Most undersized home battery systems fail for the same reason: somebody picked a capacity from a catalogue instead of measuring the load. This guide walks through the sizing calculation we use when quoting, with three worked examples and the numbers behind them.
Sizing a home battery is arithmetic, not guesswork. There are five inputs — critical load in watts, daily energy in kWh, usable depth of discharge, round-trip efficiency, and surge current — and if you get those right the answer falls out. Get one wrong and the system either browns out on the first cold evening or costs 40% more than the customer needed.
Step 1: Separate critical load from total load
The first mistake is sizing to the whole house. A backup system is not meant to run the whole house; it is meant to run what the customer actually needs during an outage. Ask them to list it, then confirm against their bill.
| Load | Running power | Surge multiplier | Typical daily energy |
|---|---|---|---|
| LED lighting (10 points) | 50–100 W | 1× | 0.3–0.5 kWh |
| Refrigerator (400 L) | 120–180 W | 3× (compressor start) | 1.0–1.5 kWh |
| Router + ONT | 10–15 W | 1× | 0.25–0.35 kWh |
| Laptop | 60–90 W | 1× | 0.3–0.4 kWh |
| TV (55″) | 80–120 W | 1× | 0.2–0.4 kWh |
| Microwave | 900–1200 W | 1× | 0.2 kWh per 10 min |
| Kettle | 1500–2000 W | 1× | 0.1 kWh per boil |
| Water pump (0.75 kW) | 750 W | 3–5× | 0.3–0.6 kWh |
| Air conditioner (1.5 kW) | 900–1500 W | 2–3× (inverter type: 1×) | 3–8 kWh |
| Induction hob | 1800–2200 W | 1× | 0.6–1.0 kWh |
Two things matter here and both are easy to miss. First, running watts are not starting watts. A 750 W pump can pull 3.5 kW for a fraction of a second, and an inverter that cannot deliver that surge will trip even though the battery is full. Second, daily energy is not running watts × 24. A fridge drawing 150 W runs maybe a third of the time, so its daily consumption is roughly 1.2 kWh, not 3.6 kWh.
Step 2: Convert to daily kWh
Multiply each load’s running watts by its actual hours per day, not by 24. For cycling loads — fridges, pumps, AC — use a duty cycle estimate and then check it against the customer’s electricity bill. If your calculated critical load is more than about 40% of their total daily consumption, the list is too generous.
For reference, average household consumption is roughly 10–15 kWh per day in most of Europe, 25–30 kWh per day in the United States, and 8–12 kWh per day in urban Southeast Asia. A backup system is typically sized to cover 30–50% of that, not all of it.
Step 3: Apply depth of discharge and efficiency
Nameplate capacity is not usable capacity. Two derating factors apply:
- Depth of discharge (DoD). LiFePO4 can safely use about 90% of nameplate. Lead-acid should not go below 50%.
- Round-trip efficiency. Expect 92–95% for a lithium system including the inverter; 80–85% for lead-acid.
The formula:
Required nameplate (kWh) = Daily critical load (kWh) ÷ (DoD × efficiency)
For 5 kWh of daily critical load with LiFePO4 at 90% DoD and 93% efficiency:
5 ÷ (0.90 × 0.93) = 5.97 kWh
So a 5 kWh load needs roughly a 6 kWh battery — a number no catalogue page will tell you, and the reason a “5 kWh system” sized by eye comes up short.
Step 4: Match continuous inverter output
Battery capacity and inverter power are separate decisions, and the smaller of the two is what the customer experiences. Add up the loads that might run simultaneously — not every load in the house — and add 20% headroom.
A practical check: if the customer wants to run a kettle and a microwave together, that is 1.5 + 1.0 = 2.5 kW, so a 3.6 kW inverter is right and a 2 kW unit is not. Our home energy storage range pairs 3.6 kW inverters with 5–15 kWh banks and 12 kW inverters with 15–61 kWh banks for exactly this reason.
Step 5: Size for autonomy, not for one night
Ask how many days of independence the customer expects. One night is typical for grid-tied backup; two to three days matters for off-grid or unreliable-grid markets.
Autonomy days multiply the battery, not the inverter. A customer wanting two days on 5 kWh per day needs 12 kWh of nameplate, not 6 kWh.
Three worked examples
Small apartment, grid-tied backup
Critical loads: fridge 150 W, lighting 60 W, router 12 W, laptop 70 W, phones 20 W. Daily energy ≈ 2.2 kWh. Inverter peak 1.0 kW (microwave).
2.2 ÷ (0.90 × 0.93) = 2.63 kWh for one day. A 5.12 kWh wall-mounted unit gives about two days of autonomy on this load, paired with a 3.6 kW inverter. This is the smallest configuration that still feels normal during an outage.
