What size inverter would be good for a small shop

Re: What size inverter would be good for a small shop

You are looking at shop power--And I assume fairly significant loads running for longer periods of times (pumps, compressors, as well as table saws, joiners/planers etc. And, there are a bunch of inverters in the $1,700 to $2,200 range--Start with a somewhat bigger inverter/battery bank and then see how this matches against your needs/expectations/bank account...

You won't hurt my feelings if you say cut the price by 1/2--Just a point on paper for discussion (we are all volunteers here except the Admin--Wind Sun), so we do not have any dogs in the hunt based on what you eventually choose (or where you purchase the equipment). We do start with our host because they are a good wholesaler/retailer with good pricing and service--and to support them for the "free" support of this forum (that they created--not any of us).

Start with a nice 2kW inverter with 120 VAC output. Also, this is an Inverter/Charger--So you can plug your genset into the AC power input for backup power during poor weather/heavy operation--genset will run the loads and also recharge your battery bank (note prices are from our host's website--Just putting in prices for information--and they will change over time):

Magnum MS2012 2000 Watt Sine Wave inverter w/charger
Regular Price: $2,149.00; sale price: $1,719.20


I would choose 48 volts as this is the most cost effective for larger power systems (less copper for wiring, solar charge controllers can manage larger arrays without having to buy more controllers at the higher voltage). But to keep the system costs bit less of a shock, I will start with 24 volt battery bank. Note there are larger inverters for not much more money here too--But the will need much larger battery banks to support those higher output ratings--No longer a "midrange" system.

So, assuming flooded cell battery bank (cheaper, rugged, a bit more forgiving of mistakes)--We have two numbers to look at... The maximum sustained power (watts) and maximum surge power. Sizing the battery bank based on power would give us:

• 2,000 Watts * 1/24 volts battery bank * 1/0.85 inverter efficiency * 8 hour battery discharge rate = 784 AH @ 24 volt battery bank

Yes, this is a large battery bank--But very easy to cut down if you, for example, only want 1,000 watt maximum continuous output--Cut the battery AH by 1/2 to 392 AH @ 48 volts--Anyway, to continue.

The rough maximum surge current from such a battery bank would be:

• 24 volts * 784 AH * maximum surge = 7,526 watt maximum surge (well more than this inverter can supply)

Next, we would want to recharge this battery bank with solar power... The normal recommendation is around 5% to 13% rate of charge, which would be:

• 784 AH * 29 volt battery bank charging * 1/0.77 panel+charger deratings * 0.05 rate of charge = 1,476 watt array minimum
• 784 AH * 29 volt battery bank charging * 1/0.77 panel+charger deratings * 0.10 rate of charge = 2,953 watt array nominal
• 784 AH * 29 volt battery bank charging * 1/0.77 panel+charger deratings * 0.13 rate of charge = 3,839 watt array cost effective maximum

And, such solar arrays on a reasonably sunny 4 hour of "noon time equivalent" power per day (by the way, where will system be--You are currently posting from Europe--Some areas have notoriously poor sunshine at times):

• 1,476 watt array * 0.52 system derating * 4 hours of sun per day = 5,018 WH = 5.0 kWH per day
• 2,953 watt array * 0.52 system derating * 4 hours of sun per day = 10,039 WH = 10 kWH per day
• 3,839 watt array * 0.52 system derating * 4 hours of sun per day = 13,048 WH = 13 kWH per day

And you need a charge controller--A high end MPPT charge controller, such as the Midnite Classic 150 volt can output a maximum of 94 amps at 24 VDC--(around $610). A single controller could manage (again, cost effectively) a solar array of around:

• 94 amps * 29 volts charging * 1/0.77 panel+charger deratings = 3,540 watt rough maximum cost effective array @ 24 volt battery bank

So a single "large" MPPT charge controller can handle most of the suggested configurations above (300 watts less than the 13% rate of charge array--not a big difference).

Now--You can see that the loads will guide us to battery bank sizing, which then forces the solar array size (for proper charging).

If you average loads are only 1kW continuous, and you only need 2.5 kWH per day--You could cut the battery bank and solar array by 1/2 and still have more than enough battery bank for starting surge for the battery bank.

Anyway--All of the above gives you some ideas and what a "generic" system could do if you expected to run it at its limits.

The simplest way to reduce cost would be to limit the typical maximum inverter to 1,000 watt maximum continuous load (around 1 HP) and cut battery bank AH and Array Wattage by 1/2...

You could also save money by going with an Inverter Only and use a separate AC battery charger (grid or backup genset).

And--A big driver in this system is the questions of how many kWH per day you will be running the system.

A smallish off grid energy efficient home/cabin can be pretty comfortable on 3.3 kWH per day (I would use this number as the dividing line between "smallish" and "larger" off grid power system)--And this system is larger than that--So if you need to use power for your living area too--You might want to add those power requirements for your shop too.

Also, as I asked before, the actual amount of kWH per day from any solar array is very dependent on where the system will be installed and how much sun you get (by season). it is very easy for some folks to get less than 2 hours per day of sun in winter (far north, lots of cloudy weather)--And others to get >6 hours per day (high desert, far north, no-marine layers, etc.).

Hope this helps.
-Bill