Solar/battery ratio?

Is there a rule of thumb between the amount of installed Solar and battery size? I know there are lots of dependancies but was wondering if there is a starting ratio from where to work. I currently have 16 x 550w panels with a 21kWh battery and the solar is battling to get the battery from 50% to 100%. Come to think of it, it is actually a dumb question because power usage during what hours etc. plays a major role. At this stage I think I must dig into the piggybank and just get another 8 panels. :frowning: Apologies for rambling on.

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As mentioned elsewhere: your panels should match your load plus a margin (say 25%-50%). Then your battery size should not really matter - you will on average have more power coming in than going out, so your batteries should be charged fully most days. You can make it a lot more complicated and there are tools available to help you get your system as optimal as possible, but generally panels are cheap enough to buy more and spare the effort of doing the math.


It depends, on your requirements…. Do you want the battery to last for blackouts or for 4 hours or the night?

You can look at my planning and design posts on here to see some of the choices and compromises to be made.
Energy flow for summer, below that for winter


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It’s a bit involved if you have the appetite, but you can keep it simple…. You may want to ensure, as the first criteria, that you have enough panel to fully charge the battery, say in 4-6 hours. That ensures that on bad weather days you still have a chance of some solar charging.

Normally you are roof constrained for the amount of panels, and money constrained too. From that figure out how much battery you can charge.


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Thanks to all the annswers. Yes my problem is that the installation was done beginning of the year where it worked great but now that the suns trajectory is moving I am getting less sunlight with the biggest culprit being large trees. I am currently using 2 x 250/85 mppts and not sure I can hook up more panels on them or need to upgrade them as well?

There are a couple of angles to look at battery capacity from:

  1. Certainly, what you need energy-wise is probably the biggest influencer in most systems.

  2. From a technical aspect, a battery acts as a damper or stabilizer on the system.
    It can absorb the highs and supply the lows so that the effects of peaks and troughs are lessened.
    This comes into play, even more, when AC-coupling is done with PV inverters are they aren’t as responsive.
    If you go too small battery-wise you may run into stability issues if big loads are switched.

You couldn’t for example, switch out your battery whilst running and hope to have a stable system. ( Although it may run for some 10’s of minutes, just from the MPPTs).

In general, these stability aspects are already met without a thought, just by having sufficient capacity to cater for one’s needs.

Easy enough to find out. Measure or check your records if you have VRM. I find people tend to under-panel, and most installations can benefit from over-panelling.

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Instead of thinking that the PV is struggling to get your battery from 50% to 100% in winter, you might want to look at what you are using and what you need for that. If your has BatteryLife enabled (Victron ESS) then it will handle that for you.

Seasons change and your system usage will change with that. What is your goal and what is the ROI you want from that.
Can you get a better return by trimming some trees?
Or by replacing older high usage appliances with more effecient ones? (You can also score more browny points this option as well :slight_smile: )

As for your question on the ratio I would say it is dependant on your requirements, but I would say a general rule of thumb that works well for a houshold is to have PV that match your inverter power and with x3 or x4 the battery capacity.

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Yeah, that is me, 560ah bank with 4.2kw array. I cannot run the house AND recharge the batteries.

So, I fit my loads to the panels. Using ESS I can go from 100% SOC to about 80% SOC and the system will recharge, by end of the day, with daytime loads using the 4.2kw array. I need more panels. yes!

BUT, that is on good sunny days only. Come winter, it could maybe do 100% SOC to 90% SOC.

Can we cater for every single day of the year, for all the bad weather days, the season, trees?
Nope, relax on those days, just let it go! :slight_smile:

Focus on the 365 days saving, not the 30 days a year the sun is battling, enjoy the rain on bad weather days, we need that too.

In the old days, with lead acid batteries, there was an explicit ratio. You didn’t want to charge the batteries at more than 15% or so of their Ah rating, so in an off-grid (farm) situation that meant the batteries had to be 5 times bigger than the PV array. If you are also powering loads, then you’d add that on top.

