I went to the Fully Charged show last year and am interested in solar panels and home batteries / inverters, and (whilst plucking up the courage to invest) thought that I'd start with making a small semi-transportable powerpack. I then used another project on instructables to read around the subject (Off Grid Power Supply by JON-A-TRON, https://www.instructables.com/id/Off-Grid-Power-Su... ). Have a look at the comments and his responses which are valuable.

The hard work is really the box I made for it to go in, which has a battery compartment and a components compartment.

Supplies

The components were slowly assembled and added to the box as I built (quite slowly, hoping not to make decisions that I regretted).

Inverter - from https://www.sunshinesolar.co.uk/Item/VPS300
AGM (Absorbent Glass Matt) deep cycle lead acid battery, ideally of approx 100Ah (but I have not spent the money on this yet - pretty expensive https://www.sunshinesolar.co.uk/Item/AGM110. Instead I am using an old 70Ah car battery, but you should be careful if using these - they emit hydrogen when charging so need to be kept in a well ventillated area.
blade fuse holders https://www.amazon.co.uk/gp/product/B07GBR5MR7/ref...12v
utility outlet https://www.amazon.co.uk/gp/product/B019XOQ42G/ref...5v
Voltage Regulators https://www.ebay.co.uk/sch/sis.html?_id=202660224...
USB sockets https://www.ebay.co.uk/sch/sis.html?_id=133228771...
Voltmeter https://www.ebay.co.uk/itm/White-car-LCD-12-48V-L...

It also has a couple of tiny 12V solar panels to trickle charge the battery. I got these ages ago and I re-purposed them from another project.

Step 1: General Design

The separating bulkhead is removable and the inverter is mounted on it (allowing me to easily remove the inverter for use in the car or elsewhere). I decided to add an access door to keep the electronics inside, although the design is not really properly weatherproof. The outputs are:

12V (utility car socket) - straight from the battery, fused.
5V (USBs) x 5off - via the small 1.2A voltage regulators, the feeder from the battery is fused.
240V AC - the 300W inverter, fused.
1 additional 5V (1.2A) USB that is provided from the inverter.

Step 2: Box / Enclosure

Made from wood I already had in the shed, I started with a very sturdy base, and 12mm ply sides and lid/top. I used some 90deg oak angle/coving pieces to strengthen the edges as well as hide my workmanship. The lid is secured using some padlockable stainless steel clamps that I had.

Step 3: Wheels

The wheels and axle came off an old weber bbq, and I mounted these through a frame that I cut off an old airline carry-on bag. This means I have an extendable handle that I can trolley the powerpack around with.

Note that the vertical orientation is ONLY possible if you have a AGM battery - other ones may likely leak (or the cells may not be submerged in the electrolyte).

I added the varnished lumps of oak as props to keep it level when in the horizontal or vertical position.

Step 4: Inside

It gets a bit messy on the inside.....

Step 5: Operation

It works well, but I'm not really sure there's a sound use for it. Maybe for a festival or camping, but it's nearly too heavy to lift into a car. The 2 x 12V solar panels were a bit of an afterthought, and they are connected in series, delivering 24V to the battery (via a diode). This is a bit of a bodge, as you should really charge though an intelligent charger to prevent overcharging (damaging the battery or the other components), but I read up on it and the charging current of the panels is so low and the battery capacity so large in comparison that it instantly draws down the voltage to the battery terminal voltage.

You can test the charging voltage using the 2 switches, and also test the battery voltage.