Table of Contents

Sizing an off-grid solar inverter starts from the appliances that must run at the same time, not from the wattage of the panels and not from the number of batteries. The inverter’s continuous AC rating has to cover that simultaneous load. A separate surge figure on the inverter has to cover motors that draw more for a moment than they draw while running, and that surge figure has to be read from the inverter and from the motor, not invented as a multiple. The battery voltage has to sit inside the inverter’s DC window for the whole charge range, not just at the nominal label. Hours of autonomy, the question of how long the site lasts after sunset, are a battery and array problem. They are already laid out in living off grid. This article is only the inverter.

A grid-tied inverter is the wrong object. IEEE Std 1547-2018, in the National Renewable Energy Laboratory primer, requires a grid-connected unit to stop energizing a line that has become an island, within 2 seconds. An off-grid site has no utility to synchronize to. The inverter has to form the AC supply itself. A unit listed only as an interactive inverter will sit there, in the sun, and produce nothing useful once there is no grid to follow. Backup equipment that can island selected circuits of a grid-tied house is a third design. It is not sized by the off-grid method below.

Add the loads that overlap

List every AC load that will run together on the worst ordinary hour, not the sum of every nameplate in the building. A workshop that never runs the saw and the welder together should not size the inverter for both. A house that runs the well pump while the fridge and the lights are on should size for all three. The continuous rating of the inverter, at the temperature where it will actually sit, has to be at least that overlap. Afternoon derating is real. A Sandia and NREL thermal study found inverter temperatures rising with dissipated power, season, and wind. A continuous rating quoted at a cool laboratory temperature can be unavailable in a closed shed in summer. Read the derating curve. If the curve is missing, the size is not finished.

Motor loads need a second number. The running watts and the starting watts are different, and only the nameplates or the maker’s data sheet carry them. If the inverter publishes a surge rating for a stated number of seconds, compare that pair with the motor’s pair. If either side is blank, do not guess a multiplier and buy. Pumps are the usual surprise. The US Department of Agriculture’s solar-pump note sizes the pump from flow and total dynamic head first, then picks a pump curve, then sizes the array to the pump’s power. An inverter, if the pump is AC, has to cover that pump power while the other AC loads in the same hour are on. A DC pump that does not need an inverter at all is often the simpler off-grid water design. The note’s livestock water quantities are regional examples. They are not a household appliance list.

Match the battery, then correct for conversion

Off-grid inverters are sold against a battery voltage. A nominal match, both labeled 48 volts for example, is not enough if the battery’s full and empty voltages fall outside the inverter’s operating window. The window is on the inverter manual. The battery’s voltage range is on the battery manual. This article does not publish a universal window, because the safe overlap is those two documents. How the battery is coupled, DC or AC, changes which inverter is the one that forms the grid. The equipment patterns are in how independent systems are classified.

Conversion efficiency means the battery must supply more DC power than the AC load consumes. Use the efficiency at the load you actually run, not the peak efficiency at an ideal point. The performance-index warning is the same as for grid-tied units: one peak percentage is not the day. If the manual does not give efficiency near your continuous load, say so in the sizing note instead of inserting a round loss factor. Night-time idle draw matters on a small system, because it is a load that runs when the sun does not. It belongs in the battery-hours calculation in the off-grid living article, and it has to be taken from the inverter’s own standby figure.

What the inverter size does not decide

It does not decide how many days of bad weather the site can ride through. That is storage and a generator. It does not decide the array. An inverter sized for a large welder does not make the welder run at night if the battery was sized for lights. It does not replace a charge controller or the DC protection between the array and the battery. Pacific Northwest National Laboratory’s warning still applies off the grid: modules produce in daylight even when an inverter is off, and only trained people should connect or disconnect that wiring. A larger inverter does not make those connectors safe to unplug.

Monitoring for this kind of site is not a grid performance ratio. It has to show battery state and, if there is a generator, how many hours it ran. That instrument is described in off-grid solar monitoring. An inverter screen that only shows AC watts out will hide the slow failure mode, which is a battery that ends each day a little lower.

A sizing sequence that stays inside the documents

Write the overlapping AC loads, with running and starting figures from their own plates. Reject any inverter whose continuous rating, after temperature derating, is below the running overlap, or whose stated surge does not cover the motors that start during that hour. Reject any inverter whose battery window does not cover the battery you will use. Apply the manual’s efficiency and standby draw when you hand the load to the battery calculation. If the largest load is a pump, do the USDA sequence for head and flow before you let the inverter wattage drive the design. If a number is missing, stop. A confident kilowatt picked to look standard is how off-grid systems trip on the first motor start or die quietly in the first cloudy week.

Frequently asked questions

Can a grid-tied inverter be used off-grid if the array is large enough?

No. Under IEEE 1547-2018 a grid-tied unit stops energizing an islanded line. Off-grid operation needs an inverter that forms the AC supply. A larger array does not add that function.

Should the inverter equal the sum of every appliance nameplate?

Only if those appliances run together. Size the continuous rating to the overlapping hour, and match surge to the motors that start in that hour, using the figures printed for both devices. Do not apply a universal starting multiple.

Does a bigger inverter mean more days of backup?

No. Days of backup are a battery question, covered in the off-grid living discussion. An oversized inverter can even add standby draw. It does not store energy.

Sources and references

Why a grid-tied inverter will not carry an off-grid load: NREL/Sandia IEEE 1547-2018 primer. Temperature and continuous capability: Thermal study of inverter components. Sizing a pump from flow and head before sizing the supply, and keeping the array near the load: USDA NRCS Technical Note 28, 2010. Live DC wiring: Pacific Northwest National Laboratory.