Battery Bank to Grid-Tie for Overnight Usage?

[RobertReadman]

Hello,

I wasn’t sure if to start this here or in "Batteries and Inverters" (but that said for off-grid/semi-off grid, please move topic if this is wrong)

My current set-up is as follows, a 3.6kWh solar pv system to an SMA SunnyBoy 4000TL-20 Grid Tie inverter.

What I want to achieve is to fully reduce what the premises is pulling from the grid after sun down.
The Solar PV array nearly always produces around 500W on the most cloudiest days, output can be seen on SunnyPortal here...
http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?plant=cb1b494f-d3fe-4cca-9395-39575f06ceee&splang=en-GB

What I want to do is connect a battery bank to a G83 grid-tie inverter to supply the load in the house and also export slightly any excess to the grid (without going through the generation meter)

My idea is as follows... out in the garage/shed or workshop, to build a small battery bank with a grid-tie to take the load of the property after sundown to reduce the 1-2 units used from 9pm to 9am.
This is an average overnight consumption of 166watthours per hour (based on 2 units), with the load being a Fridge, a small Chest Freezer, and the TV (for a limited time only), and a few lights etc.

The devices would be connected as follows...

13A socket.
>
Time Unit, set 9:30am > 18:30pm
>
Battery charger 4A @ 12v / 2A @ 24V
>
Controller
>
Battery Bank  (4x 110Ah 12v Deep Cycle Leisure Batteries) Providing 12v 440Ah or 24v 220Ah.
>
Controller
>
G83/1 certified Grid-Tie Inverter
>
Time Unit, set 9:00pm to 9:00am.
>
13A socket.

I have done a search on the forum, and the answers given before were, don't do it, its too expensive for the gain etc...
Now while an SMA SunnyBackup system would be nice and tidy, the cost would most certainly never pay for its self.

I'm looking for a safe approach as a project to do.

I would love some feedback on the idea of using time switches to charge the battery bank in the day / release the energy at night.
Also what about using batteries into the Grid-Tie Inverter? any recommended controller / limiter.
If I could make it draw 150w from the batteries into the inverter to stop / slow the take from the grid that would be great.

Many Thanks,

Rob.

[rogeriko]

I just installed a system to do exactly as you say. 5kw sunny island, 5kw sunny boy, 1000ah 48volt batteries, 22 Sanyo 230w panels. The house/villa uses about 18kwh overnight and the sunny boy produces over 35kwh per day. This is all used to run swimming pool pumps etc and the mains electric cuts in for about 4 hours every morning. Total cost around 20000 euros.  As you see it is not cheap to do what you want, you cannot connect batteries to the sunny boy you must have a sunny island as well.

[RobertReadman]

Hello,

On the setup I proposed, I wouldn’t be using the SunnyBoy, the solar PV install would stay as is. I would get a different grid-tie inverter, however I would not want to take the premises off-grid so I am looking for a method for a battery bank to supply a small grid-tie inverter, which the household use would automatically pull from instead of pulling from the grid, with a very small amount of power from the batteries exporting.

Hope that makes sense,

Many Thanks,

Rob.

[clockmanFR]

I think your best cost option is a new system.

As Rogeriko says leave the original alone.

Get a panel or panels 250w and mount where appropriate. A controller to charge your battery's safely. A 1Kw pure sine wave inverter (fridges need pure Sine). And DPDT switches or H/Duty relays with on/off/on for switching your supply, rated accordingly.  (relay could be operated by timer).

Probably not cost effective if purchased new from suppliers.

But, by shopping around and a bit of inventing it could be viable.  ie, use a APC UPS (uninterruptable power supply)1500KVA computer inverter at S/hand of *bay for under £100, (uses 24v).

[Justme]

Why not go for a side by side setup?

Keep the existing grid connected system as it is & add an off grid system (charger, inverter & batteries) connected to your CU via an automatic change over as used on generators.

Keep any important loads on a small CU direct to the mains.

Switch the off grid system in as soon as the PV output stops meeting demand or via time a clock. Have it set to switch out either when the PV takes back over or when the battery bank is at X% SOC. You will need a proper deep discharge battery bank (no not those small leisure or truck one but real forklift 2v cells).

Its not going to be cheap if you spec it to last. Remember if you set it up to use 50% each night then you will soon use up the max cycles of the batteries. Better to try to only use 20-30% per day if poss.

[Alan]

Quote from Rob  “  I would not want to take the premises off-grid  “

The down side is that batteries are costly and they don't last.

How about an exercise bike with a generator fitted. Use it to charge some super capacitors.

The capacitors would allow the Lady of the House to stop pedalling for a few seconds at a time and still maintain a constant feed to your other grid tie inverter.

Head down time.

Regards

Alan