Thoughts on these panels as a used buy

[Hedges]

The 9 panels I suggested would only work with a 200V or 600V charge controller. Or all panels in parallel for 60V.
Any even number would be good for 100V.

You need to determine watt hours to run a machine cycle, continuous watts while running, and surge watts to start a motor.
Then you can select battery size and inverter.

PV/battery backup is nice during grid outages. You just have to design it and size it for your loads.
Mine is huge (except relatively small battery), runs A/C and everything during daylight.

 

[Hedges]

A 2kW inverter would be sufficient to power the microwave. Or the refrigerator (or any other appliance which plugs into a standard 120V outlet with the exception of larger motors.) 12V is sufficient, but higher voltage is always preferred (1800W from 12V is 150A, even more due to inefficiencies so design for 200A). Higher voltage battery also saves money on charge controller, although a 60A unit will about handle your 4 panels. If 24V battery, twice the wattage from same charge controller.

No-load draw by the inverter could consume most of your production especially in winter. You can turn it off manually, or get one with a sleep mode which only wakes up to check for loads by producing AC periodically.

For any load more than about 400W, battery usable capacity needs to be sufficient to power the load (PV production is much less morning/afternoon, assuming panels are oriented South.)

The shed roof slope will make much less power in winter. Unless that matches your consumption, a tilted rack maximizing winter production would work better. You can check insolation calculators. If a 45 degree slope, almost 50% more panel area would fit within shed footprint.

If you also keep battery charged from grid, then you can get by with one having much shorter cycle life. Lithium is the way to go for many systems but for small systems and those which rarely cycle, AGM is economical and simple.

A 100 Ah lead-acid battery might deliver 200A for 3 minutes to power microwave, but about 50A would be max advisable for 30 minutes. About 500W appliance. That covers refrigerator with occasional defrost cycle. Probably dishwasher if heater not used.
Quality 100 Ah 12V AGM is about $350. Cheaper brands maybe half, same for FLA.

If you want to DIY lithium, that may come in between $350 to $600 for 100 Ah to 280 Ah. The catch is supporting 200A, hefty BMS or maybe just remote enable of inverter (so get one with remote wire connection).

 

[Hedges]

Oh, Lead acid batteries have a limitation on maximum charge current. The SunXtender I use want 0.2C charge rate (although vendor says they can tolerate much higher, within voltage and temperature limits.) Other brands/types are more strictly limited.

If your PV delivers 800W, you want 4000 Wh of lead-acid battery, around 300 Ah at 12V nominal.
That drives you to a larger bank, significant money (around $1000 for quality AGM, $500 for FLA golf-cart batteries.)
2000 to 3000 Wh usable, depending on battery specs and cycle life desired.

So even for this size system, DIY LiFePO4 is looking like a no-brainer. It saves money up front and gives more Wh capacity and 5x the cycle life.
Probably 150 Ah or larger will be happy at 800W charging (0.5C), except when cold. Set BMS low-temperature cutout at whatever temperature limit shows 0.5C as acceptable.

 

[dwford36]

A 2kW inverter would be sufficient to power the microwave. Or the refrigerator (or any other appliance which plugs into a standard 120V outlet with the exception of larger motors.) 12V is sufficient, but higher voltage is always preferred (1800W from 12V is 150A, even more due to inefficiencies so design for 200A). Higher voltage battery also saves money on charge controller, although a 60A unit will about handle your 4 panels. If 24V battery, twice the wattage from same charge controller.

No-load draw by the inverter could consume most of your production especially in winter. You can turn it off manually, or get one with a sleep mode which only wakes up to check for loads by producing AC periodically.

For any load more than about 400W, battery usable capacity needs to be sufficient to power the load (PV production is much less morning/afternoon, assuming panels are oriented South.)

The shed roof slope will make much less power in winter. Unless that matches your consumption, a tilted rack maximizing winter production would work better. You can check insolation calculators. If a 45 degree slope, almost 50% more panel area would fit within shed footprint.

If you also keep battery charged from grid, then you can get by with one having much shorter cycle life. Lithium is the way to go for many systems but for small systems and those which rarely cycle, AGM is economical and simple.

