Summary: The biggest mistake buyers make when comparing home batteries is trusting the nameplate capacity number. A "10 kWh battery" often delivers only 9 kWh — or less — in real use. The difference comes from depth of discharge (DoD), round-trip efficiency, voltage derating, and temperature losses. This article shows you how to read a supplier datasheet correctly and calculate the actual energy you will get.
The nameplate number is a marketing number
On a datasheet, the nameplate capacity looks like this:
- Nominal capacity: 10 kWh
- Nominal voltage: 51.2 V
- Rated current: 100 A
Datasheet specs assume 25°C lab conditions, fresh cells, and 100% DoD—rarely true at home.
The four numbers that matter
When you compare two batteries, do not compare the nameplate kWh. Compare these four:
1. Usable capacity This is the energy you can actually draw. Most LiFePO4 batteries allow 90% DoD. A 10 kWh nameplate battery delivers about 9 kWh. Some cheaper brands only allow 80% DoD, delivering 8 kWh.
Check the datasheet for:
- "Usable energy" or "Available capacity"
- "Depth of discharge (DoD)"
- "Cut-off voltage"
If the datasheet only shows nominal capacity and no usable capacity, ask the supplier directly.
2. Round-trip efficiency This is the percentage of energy that comes back out after charging. A typical LFP battery has 93–95% round-trip efficiency.
So 9 kWh usable × 94% efficiency = 8.46 kWh delivered to your home.
Cheap batteries with lower-quality BMS may have 88–90% efficiency. At 89% RTE, an 11% loss on a daily 10 kWh cycle ≈ 400 kWh lost per year.
3. Continuous discharge current This is how much power the battery can deliver at once. A 5 kWh battery with a 100 A discharge rating delivers 5.12 kW continuous. That is enough for essentials but not for running an electric oven.
Check:
- "Continuous discharge current"
- "Peak discharge current" (usually 2× continuous, for 5–10 seconds)
If your home backup needs 3 kW, make sure the battery's continuous power is at least 3.5 kW.
4. Cycle life at specified DoD A datasheet might say "6,000 cycles." But at what DoD?
- At 100% DoD: 3,000 cycles
- At 80% DoD: 6,000 cycles
- At 50% DoD: 10,000+ cycles
Always check the cycle test condition. A "6,000 cycle" battery cycled at 100% DoD is not as durable as the marketing suggests.
Worked example: comparing two 10 kWh batteries
| Brand A | Brand B | |
|---|---|---|
| Nameplate capacity | 10 kWh | 10 kWh |
| DoD | 90% | 80% |
| Usable capacity | 9 kWh | 8 kWh |
| Round-trip efficiency | 94% | 89% |
| Delivered energy | 8.46 kWh | 7.12 kWh |
| Continuous power | 5.12 kW | 3.5 kW |
| Cycle life | 6,000 @ 80% DoD | 6,000 @ 100% DoD |
Brand A looks slightly more expensive on paper but delivers 19% more energy per cycle. Over 6,000 cycles, Brand A gives you 8,000 kWh more energy — that is the real comparison.
Temperature derating
Most datasheets list capacity at 25°C. At 0°C, LFP capacity drops by about 10–15%. At -10°C, it drops 20–30%. If you live in a cold climate, the battery's real usable capacity is lower.
Check the datasheet for a "capacity vs temperature" curve or table. If it is missing, ask for it.
Why suppliers hide usable capacity
Some suppliers only show nominal capacity because:
- It looks bigger on the spec sheet
- It makes the price per kWh look lower
- It differentiates against competitors who show usable capacity
Require datasheets that list both nominal and usable capacity.
The questions to ask
When comparing quotes, ask:
- What is the usable capacity at 90% DoD?
- What is the round-trip efficiency?
- What is the continuous discharge current?
- How many cycles at what DoD?
- What is the capacity at 0°C?
Treat unanswered specs as unverified marketing claims.
A simple formula for real capacity
Real usable energy = (Nameplate kWh × DoD) × Round-trip efficiency
Example:
- Nameplate: 10 kWh
- DoD: 90%
- Efficiency: 94%
- Real delivered: 10 × 0.90 × 0.94 = 8.46 kWh
Use this formula when comparing quotes. The cheapest nameplate kWh may not be the cheapest usable kWh.
Bottom line
Do not buy a battery based on the nameplate number. Read the datasheet, calculate usable energy, and compare that number. A 10 kWh battery that actually delivers 8.5 kWh is better than a 12 kWh battery that only delivers 9 kWh due to lower efficiency and tighter DoD.
Ruibit's Dawnice HZEB-LCT datasheets show both nominal and usable capacity, along with round-trip efficiency and cycle life at 80% DoD. Ask us for the full datasheet before comparing.
FAQs
Q: Is usable capacity always 90% of nameplate? Most LFP batteries are 90% DoD. Some are 80%, and premium ones can reach 95%. Check the datasheet.
Q: Why is round-trip efficiency not 100%? Energy is lost as heat during charging and discharging. A 94% efficiency means 6% of energy becomes heat inside the battery.
Q: Does the inverter also reduce efficiency? Yes. The hybrid inverter has its own efficiency (typically 97%). Total system efficiency = battery efficiency × inverter efficiency ≈ 91%.
Q: Can I increase usable capacity by discharging deeper? You can, but it shortens cycle life. Going below 90% DoD on a regular basis cuts cycle life by 30–50%.
Q: What is the best way to compare battery prices? Compare "price per usable kWh delivered," not "price per nameplate kWh." Divide the total price by the real delivered energy number.
