Calculator KB-21
Power Supply Efficiency Calculator
The difference between 85% and 95% efficiency is noticeable both in heat and on your electricity bill.
Result
In brief: Calculate power supply losses based on efficiency: input power, heat generated, and energy and cost losses per year at your electricity rate. Example: Dissipated as heat: 13.33 W.
Calculation example
| Output voltage | 12 V |
|---|---|
| Load current | 10 A |
| Efficiency | 90 % |
| Hours of operation per day | 24 ч |
| Price per 1 kWh | 4.32 |
| Dissipated as heat | 13.33 W |
|---|---|
| Output power | 120 W |
| Power drawn from mains | 133.33 W |
| Annual losses | 116.8 kWh/year |
| Annual cost of losses | 505 |
How it is calculated
Pheat = Pin − Pout
Everything the power supply draws from the mains but doesn’t deliver to the load turns into heat inside the case. This figure helps determine whether the cooling is adequate and whether forced airflow is needed.
What to look for
- Efficiency depends on the load: it usually peaks at 50–80% of rated power and drops noticeably at low currents.
- The figures in product descriptions are often inflated. You can easily check the actual efficiency with two wattmeters or by looking at our power supply reviews.
Frequently asked questions
How much heat does a power supply with 90% efficiency produce?
Pheat = Pout · (1 / η − 1). At 12 V and 10 A, that’s 120 W output and about 13.3 W of heat.
How much does higher efficiency save?
A 120 W power supply running 24/7 loses 13.3 W at 90% efficiency and 6.3 W at 95%. The 7 W difference adds up to about 61 kWh per year.
At what load does a power supply operate most efficiently?
Usually at 50–80% of its rated power. Efficiency drops noticeably at low loads, so a power supply with a huge capacity margin isn’t always the best choice.
