Solar backup system, advice needed

Question

Here in South Africa we are experiencing frequent power blackouts, as I work from home on a small property (generators not an option) this is rather disruptive to my work as a programmer. So bearing this and the rapidly rising electricity prices in mind I need to find a solution.

I've searched already and found very similar questions, but most of them are centered on e.g. the US and give advice that won't work for here. A few key differences.

1) There are no rebates here for installing solar

2) There is no 'grid tie' allowed - that is feeding back into the grid is not only not paid for but is illegal.

3) The blackouts - we experience and probably will for the next few years 'rolling blackouts' of 2 - 4 hours at various time intervals - almost always during daylight hours - due to power shortages.

Also solar and especially batteries are very expensive here and I definitely can't afford a complete off grid system up front, and would probably want to buy any system I do get in pieces.

So here are my needs in order of priority:

1 - I need to get my critical systems for work: computers/routers etc. onto battery power with enough juice to last at least 2 hours ASAP the solar portion can be added on later.

2 - I would like to power at least that part of the system with solar, this will help offset my power costs and also increase the length of power outage I can go through should things get worse.

3 - I would like to be able to use any excess power to offset the costs of the rest of my household items, and possibly add more solar in future to cover more of it, but only want to generate around 80% as any excess generation would go to waste (can't feed back into grid) - and I don't want to waste money on batteries I don't need.

Below is roughly what I think a system that best fits my situation is like.

So a few questions:

1- Any recommendations on where you would change/improve/modify this system to get better value for money, improve safety, improve efficiency or otherwise improve on it somehow?

2 - How to stop the feedback into grid?

3 - What can I use for the 'splitter' portion to ensure that power goes first to the critical system and that only excess goes to the non critical stuff?

4 - At the 'combine' portion, how do I ensure that the non-grid power is favoured first before the grid power (when both are available)

5 - How can I ensure that charge controller only takes from the AC if batteries are below a certain level, do some charge controllers already have this feature or do I need an other component for that?

6 - Given that routers, computers etc. all run DC internally would it be worthwhile looking at having them all draw directly from the battery to avoid the cost and efficiency loss of the inverter - or is it not worth the hassle?

Answer 1

I think there are two levels of Battery backup here, First you can do a full move into Solar. in which case you would need a LOT of changes to your current setup (energy efficient devices, LED Light Bulbs, Solar Geyser, LED PC Screens, LED TV, Alternatives to tumble-dryer, gas stove etc. ) I dont think its as easy as spending lots of money on a solar implementation.

In terms of your PC Issue, a nice, relatively cheap option would be to only backup your pc equipment. I am in the process of doing that with my Modem, Router, Switches (all 9v). Laptop and two Screens (LED 19v). I will use a simple DC Charge controller and Fail-over circuit (HERE) and then will have a LION batterypack made up specifically for the two 19v screens, The laptop will just get an extra-size battery

Otherwise the alternative is just to go the route of a large UPS, But I am trying to use source voltage current to avoid the need for a pure sine wave converter (since my pc and monitors are really sensitive to different wave patterns) - for that reason, I would like to get everything on DC...

This is not an implemented solution, but I am in the process of designing (also in RSA). so a good (cheap) solution would be great

Answer 2

With grid-tie being illegal, there's really not a way to do what you have diagrammed, as far as I know, and I spent several years working towards a large and complex off-grid system before local conditions changed and it became practical for me to grid-connect. In particular, the "draw from solar, supplement from grid" (or questions 2 & 4) part does not work - for non-grid-tie it has to be one or the other with a transfer switch, it can't be both (well it can be both if you double-convert, but all the AC would come from the inverter - and you'd have efficiency loss X2 for the double conversion.)

My thoughts on this type of system lead me towards the following general idea, which I may still implement alongside my grid system.

• Pick a system voltage - I'm leaning towards 24.
• Build battery bank - despite various fancy options, "golf cart batteries" (~225 AH/6v) seem to win the "value for money" and WIDELY available categories.
• Do not neglect adequate DC-rated fuses on the bank.
• Attach a good grid charger. A bad charger will murder batteries (as a UPS owner you may already know this too well - many UPSes seem to have terrible chargers and terrible battery life as a result.) One possible approach is to use a very basic (but preferably efficient) AC-DC supply and use that as input to an "offgrid" charge controller; then you'll have the charge controller you'd eventually need for your solar panels, and they are normally excellent battery chargers.
• Use DC-DC converters to get other DC voltages you need as directly/efficiently as possible. i.e. Question 6 - yes, I think it's worthwhile where at all practical. The convenience of AC comes at a considerable efficiency cost.
• Add a charge controller (if you are not using one as a battery charger) and solar panels when possible.
• If the battery bank/solar capacity seems adequate, add an inverter for AC loads.

Control your use of inverter .vs. grid power by choosing where to plug loads in, or with a transfer switch. Plug control leaves the control system in your head and is low-cost.