Detached house, 8 kWh critical load
Critical loads: fridge, lighting, router, TV, laptop, pump, one AC circuit. Daily energy ≈ 8 kWh. Simultaneous peak 4.5 kW.
8 ÷ (0.90 × 0.93) = 9.55 kWh per day. One 10.24 kWh unit covers a single day; two stacked units at 20.48 kWh cover two days comfortably. A 12 kW inverter handles the AC surge without derating.
Small commercial, 40 kWh critical load
Retail unit with refrigeration, lighting and POS. Daily energy ≈ 40 kWh. Peak 18 kW at opening.
40 ÷ (0.90 × 0.93) = 47.8 kWh. A 46.08 kWh rack, or 61.44 kWh for half a day of extra margin, with a 12 kW inverter per phase. For loads above 100 kWh per day, a commercial cabinet in the 106–215 kWh range is the right format.
Why we standardise on 51.2 V
Almost every low-voltage residential system in the 5–60 kWh band now uses a 51.2 V nominal bus — sixteen LiFePO4 cells in series, 40–58.4 V operating range. The reason is practical: it stays under the 60 V DC safety threshold that triggers extra enclosure and training requirements in most jurisdictions, while keeping current manageable.
At 51.2 V, a 12 kW inverter draws about 235 A, which is why cable gauge and breaker selection matter as much as cell count. Above roughly 60 kWh, the current becomes impractical and systems move to high-voltage stacks at 204–512 V.
The advantage of a single bus voltage is expansion. A customer who starts with 5.12 kWh and adds capacity later needs the new module to sit on the same bus — which is why stackable platforms beat one-off enclosures for resellers. Our 51.2 V range runs from 5.12 kWh to 61.44 kWh on the same architecture, in both wall-mounted and stacked formats.
Chemistry: LFP versus LMFP
Both are lithium iron phosphate family chemistries, and the choice depends on the application rather than on which is “better”.
| LFP (LiFePO4) | LMFP (lithium manganese iron phosphate) | |
|---|---|---|
| Energy density | Baseline | 15–20% higher |
| Cycle life | 6,000+ cycles to 80% | 3,000–4,000 cycles typical |
| Thermal stability | Excellent | Very good |
| Low-temperature performance | Limited below 0 °C charging | Slightly better |
| Best fit | Stationary storage, daily cycling | Portable power, weight-sensitive products |
In practice: stationary home storage should be LFP, because cycle life dominates the economics over ten years. Portable power stations benefit from LMFP, because a lighter unit at the same capacity is a selling point the end user can feel. Our portable power range uses both, with LMFP on the models where weight matters most.
Five mistakes that show up in every bad quote
- Sizing to nameplate instead of usable capacity. A 5 kWh battery delivers about 4.2 kWh. Quote the usable number.
- Ignoring surge. The battery is fine; the inverter trips. Check the pump and compressor multipliers.
- Sizing for the whole house. The customer’s bill is not the system’s load. Ask for the critical circuit list.
- Forgetting the charger. A 5 kWh battery charged at 1 kW needs five hours of sun or grid. Match charge power to the recharge window the customer expects.
- No expansion path. Customers add load. If the platform cannot stack to at least twice the initial capacity, you will lose the second sale.
Frequently asked questions
How many kWh do I need to run a house for a day?
For a typical European home, 5–10 kWh of usable capacity covers essential circuits for 24 hours. US homes with air conditioning usually need 10–20 kWh. The number that matters is the critical load list, not the total bill — calculate that first, then divide by 0.84 (90% DoD × 93% efficiency).
Can I add batteries to a system later?
Only if the platform supports it. The new module must share the same nominal voltage, the same BMS communication protocol, and — ideally — the same cell chemistry and age profile. Mixing old and new modules on one bus causes imbalance and shortens the life of both.
What size inverter do I need?
Add the loads that will run simultaneously, add 20%, and check the surge requirement of any motor load. A 3.6 kW inverter suits most apartments and small homes; 12 kW suits a detached house with air conditioning.
Is a 51.2 V system safe to install myself?
51.2 V sits below the 60 V DC threshold used in many jurisdictions, which is why it is the standard for residential. That does not make it risk-free — a lithium bank can deliver thousands of amps into a short circuit. Installation should follow local electrical code, and in most markets requires a licensed electrician for the AC side.
How long will a home battery last?
A quality LFP system cycling once a day should retain 80% capacity after 6,000 cycles, which is roughly 15 years of daily use. In practice, calendar aging and temperature matter as much as cycle count; keeping the bank between 10 °C and 30 °C has more effect on lifespan than shallow-cycling it.
Sizing a project right now? Send us the load list and we will return a capacity and inverter recommendation with the calculation shown. Get in touch, or browse the home energy storage range and commercial & industrial ESS.