Off-grid inverter: What size do you need?

Updated 2026-10-25

An off-grid inverter turns the direct current (DC) stored in your battery bank into the alternating current (AC) that household outlets and appliances use. To size one, its continuous rating has to cover everything you’ll run at the same time, plus a margin of about 25%. Its surge rating has to cover the brief jolt of power that motors draw when they start. Then match its input voltage to your battery bank and choose a pure sine wave model.

This guide explains how to add up your loads, gives a worked example, and covers sine wave types, battery voltage, the difference between plain inverters, inverter-chargers and all-in-one units, how much power an inverter draws while idle, and safe installation. It covers battery-based off-grid inverters only. Grid-tied string inverters and microinverters, which feed power straight into the grid without batteries, are a different product.

What size inverter do you need

Every inverter has two power ratings, and you need to check both:

• Continuous rating is the power the inverter can supply for as long as you need it, in watts. It must be at least the total running watts of everything that might be on at the same time. Add a margin of about 25% so the inverter isn’t running flat out, and so you have room for new loads.
• Surge rating (or peak rating) is the higher power it can supply for a few seconds while a motor starts. Fridges, freezers, well pumps, air conditioners and power tools can draw several times their running watts for a moment when they start. The surge rating must cover the largest of those starts on top of everything else that’s running.

Find the running watts on each device’s label or datasheet, or measure them with a plug-in watt meter. Starting watts are harder to find: some manuals list them, and a clamp meter with an inrush setting can measure them. Without a figure, allow about three times the running watts for motor loads, and more for well pumps.

List your loads in a table with both columns:

These figures are hypothetical, so use your own devices’ ratings. Devices without motors start at about their running watts.

Worked example

Using the load table above, here’s how to size an inverter for a cabin:

Continuous rating: if everything ran at once, the running watts would total 120 + 250 + 60 + 40 + 1,000 = 1,470 W. Add 25%: 1,470 × 1.25 ≈ 1,840 W. A 2,000 W inverter covers it.

Surge rating: the worst case is the fridge and the pump starting at the same moment while everything else runs. That’s 600 + 750 + 60 + 40 + 1,000 = 2,450 W for a few seconds. Many 2,000 W inverters are rated to surge to about twice their continuous rating, but check the datasheet. The figure varies between models, and so does how long the inverter can hold it.

What made the difference: without the microwave, the same cabin would need only about 600 W continuous, and a 1,000 W inverter would do. One large appliance often decides the inverter size, so it’s worth asking whether you need it on the inverter at all. A propane stove or a generator for occasional heavy use can let you choose a much smaller inverter.

Pure sine wave vs modified sine wave

Inverters produce one of two kinds of AC:

• Pure sine wave inverters make smooth AC that matches what the grid supplies. Every device that runs on grid power runs properly on it.
• Modified sine wave inverters approximate AC with a stepped waveform. Simple resistive loads like incandescent bulbs and heaters run on it, but many other devices don’t run well.

Devices that need pure sine wave, or run poorly without it, include:

• Fridges, freezers, pumps and other motors, which can run hot, buzz and wear out sooner
• Microwaves, which may heat slowly
• Laptop and phone chargers, which can run hot or fail
• Variable-speed tools and appliances
• Medical devices such as CPAP machines
• Audio equipment, which can hum

Modified sine wave inverters cost less, but a typical off-grid system runs at least one of these devices. Choose pure sine wave unless the inverter will only ever power simple resistive loads.

Match the inverter to your battery voltage

An off-grid inverter’s DC input voltage must match your battery bank: 12 V, 24 V or 48 V. The current it draws from the batteries is roughly its output watts divided by the battery voltage, plus about 10% more for its own losses:

High currents need very thick, expensive cable and large fuses, and they waste more energy as heat. That’s why larger inverters belong on 24 V or 48 V banks. A rough guide: 12 V for inverters up to about 1,000 W, 24 V up to about 3,000 W, and 48 V above that.

Your batteries also have to deliver that current. Lithium batteries with a built-in battery management system (BMS) have a maximum continuous discharge current, so a big inverter may need several batteries in parallel even if you don’t need their extra capacity. Our guide to building a solar battery bank covers choosing a system voltage and wiring the bank.

