How to Size a Solar Inverter

One line: Sizing runs in one direction — load first, then inverter, then battery voltage, then PV. Getting that order wrong is what produces oversized inverters and undersized battery banks.

The four numbers you need

Before looking at any model, gather four figures:

1. Connected load (W) — everything that could run at once.
2. Surge load (W) — the starting current of the biggest motor or compressor.
3. Daily energy (kWh) — load × hours per day.
4. Autonomy (days or hours) — how long the system must run with no PV input.

Everything else follows from these.

Step 1 — List the loads

Write down every appliance with its running wattage and the hours per day it runs. A residential example:

Two figures come out of this list:

• Connected load: 1480 W — this sizes the inverter.
• Daily energy: 4.85 kWh — this sizes the battery.

Step 2 — Size the inverter for load and surge

Choose an inverter whose rated output exceeds the connected load, then check surge on top.

Our example has 1480 W of connected load. The water pump is the surge risk: a 750 W pump can draw 2–3 times its rating for a second at start-up — up to 2250 W momentarily.

The 2000 W unit technically covers a 1480 W running load, but leaves almost nothing for surge. The 3600 W model is the smaller sensible choice; the 5500 W model leaves room for load growth without over-specifying.

Do not size to the total of everything that exists. Size to what runs at the same time. A welder and a pump that are never used together do not need to be added.

Solar inverter size chart — PV inverter size by load

The chart below maps a PV inverter size to the kind of load it usually carries. Use it to shortlist a model, then run Steps 3 to 5 to confirm battery voltage, battery capacity and array size.

Two caveats before you use the chart:

• The load column is a starting point, not a rule. A pump, compressor or welder adds surge, and surge decides whether a size is adequate — check the surge figure in Step 2.
• Inverter sizing and battery sizing are two separate decisions. The chart gives the PV inverter size; it says nothing about how many kWh of battery you need. That comes from daily energy and autonomy in Step 4.

When your load falls between two rows, size up if it contains motors, and take the lower row if it is resistive only (lighting, heaters, electronics).

Step 3 — Choose battery voltage

Battery voltage is set by the inverter suffix — O = 12 V, T = 24 V, F = 48 V. It is not a free choice; it must match the inverter.

The reason is Ohm's law. A 5000 W load at 12 V draws over 400 A; the same load at 48 V draws about 104 A. High current means thick, expensive cable and heat. Above roughly 5 kW, 48 V is the practical choice — which is why the HE200-12000-F and HE300-12000-F are both 48 V machines.

Matching battery voltage to the inverter is not optional. Connecting the wrong battery voltage can destroy the unit's capacitor bank — the manufacturer's manual states this as a hard warning.

Step 4 — Size the battery

Usable capacity (kWh) = daily energy (kWh) × autonomy (days) ÷ depth of discharge

Our example uses 4.85 kWh per day. For one day of autonomy at 80% depth of discharge:

4.85 × 1 ÷ 0.8 ≈ 6.1 kWh

So the battery bank needs roughly 6 kWh usable. Two 5.12 kWh packs in parallel (10.24 kWh gross) give comfortable headroom; a single 16 kWh pack suits two days.

Respect the charge-rate limit as well:

• Lead-acid: charge at no more than 0.2C. A 24 V 200 Ah bank → 200 × 0.2 = 50 A maximum.
• Lithium: charge at no more than 0.5C. A 48 V 200 Ah bank → 200 × 0.5 = 100 A maximum.

Set the inverter's charge-current parameter to respect this. Over-current shortens battery life sharply.

Step 5 — Size the PV array

Array size (W) = daily energy (kWh) ÷ peak-sun hours

At 4.85 kWh per day with 4 peak-sun hours:

4.85 ÷ 4 ≈ 1.2 kW

Then add for losses — wiring, temperature derating, inverter efficiency and non-ideal orientation typically cost 20–30%. A realistic target is 1.5–1.6 kW of array.

Check it against the inverter's PV limits:

Practical panel wiring, using 540–700 W panels with 45–55 V open-circuit voltage: string 4 to 8 panels in series, then parallel as needed for array size. Exceeding 500 V open-circuit risks immediate damage.

Worked example — small off-grid cabin

Everything fits within the model's limits with margin on the load and the PV input.

Worked example — home with critical-load separation

The dual AC output is what makes this case fit the 7200-F: critical circuits go on the main output, the workshop on the secondary. When the battery runs low, the inverter drops the secondary output so the house keeps running. Sizing alone would not have selected a dual-output model — the requirement did.

Common sizing mistakes

• Sizing the inverter to the total nameplate of every appliance. Only simultaneous loads count.
• Ignoring surge. Motors and compressors start at 2–3× rated power.
• Undersizing the battery to save cost. The battery determines backup duration, not the inverter.
• Mixing battery voltages. A 24 V pack on a 48 V inverter damages the unit.
• Oversizing the PV array past the input limit. Surplus is clipped at best, and over-voltage past 500 V damages the unit.
• Assuming the model number is the wattage. A 7200 model is a 6200 W machine — see the decoder.

Next step

Send us the load list, autonomy requirement and battery chemistry, and we will come back with a model recommendation and the calculation behind it.

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