A 100 Ah lead-acid battery might deliver 200A for 3 minutes to power microwave, but about 50A would be max advisable for 30 minutes. About 500W appliance. That covers refrigerator with occasional defrost cycle. Probably dishwasher if heater not used.
Quality 100 Ah 12V AGM is about $350. Cheaper brands maybe half, same for FLA.

If you want to DIY lithium, that may come in between $350 to $600 for 100 Ah to 280 Ah. The catch is supporting 200A, hefty BMS or maybe just remote enable of inverter (so get one with remote wire connection).

I want to assume enough battery capacity to run these things whenever needed, so I'm not in a "it's a cloudy day, I guess I'll wait in the laundry" type of situation. I'd probably spring for lithium, just so I'm not needing to monitor the charge level on a regular battery.

 

[Hedges]

That's fine, just determine how many Wh you want stored.
LiFePO4 cells of 280 Ah seem to be the sweet spot now. You get about 4kW per 12V (maybe 3.2 kWh usable safely between the knees of the curve), so you could select 12V, 24V, 48V system depending on how much storage you want.

The battery itself will be a project. Getting the cells properly balanced, and making sure all contacts are good (oxide free), carry high current without voltage drop.

Inverter you can do piecemeal, or a hybrid if you prefer.

Inverter/charger and BMS that are on speaking terms would perform better. But may not be available at the budget end if that's were yours will be.

 

[sunshine]

I want to assume enough battery capacity to run these things whenever needed, so I'm not in a "it's a cloudy day, I guess I'll wait in the laundry" type of situation.

Go with a 12v system. Your posts seem to indicate that this will be ideal.
A simple conversion of your first panel to 12v (refer other posts or YT) means you can add as many dissimilar panels as you wish as long as they too get the 12V conversion up to any size your enthusiasm takes you to, at the lowest cost and highest DIY level!

 

[chrisski]

A simple conversion of your first panel to 12v (refer other posts or YT) means you can add as many dissimilar panels as you wish as long as they too get the 12V conversion up to any size your enthusiasm takes you to, at the lowest cost and highest DIY level!

I think I’m reading This wrong. Are you saying you can mix and match panels including if they don’t have the same voltage? Or I’d there some 12 volt conversion kit you run mismatched panels through to match voltage?

 

[sunshine]

Are you saying you can mix and match panels including if they don’t have the same voltage?

If the panel specs are close to each other you can mix and match, They default down towards the specs of the lowest panel.
However what I was referring to is physically modifying the panel by splicing cables and splitting the existing V into one you need.
Google ' convert solar panel to 12v'. Just remember most 12v panels are 20v. You get 2 out of a 40v panel.

 

[Hedges]

He is considering a couple different panels. Either one is about 40 Vmp and 50 Voc.
Connected 2s for 80 Vmp, 100 Voc would be OK for a 125V or 150V charge controller. Could have parallel strings of these two kinds and anything else adding up to same voltage.

12V battery means 4s BMS but highest current. So I would consider 4s for 4 cells, 8s 24V for 8 cells.

If the panel specs are close to each other you can mix and match, They default down towards the specs of the lowest panel.
However what I was referring to is physically modifying the panel by splicing cables and splitting the existing V into one you need.
Google ' convert solar panel to 12v'. Just remember most 12v panels are 20v. You get 2 out of a 40v panel.

But why would you?
An MPPT charge controller takes a range of voltage input. It will have an efficiency curve vs. battery voltage and PV voltage. With MC4 it is plug and play.
Your array configuration affects wire cost and power loss, and what shading does to the system. If single or two parallel strings, no PV fuses needed.
I run mine as high as I can (< 600Voc and about 380 Vmp into 240 VAC inverters)
MPPT SCC typically accepts from a few volts above battery to 150V, but it varies between models.

 

[dwford36]

I ended up buying the 3 Silfab 370w panels. They were brand new, still shrink wrapped to the pallet they were shipped on. $450 total. So $.38/watt. For now they are in my basement while I research and gather up everything else I need