Electrical Calculators
Tools for home electrical work, DIY projects, and cost tracking. Calculate what an appliance adds to your monthly bill, check whether a wire gauge handles the load, or size a power supply before you buy it. Practical, instant results for everyday electrical work.
Last updated: June 2026
Cost & energy calculators
Wiring & power calculators
Printable cheat sheets
One-page references for the truck or workshop wall. Open one, then print it or save as PDF.
Electrical abbreviations glossary
V, A, W, VA, VAR, kVAR, PF, RMS, MCB, GFCI, AWG, mm²: 30-plus abbreviations used across these calculators, each defined in one sentence with units and everyday context.
Electrical units: what V, A, W, and Ohm mean in practice
All electrical calculations use five quantities. Understanding what each measures makes the tools on this hub more intuitive than just plugging in numbers. For the full list of symbols and units used across these calculators, see the electrical abbreviations glossary.
| Unit | Symbol | What it measures | Analogy |
|---|---|---|---|
| Volt | V | Electrical pressure: the force pushing current through a circuit | Water pressure in a pipe |
| Ampere | A | Current: the rate of charge flow through a conductor | Flow rate in litres per second |
| Watt | W | Power: the rate of energy use. P = V × A | The pump's output (pressure × flow) |
| Ohm | Ω | Resistance: opposition to current flow. V = I × R | Narrowness of the pipe |
| Kilowatt-hour | kWh | Energy consumed over time. 1 kW running for 1 hour = 1 kWh | Total volume of water moved |
The practical billing relationship: kWh = watts × hours ÷ 1000. A 2000 W kettle running for 3 minutes uses 2000 × (3/60) ÷ 1000 = 0.1 kWh. At €0.32/kWh, that boil costs about 3 cents. Your electricity bill charges per kWh, so this formula converts any appliance into a monthly cost.
What circuit breaker and fuse ratings mean
A 16 A circuit breaker allows up to 16 × 230 V = 3680 W continuously. A 32 A circuit for an EV wallbox handles up to 7360 W. The cable in the wall must match that current: 1.5 mm² for 16 A, 2.5 mm² for 20 A, 4 mm² for 25 A, and 6 mm² for 32 A in standard conduit. The Wire Gauge Calculator confirms the correct cross-section for any specific load and run length.
What each tool does
| Calculator | What you solve | Typical use |
|---|---|---|
| Electricity Cost | Consumption in kWh + tariff rate = exact cost for any billing period | Verify a monthly bill, compare tariffs, estimate solar payback |
| Appliance Running Cost | Appliance wattage + daily hours + electricity rate = daily, monthly, and annual cost | Deciding whether to replace an old fridge or dryer, tracking standby costs |
| kWh to Euro | Energy amount in kWh + tariff = cost in any currency | Quick cost look-up when you know the kWh figure from a datasheet or meter |
| Voltage Drop (AC) | Wire length + load current + conductor material + cable cross-section = voltage drop in V and % | Checking IEC compliance for a new socket circuit or outbuilding feed |
| Wire Gauge | Load current + circuit length + allowable drop % = minimum AWG or mm2 cross-section | Sizing cable for a garage sub-panel, garden shed, or extension run |
| Power Supply | Total component wattage + headroom % = recommended PSU wattage | PC builds, workshop bench supplies, 12 V DC systems |
| Fuse Size | Load current + circuit type = recommended fuse rating, minimum wire ampacity, IEC compliance note | Sizing fuses for new circuits, checking existing protection, motor branch circuits |
| Power Factor | Any two of W, VA, PF = the third value plus reactive power (VAR) and kVA | UPS sizing, generator selection, motor circuit analysis, transformer loading |
| Voltage Drop (DC) | DC current + wire length + resistivity = voltage and power lost in the cable | 12 V LED strips, battery wiring, bench electronics |
When to use these tools
- Rewiring a room: use the wire gauge calculator to confirm the cable cross-section handles the load, then the voltage drop calculator to check the run stays within the IEC 3% limit.
- Sizing a generator or UPS: enter the total connected wattage into the power supply calculator to find the VA rating you actually need, accounting for power factor.
- Checking whether a fuse is correct: compare the circuit's calculated load current with the fuse rating; the wire gauge tool shows what current the existing cable can carry.
- Calculating a monthly electricity bill: use the appliance running cost calculator for each device, then sum the results, or feed the total kWh into the kWh to Euro converter.
