How to Choose an Inverter for Your Off-Grid System

An inverter converts the DC power stored in your batteries into AC power for household appliances. It is a critical component in any off-grid or backup power system. Getting the sizing and type wrong is an expensive mistake to correct.

Pure Sine Wave vs Modified Sine Wave

This is the single most important decision you will make when choosing an inverter.

Pure Sine Wave

Produces AC power that is identical in quality to the grid. Every AC device is designed to run on pure sine wave power. This type is required for:

• Laptops and desktop computers
• Medical equipment (CPAP machines, etc.)
• Modern TVs and audio equipment
• Variable speed motors (washing machines, some power tools)
• Anything with a microprocessor

Modified Sine Wave

Produces a stepped approximation of AC power. It works with simple resistive loads (incandescent bulbs, basic heaters) but can cause humming, overheating, or outright damage to sensitive electronics. In 2026, there is rarely a good reason to buy modified sine wave unless you have a very specific, simple application and are severely budget-constrained.

Recommendation: always buy pure sine wave.

Sizing Your Inverter: Continuous vs Peak Power

Inverters are rated in two ways:

• Continuous output (W): what it can sustain indefinitely
• Peak/surge output (W): what it can handle for a few seconds on startup

Electric motors draw 3-7x their running wattage when they start. A fridge with a 200W running draw might need 800-1000W at startup. Your inverter's peak rating must handle this.

List every appliance you might run simultaneously and total their wattages. Add 20-25% headroom, then check that the peak rating handles your most demanding motor loads.

Battery Bank Voltage: 12V, 24V or 48V

Your inverter's DC input must match your battery bank voltage.

• 12V: common in small van and camping setups, simpler wiring
• 24V: better efficiency for medium systems (500W-3kW), less cable losses
• 48V: the standard for serious home and off-grid installations above 3kW, lowest cable losses, most efficient

At higher power levels (2kW and above), 48V is strongly preferred. The same power at lower voltage requires proportionally higher current, which means thicker, more expensive cables and greater resistive losses.

Inverter/Charger Combos

For any system with both battery storage and a grid or generator connection, an inverter/charger (also called a hybrid inverter) is far more practical than a standalone inverter. These units combine:

• A DC-to-AC inverter
• An AC battery charger (for charging from grid or generator)
• An automatic transfer switch (seamlessly switches between grid and battery)

Brands such as Victron Energy, SMA and Growatt make well-regarded inverter/charger units used in UK off-grid and home backup installations. EcoFlow's PowerOcean system integrates similar functionality for home users in a more consumer-friendly package.

Grid-Tied Inverters

If you are connecting a solar system to the UK grid to export surplus energy, you need a grid-tied inverter rather than an off-grid type. Grid-tied inverters must be G98 or G99 certified in the UK (the type approval scheme for generation equipment). They automatically shut down if the grid fails, which is a safety requirement, not a fault.

Efficiency Ratings

Inverter efficiency is typically quoted at a specific load percentage. A good pure sine wave inverter runs at 90-95% efficiency under normal loads. Be cautious of units with high peak efficiency claims but poor efficiency at partial loads, as most real-world use involves partial loading.

Installation Considerations

Inverters should be installed close to the battery bank to minimise DC cable runs and associated losses. Ensure adequate ventilation as they generate heat under load. For systems above 3kW, professional installation by a qualified electrician is strongly advised.