What size inverter?

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Dominic asked Oct 24, '21

What size inverter?

I'm trying to work out what size Inverter to get, been reading up but have never used one before and finding it all very confusing so hoping someone can offer some advice.

I've got a small solar system which I want to use to power my wifi network and potentially charge batteries for tools from, if the inverter can help me do that.

This is the current setup:

1 x 130w panel

Victron 75/15 MPPT

Victron BMV-700

Pi3B running Venus OS

1 x 90 A/h battery

I'm not sure how to work out what size Inverter to use? Is it a case of just adding up all the watts of the equipment connected and getting something a bit bigger or should it be rated according to the panel?

Would a phoenix 12/250 be suitable?

I'd like to use it to power 2 routers and a POE switch. I can't figure out the best way to get the PoE siwtch (48v) running from the solar so I think an inverter I can plug it into is going to be the simplest solution.

- NBN Router: Frits!Box 7490 - 9 Watts

- Wifi extension Router: Mikrotik hapac2 - 16 Watts (max)

- PoE switch: TPLINK TL-SG108PE (6W) (this is capable of 72W according to what is connected to it, in my case it will be used to power the 2 routers above)

- Optional: Tool Battery charger: 72W https://ozito.com.au/products/fast-charger/ with 4Amp battery.

I have a couple of other things connected to load on MPPT already:

- 4 bars of XTM LED lights from BCF with unknown Watts.

- Intel NUC mini PC

- 10" laptop screen (off most of the time)

Total draw under 2A

Phoenix Inverter

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Answer 1 · 2021-10-25T00:41:25Z

Kevin Windrem answered · Oct 25, 2021 at 12:41 AM

Buy a watt meter (like the Kill-O-Watt) and use that to measure your power needs. Adding up the power numbers from the labels will usually be significantly more power than you'll actually need.

There are two numbers you need to consider: the actual power draw (watts) and the energy over time (watt-hours). Watts tells you how big an inverter you'll need. Watt-hours over the period of time you'll run on batteries before solar or other charging sources can top up the batteries.

Consider both AC and DC loads in the energy calculations.

You may also need to consider the DC load placed directly on the battery. This would be from the inverter and any DC loads.

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Answer 2 · 2021-10-25T06:51:45Z

seb71 answered · Oct 25, 2021 at 06:51 AM

With only one 130W PV panel, do not bother.

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Answer 3 · 2021-10-25T07:47:54Z

kevgermany answered · Oct 25, 2021 at 07:47 AM

Might be worth sticking in an Orion 12V dv:DC converter and running the 12V loads from that. Your Fritz box and microtik can run off that, didn't check the tplink. The Orion would isate the charge voltage from the consumers. It would also let you turn off the inverter when you don't need it, depending what you get, they can be quite wasteful.

If you can do all the other consumers through the prion, the inverter calc is just your charger, which will be less than an amp.

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Dominic commented · Oct 25, 2021 at 11:50 PM

Thanks I didn't know about these, will investigate.

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Answer 4 · 2021-10-25T23:58:52Z

Dominic answered · Oct 25, 2021 at 11:58 PM

I found one of these Belkin Inverters powered from a cigarette lighter (300w version) 2nd hand yesterday for AU $40.

http://www.belkin.com/support/dl/p75043uk_f5c412uk.pdf

https://www.youtube.com/watch?v=3EWowlb_iPQ

My battery box has a cigarette lighter socket so I mowed the lawn and tried to charge the used battery from the inverter. I charged the 2nd battery for the mower from the mains supply and started them at the same time. Both took the same time to recharge, the inverter seems to have worked fine. Since the batteries were not fully discharged it seems to have pulled about 1.5A through the charger.

I'll think I'll change tack on this and run a longer power cable directly to the hapac2 from the cabinet (stepped up to 18v) since it can take DC supply. The Fritz!Box I will power from a UPS I already have in place for my NAS which should last a few hours at least. I have the NAS setup to detect the UPS and power itself down when it becomes active so the battery can be saved for the Router.

I'll just use the inverter to charge the tool batteries and turn it off when not in use.

If I end up doing PoE later I'll come back and update.

Thanks for the replies and ideas, appreciate it.

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