What size inverter do I need? A simple guide from 3.6 to 12 kW
Inverter size is the first question we get with almost every quote request. An inverter that is too small shuts down or switches to the grid when several appliances start at once. One that is too large costs more than you need. The good news: you can make the choice with a few numbers you already have – your electricity bill and the list of appliances in your home.
1. Start with what runs at the same time
The inverter has to supply all the loads that run simultaneously, not everything you own. Think of the busiest moment of the day, for example the evening: hob, oven, washing machine, water heater, heat pump, lighting.
Add up their power ratings (you'll find them on each appliance's label) and add a 20 – 30% margin. The result is the continuous power you need.
| Appliance | Typical power |
|---|---|
| Fridge / freezer | 100 – 300 W |
| LED lighting, TV, router, laptop | 200 – 500 W |
| Washing machine / dishwasher | 2,000 – 2,400 W while heating |
| Electric water heater | 1,500 – 2,400 W |
| Induction hob | 2,000 – 7,400 W |
| Air conditioner | 800 – 2,000 W |
| Heat pump | 2,000 – 5,000 W |
These values are indicative – for your own calculation, use the ratings on your appliances' labels.
2. Allow for start-up surges
Appliances with a motor or compressor (fridge, water pump, air conditioner, heat pump, power tools) draw 2 – 3 times their normal power for a fraction of a second when they start. That's why the inverter datasheet lists peak power as well as continuous power. Lumentree inverters handle short peaks of about twice their rated power – a 6 kW model, for example, accepts 12 kVA.
3. Look at your annual consumption – it decides how many panels you need
Annual consumption (in kWh, from your bill) doesn't set the inverter size, it sets the size of the solar array. In Romania, 1 kWp of panels produces on average around 1,100 – 1,300 kWh per year, depending on location, orientation and shading.
Example: a home using 5,000 kWh a year needs roughly 4 – 4.5 kWp of panels to cover its annual consumption. Hybrid inverters usually accept more PV power than their output rating – a 6 kW S series inverter takes up to 9.6 kW of panels. You'll find the exact value in each series' technical table.
4. Single-phase or three-phase?
Check your connection: one phase (230 V) or three phases (400 V) in your distribution board. The approved power in your grid connection permit or contract also matters – the inverter shouldn't exceed it when exporting to the grid.
- Single-phase connection – choose a single-phase inverter, from 3.6 to 12 kW.
- Three-phase connection with loads on all phases (three-phase heat pump, workshop) – a three-phase inverter such as the 3S series, 8 – 12 kW.
5. Grid export or backup only?
- Prosumer (with grid export): the S, S-GR, ST-GR, P-GR, T and 3S series.
- Off-grid or backup, no export (cabins, homes without a grid connection, backup during outages): the HP and HT series.
- Outdoor installation: choose an IP65 model (S, S-GR, ST-GR, 3S). IP41 models are for indoor use.
Indicative examples
| Situation | Inverter size | Suitable series |
|---|---|---|
| Small house or apartment, 2,500 – 3,500 kWh/year, no electric heating | 3.6 – 4 kW | S 3.6 / 4 kW, S-GR 4 kW, P-GR 4 kW |
| Medium house, 4,000 – 6,000 kWh/year, electric water heater, air conditioning | 6 kW | S 6 kW, S-GR, ST-GR, P-GR 6 kW, T 6 kW |
| Large house with heat pump or induction hob, 7,000 – 10,000 kWh/year | 8 – 10 kW | S 8 / 10 kW, T 8 kW |
| Very large home, workshop, guesthouse – single-phase connection | 12 kW | S 12 kW, SUNT-12.0kW-HT |
| Three-phase connection with loads on all phases | 8 – 12 kW | 3S 8 / 10 / 12 kW |
| Cabin or home without a grid connection | 4 – 8 kW | HP, HT |
The table helps you get your bearings, but every home is different. If you need more power later, Lumentree inverters can be connected up to 6 units in parallel.
What about the battery?
The battery is chosen separately, based on how much you use in the evening and at night, or how many hours of backup you want during an outage. Lumentree LT-S batteries from 5.1 to 16 kWh can be expanded in parallel, so you can start smaller and add capacity later.
Don't want to do the maths yourself?
Send us your annual consumption from your bill, your connection type and the large appliances in your home, and we'll recommend the right inverter, battery and number of panels. Request a quote or message us on WhatsApp at +40 712 348 437.