers, big subpanels
*Copper, conservative 60 °C rating. Check your local code and insulation rating.
How to choose
Two things decide wire size: current and distance. The wire must carry the current safely (a 20 A breaker needs 12 AWG or thicker), and on long runs voltage drop matters:
V drop = 2 × L × I × R
L is the one-way length in feet, I is amps, and R is ohms per foot (chart value ÷ 1,000). Aim for under 3%. Low-voltage systems (12 V automotive, solar, RV) need much thicker wire than 120 V circuits carrying the same power.
Wire size calculator
Enter your circuit. Results update as you type.
AWG
Drop (V)
Drop (%)
Amps OK?
Drop OK?
Aluminum ampacity is derated 20%. This is a guide only; follow local electrical code.
Rules of thumb
When in doubt, go one size thicker.
Never use thinner wire than the breaker requires. The breaker protects the wire.
Bundled wires, heat, and conduit reduce ampacity.
Stranded wire suits cords and vehicles; solid suits walls.
For most home projects: 14 AWG for 15 A lighting, 12 AWG for 20 A outlets, 10 AWG for 30 A appliances. For anything inside walls or on a panel, follow local code or hire a licensed electrician.
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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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.
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.
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