Is a smaller model greener? The energy cost of 4-bit LLMs
We compare the two 4-bit post-training quantization formats, FP4 and NF4, on energy use as well as accuracy. The early results cut against the assumption that smaller means greener.
- Model
- Mistral-7B in FP16, FP4 and NF4
- Hardware
- NVIDIA RTX PRO 6000
- Quality
- MMLU, WikiText-2 perplexity, hidden-state similarity to FP16
- Cost
- Throughput, and GPU power from NVML (idle-subtracted, PUE-scaled)
- Protocol
- Five trials in randomized order
Findings so far
- 4-bit cuts peak GPU memory from 14.5 GB to about 4.6 GB and raises throughput from 71.1 to about 90 tokens/s.
- It doesn't save energy. A full MMLU run uses about 5% more than FP16: 0.104 kWh (FP4) and 0.105 kWh (NF4) against 0.0994.
- NF4 beats FP4 on accuracy (0.491 vs 0.487), perplexity and hidden-state similarity, and uses slightly more energy.
Mistral-7B at three precisions
Preliminary results. Hover or focus a bar for details.
- FP16
- FP4
- NF4
Peak GPU memoryGB, lower is better
4-bit: −68% vs FP16
Throughputtokens/s, higher is better
4-bit: +27% vs FP16
Energy for a full MMLU runkWh, lower is better
FP4 +4.6% · NF4 +5.6% vs FP16
MMLU accuracyhigher is better
NF4 leads FP4 by 0.004
View as table
| Metric | FP16 | FP4 | NF4 |
|---|---|---|---|
| Peak GPU memory (GB) | 14.5 | ≈4.6 | ≈4.6 |
| Throughput (tokens/s) | 71.1 | ≈90 | ≈90 |
| Energy, full MMLU (kWh) | 0.0994 | 0.104 | 0.105 |
| MMLU accuracy | n/a | 0.487 | 0.491 |
4-bit memory and throughput are approximate and shared by FP4 and NF4. FP16 MMLU accuracy isn't reported here yet.
NextPhi-3, Gemma 3 and Qwen 2.5 7B, then a paper.