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bob-thompson asked

MPPT 75/15 voltages

Quick question. I have two 100 Ah AGM batteries fed by an MPPT 75/15 and a 400watt PV in my house. I use very little energy from the batteries day by day.

To maximise the solar electricity generated, I have considered either, buying a small 12 volt to 230 volt invertor and plugging my Starlink internet system into it, or finding a gadget that would feed in 230 volts into the house electrical system when the batteries are full, therefore something that would need to be able to measure and switch.
Would any old inverter off ebay be the solution to the first option?
Is there a gadget that can do the second option?

MPPT SmartSolar
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matt1309 avatar image matt1309 commented ·
Hi @Bob Thompson


If you have a relay or some for of switch you can programmatically activate when the battery is full. (voltage may even be a good enough metric for AGM batteries), then i imagine any inverter with a switch might be ok.


I'm surprised you cant power starlink on 12v? Saything that you'd need to make sure you never needed to switch it back to AC power in the event battery died.


Second option, multiplus ii do exactly this. They're expensive but what you describe of feeding into the grid, or even seperate off grid 240vac circuits is possible with a multiplus ii

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bob-thompson avatar image bob-thompson matt1309 commented ·

Thank you Matt. The Starlink works off around 48 volts as standard so 12 volts is not enough, I am a newby at this lark!

I looked at the multiplus ii but it would be mad to spend that amount of money, it is about the annual electricity bill for my house. This is just playing and learning for me.

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matt1309 avatar image matt1309 bob-thompson commented ·
No worries.

Sounds like option 1 is the best option then.


If you've got plans to expand your solar I'd definitely look into investing in a multiplus ii, it has a lot of options/flexible so will cover all the configurations you mention. Really good if you've got specific/niche plans/want to customise a lot.


It's similar price proportionally for me about 1 years worth of electricity for the inverter. And then I spent another 3 years worth of electricity bills on panels/LiFePO4 batteries. Where i live labour costs are the expensive part. I installed myself so without labour costs my ROI is on track for just under 5 years (if where you live you can use cheap electricity overnight to charge batteries during the winter and then use that cheap power during the day). The ROI gets close to 3 years.

If i paid for labour it would probably double the time it takes for me to get my money back.


A cheaper route for pushing the 230v into the house is difficult because you need an inverter that auto shuts off if grid goes down (to prevent back feeding power into the grid). A few hybrid inverters have them but they tend to be more expensive systems.


How much excess power do you have? One way i use my excess power is with a water heater (immersion heater). I have a multiplus but i imagine you could install a water heater and have any inverter turn on to power that water heater whenever you have a fully battery. Or like you said go the starlink route. If you've got loads i'd look into water heater. if it's not so much then starlink sounds better to me.

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k-k answered ·

A 12VDC to 48VDC (buck-boost-)converter could be the solution for Starlink....

If Starlink needs 60W your system must deliver 24h * 0,06kW = 1,44kWh || so 2 AGMs à 100Ah * 12V = 2,4kWh - how many sunshine-hours do you have to have them always filled. (Assuming 60W 24h, but if winter-cold ~150W, summer = less)

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