Utility bills hide in plain sight. The unit economics are simple once you separate power (how fast energy flows) from energy (what you pay for). This guide gives you the anchors to read any bill, size any heater, and price any appliance in seconds.
Power vs energy: the 10-second version
- Watt (W) / kilowatt (kW): power, the rate right now. A 2,000 W heater is burning 2 kW while it runs.
- Kilowatt-hour (kWh): energy, power × time. Run that heater 1 hour: 2 kWh. That is what the bill counts.
- BTU (British Thermal Unit): heat quantity, used to size heaters and ACs in North America. 1 kWh = 3,412 BTU.
The analogy: kW is speed, kWh is distance. Your bill charges per distance.
What appliances actually cost
Take your price per kWh (EU 2026: roughly 0.25-0.40 EUR; US: 0.15-0.25 USD) and multiply by hours:
| Appliance | Power | 1 hour costs (at 0.30 EUR/kWh) |
|---|---|---|
| LED bulb | 9 W | 0.003 EUR |
| Laptop | 60 W | 0.018 EUR |
| Fridge (average over day) | ~100 W | 0.03 EUR |
| Washing machine (hot cycle) | 2,000 W | 0.60 EUR |
| Space heater | 2,000 W | 0.60 EUR |
| Sauna stove | 8,000 W | 2.40 EUR |
The space-heater rule people miss: a 2 kW heater running a winter workday (10 h) costs 6 EUR per day, about 500 EUR over a heating season. That is why insulation beats any heater upgrade.
Sizing heaters and AC without the guesswork
Rough rule: 100 W of heating per square meter for a modern insulated room with 2.5 m ceilings (older buildings: 120-150 W). A 20 m² bedroom wants 2,000 W, or about 6,800 BTU. Air conditioning: rule of 60 W (or 20 BTU) per m² including heat gains, so the same room wants a 1,200 W (8,500 BTU) unit.
| Room size | Heater (W) | Heater (BTU) | AC (BTU) |
|---|---|---|---|
| 10 m² (108 sq ft) | 1,000 | 3,400 | 5,000 |
| 20 m² (215 sq ft) | 2,000 | 6,800 | 8,000 |
| 40 m² (430 sq ft) | 4,000 | 13,600 | 14,000 |
Convert any figure exactly: watts → BTU/h.
Food energy and the calorie trap
A food "calorie" is a kilocalorie (kcal), 1,000 physics calories. EU labels also show kilojoules: 1 kcal = 4.184 kJ. A 2,000 kcal diet is 8,368 kJ, which is why Australian labels look 4x bigger. Nothing is wrong; the units differ. kcal → kJ converter.
EV and battery terms
- Battery size is kWh: a 60 kWh EV pack holds the energy of about 5 liters of petrol (chemically), but uses it 3-4x more efficiently
- Charging speed is kW: a 22 kW wallbox refills 22 kWh per hour, so that 60 kWh pack takes under 3 hours
- Consumption: 15-20 kWh/100 km is typical; divide battery kWh by that to estimate range