I want to build my own off-grid system but confused with what sizes (V, I) to get.

I want to run a 24v system powering 4.4kWh/day. I have 4 hours of usable sunlight a day and space for panels and equipment is not an issue (within reason).

Looking at the calculations:

Electrics = 220v @ 60Hz
Load = 4.4kWh/day
Sunlight = 4 hrs

The PV array I then calculate at:

PV Array = 4400Wh / 4hrs = 1100W (10 x 100w panels)

The batteries I then work out at:

4400W / 24v = 184Ah / 80% discharge = 220Ah

A battery example [1] - 24v 220Ah deep-cycle li-ion (2000times(80%DOD))
* or if you want to go down building your own battery pack with 18650's - 7 in series (to get 24v) x 55 packs in parallel (to get 220Ah).

The calculation problems I cant find anyone explaining on Google is:

a) What size charge controller do I need?  I see options from 10A to 100A.  
b) What size inverter do I need? I see options up from 100W to 3000W and more.
c) Does the PV array not need to be x2 the size so it can power the house and charge the batteries at the same time during the day?

[1] https://www.alibaba.com/product-detail/Custom-24-Volt-Solar-24V-200Ah_60670679703.html?spm=a2700.7724857.normalList.25.15c91a82Albyfj


The answers to (a) and (b) depend on the peak load you want to use - i.e. the highest powered devices you want to run. Note that these might be things that you only run for a few minutes (e.g. an electric kettle), so they don't eat up the whole of your daily 4.4kWh energy budget.

The answer to (c) is "no". For example, suppose your appliances used 1100W for the entire four hours of daylight. The panels would be powering the appliances but not charging the battery at all. But that doesn't matter, because you have used all your daily energy budget in those four hours, so to stay within the design limits of your system you will not be using any electricity at all for the remaining 20 hours of the day, and it doesn't matter that the battery isn't charged!

Of course that is an extreme example to make the point easily - but the same thing will apply for any usage pattern over 24 hours.

Note, you need to include the efficiency factors of the components in your system design. The efficiency of the battery charging system won't be 100%. The inverter should have a high efficiency, but it won't be 100% either. Its efficiency might vary depending on the load (and it might be worse for light loads than for heavy ones).

Also, ask yourself whether "4.4kWh per day" is a long term average (over a week, or a month, or a year) rather than the daily limit for every day. You might have some high-power devices (e.g. a washing machine) that you don't use every day, so you need to store energy for a longer period of time - i.e. you need a bigger battery capacity.

  • $\begingroup$ Fantastic! Thank you. I calculated the load from my last electricity statement which said 131kWh/mo. $\endgroup$ – Christian Oct 18 '18 at 10:30
  • $\begingroup$ So to work out (a) and (b), do i need to find the appliance with the highest amperage/wattage? $\endgroup$ – Christian Oct 18 '18 at 10:32
  • $\begingroup$ I have just noted that the wall sockets (here in the UK) are rated at 250v 13A = 3250W. My coffee machine is 1kW, toasty maker 1.4kW and kettle 3kW. So I couldn't plug them all in to the same plug. However, if I was to create an off-grid system, it would have to handle the load of all of these being on at the same time (if only for a brief moment). So am I right in thinking I need at least a 5.5kW inverter to handle the load? $\endgroup$ – Christian Oct 18 '18 at 12:57
  • 1
    $\begingroup$ "So am I right in thinking …" that is one way to find what load you need, but it relies on you always sticking to your own "rules" which might not happen! Another way (which will give a bigger answer) would be to find out what is the maximum current capacity of the circuit breakers or fuses where the mains power connects to your house. $\endgroup$ – alephzero Oct 18 '18 at 19:10

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