Now that Lithium seems to be the standard battery, the ratio can be as much as 1:1 (PV matches battery Ah capacity). So now the other considerations kick in: How much backup do you want? Do you want to sail through the night without touching grid power, or are you okay with buying some of it from the grid? How much money have you got?


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At this stage it just feels as if the system is not utilised to its potential with the automatic adjustment of state of charge never dropping below 45% since the solar battles to get it to 100%. It would be nice to let it drop to 20% and get through the night and be more spendy in the day with the swimming pool pump and geyser while under normal conditions fill up the battery by about 1pm.

:slight_smile: Always a factor. I have effectively 10.2kW of panels and about 13kWh of battery on 12.6kWp panels in my system. For all intents I am off grid, during most of the year. My solar can mostly during daytime run the house and charge the batteries fully. It was expensive tho. Instead of getting one more set of batteries, I opted to maximise my heating efficiency, and add cooling as well, hence my Aircon Heatpump install that will happen soon. That may be enough to heat my very cold/lossy house with the current battery bank, time will tell this winter.

Adding 16kWh (~12kWh usable) may still be in my future, but that is way more expensive than adding more AC heat pumps and further improving my heating efficiency. You can see my system behaviour by clicking on my VRM link in my sig.


Ohhh i also dont mind inviting anybody that wants to login and have a look around :wink:

BatteryLife seems to have a mind of its own. Hit 100% a few times yesterday but it decides to raise my min SoC.

Would be nice if one could manually set the BatteryLife SoC and then let it take care of it afterwards.

Yes it does have it’s own mind (or logic :slight_smile: ). I would say that it was created so normal people can just forget about their batteries and just get on with life.
Now if you are watching it daily like a hawk (or you are an engineer - see other thread discussions :smiley: ) then you will switch BatteryLife off and tinker with with all day.

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Id like to leave it there and forget about it. Will give it a few more days but it just seems inconsistent.
Failing that ill just build something better in HA that takes forecast into consideration.

I know your big problem at this stage already is afternoon shading.
You mentioned big trees, will trimming the trees make a difference to the shading on the roof at all? That’s if it’s practical to trim or even remove the trees, it’s not always practical to destroy your garden, it might create other problems for you, too much sun on the house in summer etc.

Do you have more roof space on which you can put more panels, which won’t be effected by the shading so much?

Here’s another one to weigh up, having a big tree safely and professionally removed, will pretty much cost the same as a 500W panel these days.

Unfortunately its not my tree but the neighbours. I do have some roof real estate that i can add some panels that will make my charging day longer. Oh and it is a moerse tree :slight_smile:

BatteryLife will decrease the Active SOC limit by 5% whenever your SOC exceeds 85% on the day, and by another 5% if it exceeds 95%. It will do this only once, so if you cycle between 80% and 100% multiple times in a day, it will only do it the first time.

Similarly, it will increase the Active SOC limit by 5% whenever you hit MinSoc at the bottom. It will not decrease it to LESS than the user-configured minimum. And it will also only do this once a day.

So if you hit 100% every day, it should reduce your active MinSoc by 5% every day.

It also depends when you look at it. Early in the morning the Active MinSoc will tend to be 5% higher than you might expect (because it hit MinSoc last night, but it has not yet reached 85% today). If you look at it later in the day, it might be 5% or even 10% lower than it was earlier, depending on how much sun you had.

Another thing we can maybe throw in here for the experts to comment on.
@oradba69 asked me about this, I know there was a discussion somewhere on here in the past on over sizing arrays on a MPPT but I can’t find it.

The 250/85 is rated at 4900W and his 8x 550W panels on it is 4400W already. So how much oversizing is worth it and at what point does it become a waste, maybe better to rather put the additional panels on a separate MPPT?
Adding 2 more panels to the MPPT 1100W will be 12% over, adding 4 more panels 2200W will be 35% over.

Two things play a role. If the MPPT is running at high power levels for long periods of time, it heats up and derates. So that is an argument against oversizing too much. On the other hand, I find that I rarely see the full 100% of the STC rating of the panels, anyway which means just to get the designed amount out of them already justifies 10% oversizing.

So in your case I vote for adding two more panels. If you add 4 more, then rather get another MPPT.

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