What size inverter do I need?

The real work is determining the minimum possible size to do a job. Anybody can buy an inverter that's way too big and very expensive, and it will do the job, probably. But the skill is determining the minimum-sized unit to do the required work. The minimum is not a recommendation. The minimum is the minimum. To calculate the minimum, consider a number of different aspects:

The maximum surge load on the inverter and the length of the surge.

The maximum power that the inverter will sustain running for any amount of time beyond surging.

The duty cycle on the inverter, the level of certainty you need around your power: the more certainty, the greater the safety factor you need to incorporate. The safety factor is the increased inverter headroom, so you are more certain that your loading and your surging are well within the maximum limits of the inverter. It is not running close to its maximum continuously all the time. If you come back from those extreme limits where failure is just about to happen, and you back off from them significantly, your solution will be more reliable. If the consequences of inverter failure are low you can move closer to the maximum limits of the inverter and accept the higher risk that the inverter may be overloaded, due to there not being severe consequences for you if the inverter stops.

To do number one, be familiar with the wattage ratings of all your appliances and the amount of surging that that appliance typically has. Appliances of the same type will vary from model to model and manufacturer to manufacturer, sometimes significantly, due to their differing internal designs. We can apply a broad surge multiple to a category of equipment and be reasonably confident that the surge will be within that figure though in a small number of cases, it will not. So, depending on your application and requirements, you would need to incorporate an amount of safety factor in there. The more certainty that you need and reliability, the greater the safety factor you would incorporate. Refer to our articles about inverter surging and particular case studies on certain types of appliances to get an understanding of surging of that appliance and multiples that might be used for them, and use those multiples in your calculations for this aspect. The appliance guide does exactly that for coffee machines, microwaves and induction. You can use the nominal wattage of the appliance to get more accurate figures. You would need to actually measure the power being consumed. You'd have to measure the AC current to do so. A general guide on a minimum sizing of this aspect of your inverter for a known amount of power: take the maximum sustained power rating of all your appliances running at a point in time, and have that be at most 70% of the inverter's wattage. Have a little more safety factor to this e.g. 60% if you want more certainty around your solution.

Sizing around this aspect is less important. Do consider the environment. If you're pushing the inverter to its maximum continuous ability, there will be a lot of heat byproduct, so that becomes a decisive factor as well. Keeping the environment dust-free, thermal transfer working well, and the cooling efficient so the heat is ongoingly removed efficiently from all the internal components. If you are working the inverter hard and continuously, and perhaps you are challenged around cooling due to a dusty or warm environment, or you require high amounts of certainty, do increase the amount of safety factor to help you cover all those other factors that may be present.

Once you have considered all those three aspects, let your guidance be the aspect that requires the largest amount of inverter wattage, and let that be your minimum size to do the job overall. Easiest thing is to send us the list and we'll size it.

More in our Inverters FAQ, and the DC voltage drop calculator gives you the cable size for the current. If the battery side gives out first, the low-voltage cutoff page explains why the inverter trips before the bank is empty, and the efficiency guide has what an oversized inverter costs you in idle draw every day.