Inverter, inverter-charger or all-in-one

Off-grid inverters come in three common forms:

• A standalone inverter only converts DC to AC. It’s the simplest and cheapest choice for a small system that charges from solar alone.
• An inverter-charger adds a battery charger that runs from a generator or shore power. When AC power is connected, it charges the batteries and passes power through to your loads, then switches back to the batteries when the generator stops. It suits systems that need a backup generator for long cloudy stretches.
• An all-in-one unit combines an inverter-charger with a built-in MPPT solar charge controller in one box. It means fewer parts, less wiring and one manual. The drawback is that if one part fails, the whole unit has to come out, and its built-in controller limits how far you can expand the array. Check that its maximum solar input suits your panels, the same check you’d make when choosing a separate solar charge controller.

Idle draw and efficiency

An inverter uses power just by being switched on, even with nothing plugged in. This idle draw, often listed as “no-load current” or “no-load power”, can be a few watts for a small inverter and tens of watts for a large one. That adds up over a day: a hypothetical 30 W idle draw left on for 24 hours uses 720 Wh. That’s a large share of a small cabin’s daily budget.

To keep idle losses down:

• Don’t oversize the inverter. A 5,000 W inverter powering a 400 W load wastes more than one sized to fit.
• Use a power-saving mode. Many inverters have a search or sleep mode that switches off until a load turns on. It may not detect very small loads, such as a phone charger.
• Switch it off when you don’t need AC, such as overnight in a system where the fridge runs on DC.

Inverters are also less efficient at very light loads. Most run at about 90% efficiency or better near their rated load, but much lower when lightly loaded, which is another reason to size close to your real needs.

Safety and installation

An off-grid inverter draws high currents from the battery bank, so the DC side needs the most care:

• Fuse the DC input close to the battery. Fit a fuse or breaker on the positive cable as close to the battery bank as possible, rated as the inverter manual specifies. Large lithium banks often need a fuse with a high interrupt rating, such as a class T fuse.
• Size the cables for the current. Use the cable size the manual specifies for your inverter and cable length, and keep the battery cables as short as possible. Undersized cables run hot and cause voltage drop that can trip the inverter.
• Ground it. Connect the inverter’s chassis ground. Follow the manual for neutral-to-ground bonding on the AC side, which depends on how the inverter connects to your wiring.
• Give it air. Mount the inverter in a dry, ventilated space near the batteries, with clearance for its cooling fans.
• Hardwired AC is electrical work. Plugging appliances into the inverter’s outlets is simple, but connecting it to a building’s wiring falls under the electrical code. In most places that needs a permit and is a job for a licensed electrician.

For where the inverter fits in a complete system, see our guide to planning and building a DIY solar system. Once the inverter and batteries are chosen, the next step is wiring the panels: our guide to connecting solar panels in series or parallel shows each layout.

Frequently asked questions

Can I run a fridge on a 1,000 W inverter?

Usually, yes. Most efficient household fridges run on a few hundred watts or less, and a pure sine wave 1,000 W inverter with a good surge rating can usually start one. Check the fridge’s running and starting watts against the inverter’s continuous and surge ratings, and leave room for anything else running at the same time.

How many batteries do I need for a 3,000 W inverter?

That depends on how long you’ll run it and on your battery voltage. For run time, multiply the load by the hours you need, then size the bank with the formula in our battery guide. For current, a 3,000 W load draws about 140 A at 24 V, so the bank must be able to supply that continuously. With lithium batteries, that may mean two or more in parallel, depending on each battery’s discharge rating.

Is a bigger inverter better?

Not past what you need. A bigger inverter costs more, draws more power while idle and runs less efficiently at light loads. Size it to your largest realistic combined load plus a margin, and make sure the surge rating covers your motor starts.

Can I use a grid-tied inverter off grid?

No. A grid-tied inverter needs the grid to operate and shuts down when the grid is down, and most can’t run from a battery bank. Off-grid and hybrid systems need an inverter designed to run from batteries.