Three steps
- Add up what could run at the same time. Not everything you own, just what might be on together. Use the running watts below, or the label on the appliance.
- Add about 25%. Inverters run hot and loud near their limit, and they deliver less in a warm space (see "Read the watts, not the VA" below).
- Check the surge. Motors (fridges, pumps, air conditioners, saws) need several times their running watts for a second or two to start (a fridge that runs at 250 W can pull about 1,200 W). Add the biggest starter's surge to everything else that's running, and make sure that total fits under the inverter's peak rating.
| Appliance | Running watts | Watts to start |
|---|---|---|
| Refrigerator | 100–250 W | about 1,200 W |
| Microwave | 1,000–1,500 W | about 1,500 W |
| Coffee maker | 800–1,200 W | about 1,200 W |
| Induction cooktop | 1,200–2,000 W per burner | about 2,000 W |
| Window AC (8,000–12,000 BTU) | 500–1,000 W | about 2,000 W |
| Mini-split heat pump | 300–1,200 W | about 1,500 W |
| Well pump (½–1 hp) | 800–1,500 W | about 3,500 W |
| Washing machine | 300–500 W | about 1,000 W |
| Power tools | 400–1,800 W | up to 3,600 W |
| TV, laptop, Starlink | 30–150 W each | about the same |
More appliances: how much power does it use?
Worked examples
A van or RV with a microwave. Fridge 250 W, laptop 70 W, roof fan 54 W, Starlink 75 W and a 1,500 W microwave come to about 1,950 W. Add 25% and you're at about 2,400 W: a 3,000 VA inverter (2,400 W at 25 °C, 5,500 W peak, like the Victron MultiPlus-II 12/3000). Run the microwave or the coffee maker, not both. Leave the microwave out and the same rig needs only about 600 W: a 1,200 VA (1,000 W) inverter is plenty.
A cabin with a well pump. Fridge 250 W, well pump 1,500 W, TV 150 W and a laptop 70 W come to about 1,970 W, or about 2,500 W with the 25%. At start-up, the pump's 3,500 W plus everything else is about 4,000 W. A 6,000 W all-in-one like the EG4 6000XP (12,000 W surge for about 3.5 seconds) covers it with room to spare.
A full-time home. Fridge 250 W, well pump 1,500 W, mini-split 1,200 W, washing machine 500 W, microwave 1,500 W, plus TV and laptop come to about 5,200 W, or about 6,500 W with the 25%. That's past a 6,000 W unit, so this is where the 12 kW EG4 12000XP earns its price, or two 6000XPs stacked.
Read the watts, not the VA
Inverters are often named by VA (volt-amps), which is bigger than the watts you can actually run. Victron's own datasheets show the gap, and how it shrinks further in heat:
| Victron model | Name says | Watts at 25 °C | Watts at 40 °C | Peak (surge) |
|---|---|---|---|---|
| Phoenix C12/1200 | 1,200 VA | 1,000 W | 900 W | 2,400 W |
| Phoenix C12/2000 | 2,000 VA | 1,600 W | 1,450 W | 4,000 W |
| MultiPlus-II 3000 | 3,000 VA | 2,400 W | 2,200 W | 5,500 W |
| MultiPlus-II 5000 | 5,000 VA | 4,000 W | 3,700 W | 9,000 W |
A van in summer or a hot utility closet is closer to 40 °C than 25 °C. That's what the 25% headroom covers.
The battery side
Whatever the inverter puts out, the battery has to supply, plus a few percent lost as heat (Victron's best efficiency is 92–96%). At 12 volts that's a lot of current: 2,400 W ÷ 12 V ÷ 0.92 is about 217 amps, which needs very thick cable and a battery that can deliver it. At 48 volts the same 2,400 W is about 54 amps. That's why homes and cabins run at 48V: see 12V vs 24V vs 48V.
What each guide uses
- Vans and RVs: a 12V inverter-charger, sized by whether you'll run a microwave or induction. See solar for a van and RV, bus or skoolie.
- Tiny houses and cabins: the 6 kW EG4 6000XP, which makes both 120V and 240V.
- Bigger homes: the 12 kW EG4 12000XP when a well pump, heat pump and laundry could all run together.
The guides do this math for your answers, and inverter vs inverter-charger explains which kind to get.
Questions people ask
What size inverter do I need for my RV?
Add the watts of what could run together, plus about 25%. Without a microwave, a fridge, laptop, fan and Starlink come to about 450 W, so a 1,000-watt inverter is plenty. With a 1,500 W microwave the total is about 1,950 W, so about 2,400 W: a 3,000 VA unit, as long as the microwave and the coffee maker don't run at the same time.
Can a 2,000 watt inverter run a microwave?
Usually, if not much else is running. A microwave draws 1,000–1,500 W while heating. Check the inverter's watt rating, not its VA: Victron's "C12/2000" is 1,600 W continuous at 25 °C, which leaves only about 100 W for anything else when a 1,500 W microwave is on.
What size inverter do I need to run a refrigerator?
A fridge runs at 100–250 W but needs about 1,200 W for a moment to start its compressor. Almost any home inverter covers that: even Victron's smallest Phoenix (1,000 W) is rated for 2,400 W peak.
What size inverter for a well pump?
A ½–1 hp well pump runs at 800–1,500 W and can need about 3,500 W to start. The EG4 6000XP (6,000 W, 12,000 W surge for about 3.5 seconds, 120 and 240 volts) starts one easily. Go bigger only if the pump, a heat pump and the rest of the house could all run at once.
Is a bigger inverter always better?
No. A bigger inverter costs more, is louder and uses a little power just sitting on. Size it for what really runs at once, plus about 25%, and add a second unit later if your life grows. EG4's XP inverters, for example, stack in parallel.
Sources
- MultiPlus-II inverter/charger datasheet (continuous watts at 25, 40 and 65 °C; peak power; efficiency) Victron Energy
- Phoenix Inverter 1200VA–5000VA datasheet (VA vs watts, peak power, efficiency) Victron Energy
- EG4 6000XP spec sheet (6,000 W, 120/240V, surge 12,000 W for about 3.5 s) EG4 Electronics
- EG4 12000XP whitepaper (12 kW, surge 18 kW for 5 s) EG4 Electronics
- Estimating appliance and home electronic energy use US Department of Energy
Every number on this page comes from these sources. Spotted something out of date? Tell me.
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