Off-Grid Inverter/Charger Sizing: All-in-One (Sol-Ark/EG4/Victron) vs Stacked, Pure-Sine, 120/240V

The battery bank is new, sized right, fully charged — and the lights still dim every time the well pump kicks on. Add the microwave into that same half-second and a breaker trips outright. Nobody undersized the battery. They undersized the box in between it and the house: the inverter/charger, which is a power problem, not an energy problem, and gets solved with completely different math.

This is the fourth number in the sizing sequence, after load, panels and battery — and the one most guides skip. The rest of the sequence lives in the guide index; here is the inverter/charger step, done properly.

What the Inverter/Charger Actually Does

Everything upstream of it — panels, charge controller, battery — deals in DC. Your house deals in AC. The inverter/charger sits at that boundary in both directions: it inverts DC battery power into 120/240V or 230V AC to run the house, and it charges the battery from AC — a generator, or a grid connection where one exists — through a charger with its own separate amperage rating. “Charger” is not a marketing suffix; undersizing it is one of the most common, and most invisible, mistakes in a DIY build.

In a hybrid all-in-one (Sol-Ark, EG4, Growatt and most SRNE-based units), the solar charge controller lives inside the same box. In a modular build (Victron, Outback, Schneider XW), the inverter/charger and the MPPT solar controllers are separate boxes coordinated by a system controller. Both move the same three watts — generation, storage, load — they just draw the diagram differently.

Continuous vs Surge: the Number That Actually Sizes the Box

Every inverter carries two power ratings, and sizing mistakes live in the gap between them. Continuous (rated) watts is what it outputs indefinitely without overheating — a thermal limit, not a hard ceiling. Surge (peak) watts is what it can output for a few seconds to about a minute, to carry a motor through its locked-rotor moment.

Anything with a motor — a well pump, a fridge compressor, an AC condenser, a table saw — draws several times its running wattage for a fraction of a second at start-up. Manufacturers rarely put that number on the nameplate the way they put running watts. Field-typical multipliers:

Load	Running watts	Typical surge	Surge watts
Fridge / freezer compressor	150–400W	~3×	600–1,200W
1/2 HP shallow-well jet pump	600–900W	~3×	2,000–2,700W
3/4–1 HP submersible well pump	750–1,000W	3–5×	2,500–4,500W
Mini-split heat pump (with soft-start)	1,200–2,000W	~1.5–2×	2,000–3,500W
Central AC, no soft-start	3,500–4,500W	3–4×	10,000–16,000W
Table saw / shop air compressor	1,500–2,000W	2–3×	3,500–5,000W
Microwave / kettle / toaster	1,000–1,500W	~1×	negligible

The real mistake isn’t ignoring surge — most people already know the well pump kicks hard. It’s sizing for the biggest load in isolation and forgetting it stacks on whatever’s already running: the inverter has to survive the existing load plus the pump’s incremental spike above its own running watts, not the pump’s surge number alone.

Pure Sine Wave Isn’t a Premium Option Anymore

Modified-square-wave inverters still exist and still cost less — and beyond a resistive heater or an incandescent bulb, they’re a false economy. Motors run hotter and louder on a stepped approximation of AC instead of a clean sine curve. Variable-speed compressors, the kind in every modern fridge, mini-split and washing machine, can fault or refuse to start outright. Switching power supplies — laptop chargers, LED drivers — buzz and lose efficiency. Some medical equipment, CPAP machines specifically, requires pure sine on the label. Every all-in-one hybrid and every Victron/Outback-class modular unit sold for whole-home off-grid use in 2026 is pure sine already — this stopped being a live decision, and only survives as a trap in the cheapest hardware-store portables.

120/240V Split-Phase vs 230V: Why the Two Continents Wire Differently

This is where “buy a bigger inverter” stops being the answer — it’s a wiring-topology question, not a power question.

In the US and Canada, the grid delivers split-phase: two 120V legs, 180° out of phase, that combine to 240V across the two hots for big appliances — well pumps, dryers, ranges, EV chargers — while single-leg circuits run everything else at 120V. An inverter that only makes one 120V leg physically cannot produce the second, opposite-phase leg a 240V load needs; no amount of continuous wattage fixes that. The fix is either one unit built to output both legs internally (Sol-Ark and EG4 do this natively) or two identical inverters wired leader/follower and phase-locked 180° apart. That second option is what off-grid installers mean by stacking in a US system — a synchronization requirement first, a capacity add-on second.

In Europe, mains is 230V single-phase to most homes off one leg; no split-phase step, one correctly sized inverter covers the whole panel. But large parts of Germany, the Netherlands, Austria and Scandinavia wire homes three-phase (three 230V legs, 400V between phases) specifically to carry bigger loads — an induction cooktop, a heat pump, an EV charger — without overloading any one leg. Replacing that supply off-grid needs either three phase-matched inverters, one per leg, or a single three-phase-capable unit; a genuinely single-phase cabin build skips all of it. Bring a US split-phase unit to a three-phase European job, or vice versa, and the mismatch shows up at the panel as nothing fitting — not as a power shortfall.

