Batteries
What is usable battery kWh on a South African solar quote?
Usable kWh is the energy you can actually cycle after depth-of-discharge limits, Min SoC floors and inverter losses — not the sticker kWh on the carton. A 10 kWh pack at 90% DoD is about 9 kWh usable before conversion losses; a 16 kWh pack limited to 80% may be closer to 12.8 kWh. Demand usable kWh on the quote cover next to the price, or you cannot compare Stage 4 runtime or payback.
Updated 27 October 2026 · Illustrative ranges — not a formal quote, credit decision or legal advice.
Answer first: Refuse to compare batteries on sticker kWh alone. Convert every option to usable kWh with the OEM’s recommended DoD, then check continuous backup kW for fridge surges. That single habit stops most WhatsApp “who has the bigger battery” arguments.
Sticker → usable (illustrative)
| Sticker | Recommended DoD | Usable (before inverter loss) | Typical use |
|---|---|---|---|
| 5.12 kWh | 90–95% | ≈4.6–4.9 kWh | Entry wall-mount module |
| 10 kWh | 90% | ≈9.0 kWh | Small home essentials |
| 16 kWh | 80% (some OEMs) | ≈12.8 kWh | Larger SA single-box formats |
| 16 kWh | 90% if allowed | ≈14.4 kWh | Only with datasheet + warranty alignment |
Subtract another ~8–12% for inverter/path losses when sketching kettle + fridge evenings. Related: DoD · DoD desk · runtime.
Why quotes hide behind sticker kWh
Sticker figures look bigger in ads. Usable kWh is what keeps the essentials board alive and what your warranty throughput is measured against. If two quotes show “10 kWh” but one allows 90% DoD and the other forces a 20% floor, you are not buying the same product.
Savings and tariff link
More usable kWh means more energy you can shift from expensive evening import or weak noon export into self-use. On TOU metros, that shift is often worth more than two extra panels that dump at midday — export vs self-use · Cape Town TOU · monthly savings.
Owner checklist
- Sticker kWh and usable kWh both printed.
- Recommended DoD / Min SoC from the OEM PDF.
- Continuous and peak backup kW of the inverter port.
- Warranty years and throughput (MWh) where published.
- Who registers the warranty in SA.
Worked Stage 4 evening (illustrative)
Essentials average 0.9 kW for six hours → 5.4 kWh before losses. With ~10% inverter/path loss you want about 6.0 kWh usable. That is roughly a 6.7 kWh sticker at 90% DoD or a 7.5 kWh sticker at 80% DoD. A quote that only says “8 kWh battery” without DoD leaves you guessing whether Friday’s Stage 4 finishes with lights or with a dark fridge — Stage 4 battery · Stage 6.
Parallel packs and marketing maths
Two 5 kWh modules at 90% DoD are not automatically “10 kWh usable” if the BMS reserves a higher floor when paralleled, or if one module is shaded/hot and derates. Ask for the parallel manual’s usable figure and the continuous discharge of the bank, not only module stickers. SA-made and imported LiFePO₄ lines (Solar MD via its intermediary network, BlueNova OEM sheets, Pylontech and peers stocked through Herholdt’s, ARB, Voltex, ACDC, DBS) should all be compared on usable kWh + claims desk.
Warranty throughput vs daily usable
Some warranties quote years and total MWh. Daily usable × 365 × years is a rough ceiling check. If your plan is deep daily cycling for TOU arbitrage plus Stage backup, confirm you are inside the published throughput before you treat the pack as infinite — cycles & warranty · claim steps.
App SoC vs real kWh
Phone apps often show SoC % with a hidden floor. A “20%” reading may already be near the BMS cut-off. Once a season, run a known load (oil heater on a meter) from 100% to cut-off and log kWh delivered — that is your household’s real usable number.
Buying conversation script
- “Print usable kWh at the OEM recommended DoD on page one.”
- “What Min SoC will you leave programmed on day one?”
- “What continuous kW can the backup port sustain with fridge surge?”
- “Who registers the warranty and where is the SA claims email?”
- “Show me the throughput / cycle clause highlighted.”
Lead-acid leftovers and “100 Ah” folklore
Older quotes sometimes still talk in amp-hours at 12 V or 24 V. Convert to kWh first (V × Ah / 1000), then apply a much shallower lead-acid DoD (often ~50%). A “200 Ah” bank is not automatically larger than a 5 kWh LiFePO₄ module once chemistry and DoD are honest. Prefer lithium quotes that already print usable kWh.
EV charging drain vs backup reserve
If the wallbox sits on the backup board, an evening charge can consume the usable kWh you sized for Stage 4. Either schedule the car before shedding, put the charger on non-essentials, or add usable capacity dedicated to mobility — EV load management · EV battery sizing.
Degradation and year-three usable
Usable kWh falls as the pack ages. A warranty that guarantees 70% retained capacity at year ten means your Stage maths should not assume day-one usable forever. Build a 10–15% margin into the first purchase if shedding resilience is the primary job — battery lifespan · panel degradation (same honesty habit).
Resale and disclosure
When you sell, buyers and banks increasingly ask for usable capacity and SSEG paperwork, not a Facebook photo of the wall. Keep commissioning sheets and the usable-kWh figure with the title file — selling with solar.
Hybrid settings that steal usable kWh
Grid-charge caps, forced export, and “keep battery for backup only” modes can leave usable capacity idle while you buy expensive evening kWh. Align modes with your tariff: self-use first on weak export credits; reserve a documented SoC floor for Stage 4; avoid charging from peak TOU “just in case” every afternoon — grid-charge desk · Cape Town TOU.
Write the final settings into the handover pack with a photo of the screen. When a technician “optimises” remotely six months later, you will know what changed.
Quick conversion card to keep on your phone
Usable ≈ sticker × DoD × (1 − path loss). Example: 10 × 0.90 × 0.90 ≈ 8.1 kWh delivered to loads. If the installer disagrees with your DoD or loss factor, ask them to replace the numbers with the OEM PDF values in writing. That card is enough to catch most inflated Facebook comparisons before a deposit leaves your account.
Education only: usable kWh on a quote is a planning number. Confirm the OEM recommended DoD and the programmed Min SoC on commissioning day before you trust a Stage 4 spreadsheet.
Nameplate vs usable — quote test
If the cover says “10 kWh” but the fine print allows 80% DoD, usable is ~8 kWh before inverter losses. Recalculate Stage hours with usable, not nameplate — DoD · Stage 6 · runtime tool.
Quick questions
Is usable kWh the same as DoD?
DoD is the percentage rule; usable kWh is sticker × that percentage (then losses). You need both.
Can I set 100% DoD to get more usable kWh?
Only if the OEM and warranty allow it. Many LiFePO₄ sheets recommend ≤80–90% for daily cycling.
Why does my app show less than usable kWh?
Apps often reserve a floor, show SoC not kWh, or count inverter losses. Calibrate against a known load test.
Sources
Nameplate vs usable — quote red flag
If two quotes show the same “10 kWh” but different DoD / min SoC, ask for usable kWh at the settings they will leave you with — DoD · read a quote · runtime tool.