Direct answer: for a QB hybrid inverter, 24V fits small, defined load lists up to about 4 kW, and 48V fits everything larger. Battery size in kWh follows one formula: essential load watts × autonomy hours = usable kWh, then divide by the battery’s usable depth of discharge to get the nominal bank size. A 2,000W essential load running 5 hours needs 10 kWh usable — about 12.5 kWh nominal with lithium at 80% usable depth, or 20 kWh nominal with lead-acid at 50% usable depth. The rest of this guide shows how voltage changes current and cable losses, and maps the result onto the QB range from 4.2 kW to 10.2 kW.
Why Battery Voltage Matters More Than Battery Brand
Power in watts equals volts times amps. At 24V, a 4,200W inverter draws about 175A at full load; at 48V, a 6,200W inverter draws about 129A. Higher current means thicker cables, more heat and more energy lost as heat — which is why 24V systems top out around 4–5 kW in practice, while 48V systems comfortably cover 6–10 kW. The voltage decision is therefore a ceiling decision: it caps how large the system can grow without replacing the inverter.
The QB range follows this logic exactly. The QB42-24 is the 24V model at 4.2 kW; everything above it in the QB hybrid family is 48V — the QB62-48 at 6.2 kW, and the QB82-48 at 8.2 kW and QB102-48 at 10.2 kW. Buyers who expect to add a pump or an air conditioner later should start at 48V: adding batteries later is easy, but changing battery voltage later means replacing the inverter.
The Sizing Formula, Step by Step
Step 1 — List the essential loads. Only loads that must run during an outage count here: lights, fridge, router, fans, a TV, and any pump or medical device the household cannot do without. Add their watts. Air conditioning and water heating are usually excluded from the essential list unless the bank is sized for them explicitly.
Step 2 — Set autonomy in hours. Autonomy is how long those loads must run without sun: a typical evening figure is 4–6 hours, while full overnight cover is 10–12 hours. Multiply watts by hours to get usable kWh.
Step 3 — Divide by usable depth of discharge. A battery’s nominal kWh is not its usable kWh. As a planning rule, lithium banks are commonly used to about 80% of nominal, and lead-acid banks to about 50%. Nominal bank kWh = usable kWh ÷ usable fraction.
Step 4 — Check the current. Divide the inverter’s rated watts by the battery voltage. If the result pushes past what the model’s solar charge current and cabling can support, the load list — not the battery — is the problem. The QB42-24 and QB62-48 both charge solar at 120A; the QB82-48 and QB102-48 charge at 160A.
Worked Example: 2,000W for 5 Hours
| Step | Calculation | Result |
|---|---|---|
| Essential load | Lights, fridge, router, fans, TV | 2,000W |
| Autonomy | Evening, no sun | 5 hours |
| Usable energy | 2,000W × 5h | 10 kWh |
| Lithium bank (80% usable) | 10 ÷ 0.80 | 12.5 kWh nominal |
| Lead-acid bank (50% usable) | 10 ÷ 0.50 | 20 kWh nominal |
| Matching inverter | 2,000W load + starting surge headroom | QB42-24 (4.2 kW) minimum; QB62-48 (6.2 kW) with growth room |
Note what the example does not do: it does not size the solar array. The array must refill the bank on an average day while also carrying daytime loads. As a cross-check, the QB42-24 accepts up to 4,500W of PV and the QB62-48 up to 6,500W, both over a 90–430V DC MPPT range; the QB82-48 and QB102-48 each accept two 5,400W PV strings over a 90–450V DC range. If the array cannot refill the bank daily, the bank size is irrelevant — the system will start each evening partly empty.
Which QB Model Fits Which Battery Plan
| Model | Rated output | Battery voltage | Max PV array | Solar charge current | Best fit |
|---|---|---|---|---|---|
| QB42-24 | 4.2 kW | 24V | 4,500W | 120A | Small homes, cabins, shops with a defined load list |
| QB62-48 | 6.2 kW | 48V | 6,500W | 120A | Family homes and offices, room to grow |
| QB82-48 | 8.2 kW | 48V | 5,400W + 5,400W | 160A | Larger homes, clinics, workshops with motor loads |
| QB102-48 | 10.2 kW | 48V | 5,400W + 5,400W | 160A | Small commercial sites, farms, multi-load properties |
Two special cases sit outside this table. The QB30 (1.5 kW / 3 kW, 24V) must be used with a battery — unlike the QB42 to QB102 models, which can also run battery-free in daytime. And the QB-Hi upgrade series (4, 5.5, 7.5 and 11 kW, IGBT version, 5-year warranty) covers projects that need a higher-power off-grid frame than the standard QB line. For homes that also need water pumping from the same unit, the ALLINONE hybrid (7.5–30 kW, home-power plus water-pump modes, Regular and Simultaneous versions) is a separate decision — its 220V home output on the Simultaneous version is limited to 10A, so the version choice matters as much as the kilowatt rating.
Three Mistakes That Waste Battery Money
Buying nominal kWh instead of usable kWh. A 10 kWh lead-acid bank used to 50% delivers 5 kWh. Comparing nominal figures across chemistries without the depth-of-discharge divisor is the most common sizing error.
Sizing for the whole house instead of the essential list. Whole-house sizing doubles or triples the bank for loads — water heating, air conditioning — that could simply wait out an outage. Size the bank for essentials, and let the inverter’s grid-tie hybrid mode carry the rest when the grid is present.
Ignoring the refill rate. A bank that the array cannot refill daily is a bank that degrades early, because it cycles in a permanent partial state of charge. Check the model’s maximum PV array against the daily refill need before finalising the bank.
Frequently Asked Questions
How many kWh of battery do I need for a 6.2 kW inverter?
The inverter rating does not set the bank size — the essential load and autonomy do. A 2,000W essential load for 5 hours needs 10 kWh usable, or 12.5 kWh nominal with lithium. The 6.2 kW rating of the QB62-48 only sets the ceiling: the largest load the system can carry at once.
Is 24V or 48V better for a whole house?
48V. A 24V system suits small, defined load lists up to about 4 kW. Above that, 48V halves the current for the same power, loses less energy in cabling, and gives access to the 6.2–10.2 kW models most houses need once pumps and air conditioning are included.
Can a QB hybrid inverter run without a battery?
The QB42 to QB102 models can run battery-free in daytime, when PV or AC input can carry the load — but without a battery there is no night cover and no ride-through when a cloud passes. The QB30 is the exception: it must be used with a battery. For genuine outage protection, budget for at least a modest bank.
Sizing a QB system? Send IDEEI your essential load list and the hours it must run without sun, and we will recommend the model, battery voltage and bank size for your site. Contact IDEEI for QB battery sizing advice. You can also browse the full range on our products page.