- Planning a garden shed feed: the voltage drop calculator tells you whether a long cable run from the house keeps voltage within limits at the far end.
Sizing a new cable run: 6-step workflow
Use this sequence when adding a circuit, extending an existing ring, or feeding an outbuilding. Each step references the relevant calculator on this hub.
- Calculate load current. I (A) = total appliance wattage divided by 230 V. A 3000 W oven draws 3000 / 230 = 13 A. For multiple appliances on one circuit, sum the wattages before dividing.
- Select circuit breaker rating. Standard MCB ratings: 6, 10, 13, 16, 20, 25, 32 A. Choose the first standard value above your calculated load current. A 13 A load requires a 16 A MCB.
- Measure the cable run. Use the one-way distance from the consumer unit to the furthest outlet in metres. The voltage drop tools use the one-way figure and account for the return path internally.
- Find the minimum cable cross-section. Enter load current, run length, and allowable drop (3% for socket circuits, 5% for motor branch circuits) into the Wire Gauge calculator. Typical results: 1.5 mm² handles 16 A up to about 16 m; 2.5 mm² is needed for longer runs or 20 A circuits.
- Verify voltage drop. Enter the chosen cross-section, run length, and current into the Voltage Drop calculator. Confirm the result stays within IEC 60364 limits (3% for lighting and socket circuits).
- Confirm fuse or MCB rating matches cable ampacity. Use the Fuse calculator to check that the breaker rating does not exceed the cable's rated ampacity. A 16 A MCB requires cable rated for at least 16 A continuous (1.5 mm² in conduit = 16 A for most installation methods).
Household appliance wattages: real figures
Knowing typical wattages helps you check whether a circuit is overloaded, estimate bills before switching tariffs, and understand where most of your electricity actually goes.
| Appliance | Typical power | Typical daily use | Monthly cost at €0.32/kWh |
|---|---|---|---|
| Electric kettle | 1500 to 3000 W | 10 minutes | approx. €0.80 |
| Microwave | 700 to 1200 W | 15 minutes | approx. €1.00 |
| Refrigerator (A+++ class) | 100 to 150 W average | 24 hours (cycling) | approx. €3.50 |
| Washing machine (60°C cycle) | approx. 0.8 kWh per wash | 1 wash | approx. €7.50 (30 washes/month) |
| Electric shower | 7500 to 10500 W | 8 minutes | €5 to €8 |
| EV home charger (7.4 kW) | 7400 W | 6 hours average overnight | approx. €55 per 10 000 km driven |
| Electric space heater | 1000 to 2000 W | 4 to 6 hours | €60 to €120 in winter |
| Laptop and monitor | 40 to 120 W | 8 hours | €3 to €6 |
Heating devices (shower, kettle, space heater) dominate consumption. A 10-minute electric shower uses more energy than a laptop running for an entire day. The Appliance Running Cost calculator calculates the exact monthly and annual cost for any device at your tariff rate.
12 V systems: campers, vans, and off-grid setups
Mobile 12 V systems follow the same V = I × R principles as home wiring, but voltage drop matters far more. Because voltage is roughly 19 times lower than mains, current is 19 times higher for the same power. A 100 W device draws 0.43 A at 230 V but draws 8.3 A at 12 V.
A 0.5 V drop on a 230 V mains circuit is 0.2%, negligible. The same 0.5 V drop on a 12 V system is 4%: enough for an inverter or sensitive electronics to fall below its minimum operating voltage. Standard practice is to keep voltage drop below 3% per cable run. For a 100 W load at 8.3 A over a 3-metre run, that means round-trip resistance below 0.043 Ω, pointing to 6 mm² cable or heavier. The DC Voltage Drop calculator gives the exact figure for any gauge, length, and current combination.
| Device | Typical power | Current at 12 V | Minimum cable |
|---|---|---|---|
| LED lighting (per metre of strip) | 6 to 14 W | 0.5 to 1.2 A | 1.5 mm² |
| 12 V compressor fridge | 30 to 60 W average | 2.5 to 5 A | 2.5 mm² |
| Water pump | 60 to 120 W | 5 to 10 A | 4 mm² |
| 250 W inverter | 250 W | approx. 22 A peak | 6 mm² |
| 1000 W inverter | 1000 W | approx. 90 A peak | 25 to 35 mm² |
For a complete camper or van installation, see the 12 V Van Wiring Guide, which covers battery selection, fuse placement, solar charge controller sizing, and the full wiring sequence.