All-in-One Hybrid vs Stacked Modular: the Real Trade-Off

The brand argument — Sol-Ark vs EG4 vs Victron — is really three different questions wearing one costume: box count, failure mode, and who’s allowed to open it.

Unit	Architecture	Continuous (battery-only)	Surge (10s)	Charger
Sol-Ark 15K-2P	All-in-one hybrid, built-in 120/240 split-phase	12,000W	~24,000VA (30,000VA/100ms peak)	up to 275A
EG4 18kPV	All-in-one hybrid, built-in 120/240 split-phase	12,000W	~13,500–15,500W (27,000W is a 100ms peak)	up to 250A
Victron MultiPlus-II 5000 ×2, stacked	Modular, leader/follower synced for 120/240	8,000W (2×4,000W)	~18,000W peak	~140A combined
Victron MultiPlus-II 5000 ×3, European three-phase	Modular, one unit per 230V leg	12,000W (3×4,000W)	~27,000W peak	~210A combined

Continuous figures are battery-alone output; both flagships reach higher combined totals with solar or grid contributing. Surge is normalized to each unit’s ~10-second overload rating where the manufacturer publishes one — both all-in-ones carry a much higher sub-second peak on top of that (noted in-cell); Victron’s transformer-based design publishes only a single peak figure, so that number stands in directly. Specs current as of 2026 — confirm current listings before you spec against them.

All-in-one hybrid wins if:

• One inverter comfortably clears your continuous and surge numbers — true of most single-family homes.
• You want the simplest possible install: one box, one commissioning, one warranty call.
• Price per installed watt matters more than graceful degradation.

Stacked modular wins if:

• You want the house to keep running at reduced capacity if one module fails, not go fully dark.
• The load will grow — add a module later instead of replacing the whole inverter.
• You’re remote enough that a warranty swap is measured in weeks, not days.

That last point is the real reason serious remote builds lean modular even at a higher sticker price: redundancy is worth more than watts-per-dollar when the nearest service truck is three hours away.

The Battery Charger Amps Everyone Forgets to Size

Every inverter/charger carries a third rating below continuous and surge watts: charger amps — how fast it can push AC power, from a generator or the grid, back into the battery as DC. It’s governed by whichever of these three numbers is smallest.

1. The battery’s own max charge rate — its BMS enforces a C-rate limit, commonly 0.5C–1C on LiFePO4, so a 200Ah bank tops out around 100–200A of charge current no matter what’s feeding it.
2. The generator or grid circuit’s continuous amperage — a 250A charger fed by a 30A shore or generator connection only ever sees 30A; the charger’s rating is a ceiling, not a guarantee.
3. The charger’s own rated amps — Sol-Ark 15K, up to 275A; EG4 18kPV, up to 250A; a stacked pair of Victron MultiPlus-II units, roughly 140A combined, charger-variant dependent.

Why it matters in practice: a 250A charger at 48V is asking for roughly 12kW of DC charge power, which, after conversion losses, means a generator sustaining closer to 13–14kW continuously to feed it at full tilt. Owners running a 6.5–8.5kW portable generator against a 250A-rated charger almost always have to manually cap the input-current limit in the inverter’s settings — a real menu option on every unit in this class — so the charger doesn’t ask a small generator for more than it can sustain. Skip that step and the generator just bogs down while the charger throttles itself anyway — nothing breaks, the promised fast recharge just never happens. Sizing the generator side specifically? Run it through the Home Backup Power Sizing Calculator.

A Worked Example: From Load Numbers to a Configuration

Say the solar sizing calculator has already returned: 4.8kWh/day average load, 6.2kW continuous peak (everything running at once, no motors starting), and a 900W well pump as the largest motor in the house.

• Continuous rating needed: at least 6.2kW, with headroom — most builders target 20–30% over the calculated peak, so a 7.5–8kW-continuous-class unit clears it with room to grow. Both flagship all-in-ones above are rated 12kW continuous battery-alone, well past that line.
• Surge needed: the 6.2kW baseline already includes the pump’s 900W running draw; its actual surge (~2,700W, roughly 3×) adds only the incremental spike above that — call it 1,800W extra for half a second, so ~8kW momentary total. Surge is rarely the binding constraint once you’re looking at a real off-grid-class unit — their ratings are already enormous. It matters more on a smaller cabin system, or one running two big motors at once.
• Split-phase or 230V: read straight off the panel — a 240V well pump or dryer circuit makes split-phase, or three-phase across much of Europe, non-negotiable.
• Charger amps: sized to the actual AC source, not the biggest number on the shelf — a generator-recharge plan needs real thought here; a grid-tied-as-backup connection barely does.

That’s the whole method: continuous from the load calculator, surge from the biggest motor’s incremental spike, phase configuration from the panel, charger amps from the actual source — never from whichever number sounds most impressive on a spec sheet.

Which Inverter/Charger to Actually Buy

The two families above cover almost every off-grid build: an all-in-one hybrid where simplicity and price-per-watt matter most, or a modular stack where graceful degradation matters more than sticker price. Region changes which SKUs make sense — US builds need genuine 120/240 split-phase output; European builds need 230V single-phase, or three phase-matched units on a three-phase supply. Current picks for each: