Help with re-sizing PV prioritized house system

solomon

Hi

would like to ask your help in sizing my solar system for a house. I have some loads in the house that I would always want them to be on PV and the others on Grid.

currently i have the following:

Victron MPPT 150/45

Victron Multiplus 3000VA

6 12V 205Ah AGM batteries (Trojan SAGM 12 205) in a 24V system hence 615Ah

solar panels  ( 255W Amerisolar AS 6P30) - total 8 - 2 series then 4 parallel

My loads as follows:

always on loads (daytime+Night) - 200W

Night only loads for usually 13hrs - 400W ( normally 250W but an additional 150W for  around 3 to  4 hrs)

inverter settings settings put to prioritize PV instead of Grid:

Low input shutdown 21V

Inverter accept AC voltage  if below 21.5 for more than 10s

Inverter ignore AC when Bulk charging stage finished 

Minimum sunshine - 4hrs for 3 months and above 5 hrs for the rest of the months

Reasons for resizing:

1) challenge with this setup is that for the days with low sunshine the batteries reaches the 21.5V and charged by Grid which leads to higher power bills. I would want to avoid this

2) I plan to add additional always on load - a fridge which i have observed has a peak W of 200 but mostly 100W.

this would bring my awalys on load to 400W and night time load at 400 hence total load of 800W

Another issue i have although i think its specific to victron is that the i can charge the batteries for days with PV while filling them each day (bulk stage- absorbtion to float) but Soc always incorrect (i.e never reaches 100%). The Battery Soc only correct for those days i charge with Grid.

kindly help in proper sizing calculations.

Estragon

Some rough numbers:
150w x 4hrs =600 watt-hrs
250 x 13 = 3250
200 x 24 = 4800
Total 8650 wh/day.  Add fridge ~1500wh/day = ~10,000wh/day + ~20% inverter loss = 12kwh/day.

12kw ÷50% max battery discharge x 2 days no sun = 48kwh total storage ÷ 24v nominal = 2000ah bank.  To recharge 50% discharged bank in 1 day, 24kwh ÷ 4hrs sun ÷75% panel derate =8kw pv (~32 panel array)

6kw ÷ 24v = 250a ÷ 45a/controller = 5.55, rounded up to 6 controllers (if using the same ones).

Unless your grid power is really expensive, it's likely going to work out much cheaper to use solar to offset grid consumption than to use a battery based system.

mike95490

Be sure to calculate the cost of replacement batteries every 5 years !  Battery cost almost always overwhelms any savings by load shifting.  Simple grid tie backfeeding is often the best payback

solomon

Thank you Estragon, yes this seems like a very huge investments I might have to make do with less than ideal setup