Electrical safety: what you can DIY and what requires a professional
The calculators on this hub are planning and verification tools. Whether you can legally carry out the physical work depends on your country's wiring regulations.
- 12 V to 48 V DC (van, camper, solar off-grid): Legal to DIY in most jurisdictions. A shock at 12 V is unpleasant but not dangerous for most adults. Focus on fire prevention: correct fuse sizing and cable rated for the current load.
- Single-phase mains (230 V / 120 V): Contact with a live conductor is potentially fatal. In most European countries, new circuits and consumer unit work must be certified by a licensed electrician. Replacing a socket or switch on an isolated, verified-dead circuit is typically permitted for homeowners, but rules vary by country. Check local regulations before starting.
- Three-phase (400 V) and industrial supply: Always requires a qualified electrician. Not a DIY task under any circumstances.
Regardless of voltage: isolate at the breaker first, then verify the circuit is dead with a non-contact voltage tester before touching any conductor. Never rely on the switch position alone; a breaker can be switched back on while you are working.
Frequently Asked Questions
What is the IEC 3% voltage drop guideline and does it apply to my wiring?
IEC 60364 recommends that the voltage drop from the supply point to any fixed outlet does not exceed 3% for lighting circuits and 5% for other uses under normal load. In practice, most residential standards (including BS 7671 in the UK and equivalent national codes) adopt these figures. The 3% limit matters most on long cable runs: a 20-metre circuit at 16 A with 1.5 mm2 cable will typically exceed the limit, requiring 2.5 mm2 instead. Use the voltage drop calculator before installing cable on any run longer than about 10 metres at meaningful current.
Can I calculate my electricity cost without reading the meter, just from appliance data?
Yes. The appliance running cost calculator takes wattage and daily hours as inputs, so you do not need meter access. This approach is useful for estimating the cost of a specific device before buying, or for understanding what a standby load adds over a year. For billing verification, reading the meter before and after a period gives the actual kWh consumed regardless of what appliances ran. The two methods are complementary: appliance-level calculation explains where the energy goes; the meter reading confirms the total.
Should I buy a power supply rated exactly at my calculated wattage?
No. PSU efficiency peaks between 50% and 80% of rated load, and many components draw more at startup than at idle. Standard practice is to add 20-25% headroom to your calculated peak load. So if your components sum to 400 W, a 550 W PSU is a reasonable choice. Going much higher wastes money and slightly reduces efficiency at low loads; going lower risks shutdown under peak demand or shortens PSU lifespan. The power supply calculator applies a configurable headroom percentage so you can match your own tolerance.
What is the difference between a fuse and a circuit breaker?
Both interrupt current flow when it exceeds the rated value, but they work differently. A fuse contains a wire element that melts permanently when overloaded; it must be replaced after it operates. Fuses respond very quickly to high fault currents and are common in older consumer units, automotive wiring, and industrial applications. A circuit breaker (MCB) uses a bimetal strip for sustained overload and an electromagnet for short-circuit current; it trips a switch that can be reset after the fault is cleared. Modern homes use MCBs almost exclusively. For sizing guidance, the Fuse Calculator gives the correct rating for any load and circuit type; the same current figures apply to MCB selection.
How do I estimate my monthly electricity cost from appliance wattages?
Multiply each appliance's wattage by its daily hours of use, divide by 1000 to get kWh per day, multiply by your electricity tariff (€/kWh), then multiply by 30 for the monthly figure. Example: a 1500 W dishwasher running 1 hour per day = 1.5 kWh/day × 30 = 45 kWh/month × €0.32 = €14.40 per month. The Appliance Running Cost calculator handles each device individually and sums daily, monthly, and annual costs. For your household total from a meter reading, use the Electricity Cost calculator with the actual kWh figure from your meter or energy app.
Can I use these tools for three-phase electrical systems?
The wire gauge and voltage drop tools on this hub are designed for single-phase and DC systems, covering the vast majority of residential and light commercial applications. The Watts / Volts / Amps calculator in the Electronics hub supports three-phase AC with power factor input. For three-phase panel calculations, motor starting currents, and transformer sizing, the formulas require additional inputs (line vs phase voltage, delta vs star configuration) beyond the current scope of these tools. The Power Factor calculator covers reactive power (kVAR) and apparent power (kVA), relevant to three-phase motor and generator analysis.