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. No account, no ads, instant results.
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.
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.
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.