Formula
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
- Assumes equal useful light output and unchanged usage. No discounting, lamp replacements, tariff changes or disposal costs.
Example Calculation
- Old lamp power: 60 W
- New lamp power: 10 W
- Lamps: 10
- Daily use: 5 h
- Electricity price: 0.2 currency/kWh
- Replacement investment: 100 currency
Ten 60 W lamps replaced by 10 W lamps, used five hours/day at 0.20/kWh, save 182.50 annually.
Frequently asked questions
How do I use this calculator?
Enter old lamp power, new lamp power, lamps, daily use, electricity price, replacement investment. The result updates automatically. Use Reset to restore the example values.
What method does it use?
annual saving=(old−new) × lamps × daily hours × 365/1000 × tariff.. Assumes equal useful light output and unchanged usage. No discounting, lamp replacements, tariff changes or disposal costs.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.