How to Size a Lithium Battery for Load-Shedding
Buying “the biggest battery you can afford” is a common reaction to load-shedding — and often the wrong one. Size the bank from the loads you actually need during an outage, then match voltage and discharge rate to your inverter. Here is a practical way to do it for a South African home.
Step 1 — List essential loads
Write down each backup load in watts and how many hours you need it during a typical outage.
Worked example (evening outage): fridge 150 W × 4 h = 600 Wh; LED lights 60 W × 4 h = 240 Wh; Wi-Fi + router 20 W × 4 h = 80 Wh; TV 100 W × 3 h = 300 Wh. Total ≈ 1,220 Wh (1.22 kWh) of useful energy. That is a modest critical-loads board — not a whole-house geyser and stove.
Step 2 — Add usable capacity and losses
LiFePO4 batteries are commonly used with a high depth of discharge, but you should still leave headroom. Add about 10% for inverter losses, and another buffer if you want reserve for a second outage the same day. Rule of thumb: take your Wh need, divide by 0.9 (losses), then add 20–30% reserve for comfort.
From the example: 1.22 kWh ÷ 0.9 ≈ 1.36 kWh; with 30% reserve ≈ 1.8 kWh usable. A single 5 kWh module is already more than enough for that critical set — which is why many homes start at 5 kWh and grow later.
Step 3 — Match battery voltage to the inverter
- 48V / 51.2V LV: the usual home hybrid pairing.
- High-voltage stacks: only with HV hybrid / PCS inverters. Never force an LV battery onto an HV inverter.
Confirm CAN/RS485 compatibility on both product pages before you order.
Step 4 — Check C-rate and peak current
A kettle (~2,000 W), microwave or geyser element can trip a small inverter or pull more current than a small battery bank should deliver continuously. Size the inverter for surge and continuous power, and make sure the battery’s allowed discharge current covers those peaks. If a load is optional during outages, leave it off the backup board.
Illustrative sizes (not a guarantee)
| Bank size | Typical critical-load use |
|---|---|
| ≈5 kWh | Fridge, lights, Wi-Fi, laptops/TV for several hours; manage kettle use |
| ≈10 kWh | Same plus longer outages or a few heavier intermittent loads |
| ≈15 kWh+ | Larger homes, partial air-con/geyser backup, or multiple cycles per day |
These are planning guides. Your installer should calculate from your actual load list and inverter limits.
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See lithium batteries, Dyness batteries, high-voltage batteries, and cables & connectors. All store prices are VAT-inclusive. Ask Powerforum if you want a second opinion on sizing.
FAQ
Is 5 kWh enough for stage 6?
It can be — if the backup board is limited to essentials. Whole-house backup through long stages usually needs more capacity and a larger inverter.
Can I add modules later?
Many LiFePO4 ranges allow parallel expansion within brand/model rules. Check the maximum parallel count and whether new and old modules should be the same batch/firmware.