An off grid workshop is sized on the biggest motor start, not the daily energy. How welders, compressors and three phase tools fit on a Victron system.

A shed or workshop off grid is sized on its biggest moment, not its daily energy. A welder or a compressor might use only a few kilowatt hours a day, but it asks for a lot of power for a few seconds when it starts, and the inverter has to carry that surge or the tool stalls and the system shuts down. Get the surge right with the right inverter, a soft starter on the big motors and a sensible order of use, and an off grid workshop runs a full day's work.

Energy and power are different questions

Off grid sizing has two numbers, and a workshop pulls them apart more than a house does.

Energy is how many kilowatt hours you use in a day. It sizes the panels and the battery.
Power is how many kilowatts you draw at once. It sizes the inverter.

A household spreads its load across the day. A workshop does not. A compressor cutting in while the welder is running and the dust extractor is on is a big number for a short time, and it is that number the inverter has to meet. The battery and panels can be modest if the shed is only busy a few hours a day; the inverter cannot.

The surge: why motors are the hard part

An electric motor connected straight across the supply draws a burst of current while it gets up to speed. The IEEE puts it at up to six to ten times its rated running current for several seconds. A compressor motor that runs on 10 amps can ask for 60 to 100 amps as it starts, which is the moment an undersized inverter trips.

Inverters are rated for this with a peak figure as well as a continuous one. From Victron's own published figures for the 230 volt, 48 volt models:

MultiPlus-II 48/5000: 4,000 W continuous at 25 degrees, 9,000 W peak
MultiPlus-II 48/10000: 8,000 W continuous, 18,000 W peak
Quattro 48/10000: 8,000 W continuous, 20,000 W peak
Quattro 48/15000: 12,000 W continuous, 25,000 W peak

The continuous rating falls as the cabinet warms: the MultiPlus-II 48/5000 is rated at 3,700 W at 40 degrees. A shed in a Central Victorian January can see that, which is why the inverter goes somewhere shaded and ventilated, not on a west facing tin wall.

Soft starters change the sum

A soft starter sits between the supply and a motor and brings the voltage up gradually instead of all at once, which limits the inrush current and the mechanical shock of a hard start. On an off grid system that is often cheaper than buying a bigger inverter for a surge that lasts two seconds.

They suit compressors, bore and transfer pumps, saw benches and dust extractors. They do not suit every motor: a reduced voltage start also reduces starting torque, so a machine that has to start under heavy load may need its full start. Each big motor gets checked on site, and a soft starter goes on where it makes the difference between one inverter size and the next.

Welders

Welders are a different kind of load. A modern inverter welder does not have a motor's starting surge, but it draws hard and in bursts for as long as the arc is struck, and a big machine can sit near the top of a single phase inverter's continuous rating.

The figure to look for is on the machine's rating plate: the rated input power in kVA and the maximum input current, often marked I1max. Compare that against the inverter's continuous rating at the temperature the shed will be, leave headroom for anything else that runs at the same time, and remember the duty cycle. A welder used for an hour of fabrication on a sunny afternoon is an easy load; a full day of heavy stick welding is a generator day.

Three phase tools

Some workshop machines are three phase only: bigger lathes, mills, saw benches and compressors. Off grid there are two ways to run them.

A three phase inverter stack. Three Victron inverter chargers of the same model can be configured to give a three phase supply, and Victron's datasheet allows sets of three to be paralleled for more. This is the full answer when there are several three phase machines or one big one. Our article on when an off grid system needs three phase covers the design and the trade offs.
Change the machine or its drive. Where there is only one small three phase machine, it can be cheaper to change the drive arrangement than to build a three phase system around one tool. We look at the motor plate and the use before recommending either.

Let the generator help, not fight

On a Victron system the generator and the battery work together. The inverter charger can be told the generator's current limit, and it uses whatever the shed is not using to charge the battery. When a load needs more than the generator can give, Victron's PowerAssist tops up from the battery so the generator is not overloaded. That means a heavy fabrication day can run on a modest generator with the battery covering the peaks, rather than needing a generator big enough for the worst moment. Our generators page covers sizing and auto start.

Habits that keep a workshop running

The cheapest kilowatt is the one you do not draw at the same moment as another. A few rules of thumb:

start the compressor before you strike the arc, not during
run the heaviest work in the middle of a sunny day, when the panels are carrying it
fill the air receiver in the morning so the compressor rests through the afternoon
put the big tools on circuits the monitoring can show you separately

With remote monitoring on the system, the surges show up in the data, so the next conversation about a bigger tool starts with what the shed is actually doing. That is part of why we build off grid on Victron.

Bring us the list: each tool, its rating plate, how long it runs and what runs together. The sizing tool gives a first answer for the energy, and we size the inverter and the soft starters from the plates. Send us the list and your address and we will come back with a right sized design, or get in touch here.

Questions

Can you run a welder off solar and batteries?

Yes, if the inverter's continuous rating covers the welder's input with headroom and the battery can supply it for the time you weld. Check the welder's rated input in kVA and its maximum input current against the inverter at shed temperature.

What size inverter do I need for an air compressor?

Size it on the start, not the run. A motor can draw six to ten times its running current for a few seconds, so either choose an inverter whose peak rating covers that, or fit a soft starter to bring the start down.

Do I need three phase off grid for a workshop?

Only if you have three phase machines you cannot change. A single phase system with soft starters handles most sheds; a three phase Victron stack is the answer for several three phase machines or one large one.

Can a battery let me use a smaller generator?

Yes. A Victron inverter charger can limit what it takes from the generator and cover the peaks from the battery, so the generator only needs to match the average, not the worst moment.

Sources

Written by the Trentham Energy team.

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Powering a shed or workshop off grid: welders, compressors and three phase tools