Dumb heating element and water pump

Right now it's just the manifold capped off holding pressure. I can rig up a pump pretty easily to circulate water, just the main problem is making it warm.

You have a point, I should just experiment with ac power first. But it's not good for the long term. Even if a 1000w heating element is running 24/7 that's ~9a or 24kwh per day, for 30 days, at $0.19/kwh that's $136.8/mo. That's a lot of juice.

1000w is also fairly low, which is why I was thinking 2-3 strings of panels each being 1200w each that will blast it with heat. Like I said I am not looking for a balmy 70f, I just don't want my feet to be freezing on an ice cold concrete floor.
I see where you are going, I will tell you about the two folk I know with the systems. Now I don't know their bills, or time to get "warm" but I do know a little bit about two systems on each end of the extreme.

Guy one lives in northern ID almost Canada. His system in in a VERY nice hanger, like airplane hanger. This guy has all kind of cool toys, those $40 four person side by side hot rods, naturally airplanes. I think he lights cigars with $100 bills. His system in commercial, and only has it off for a few months in the spring-fall. When outside temps are in the 50's it is getting cold at night. His runs all the time, and the floor is "warm" to the touch. It is not "hot", and warm is not the best word. Lets say you put your hand on the wood kitchen table in your house, thats it.

The other guy is a "home made" setup using a big hot water heater. I think he said he has 3-4 different "loops", not sure if that is a thing, I really don't know much about it.

When he cranks his up again around this time of year here in middle MO. He has it set to pretty hot. You don't want to hold your hand under the water hot. he says that will warm up the slab quicker. I never asked how quick. Then he will turn it down to what I would call lukewarm. Personally I think the floor is still cool to the touch, much cooler then guy 1, but it is warmer then my floor, when on a 50f day and you open the shop it is flat cold in there, and if you leave a bottle of water on the floor it is the same temp as you take it out of the fridge. His is on 24x7 in the winter. Everything he has is electric. I don't know what the bill did after he hooked that up, that would be tacky to ask. You don't ask if bills go up, but you do ask if bills go down....solar people tend to like to talk about that.

I think it would have to, I have never seen the light on the water heater off, but I don't look every time I am over there. And that pump is always running, you can hear that in one part of his shop. He is a big time wood worker with vac hoses in the floor and this big fancy dust collection deal he setup. Stuff to keep the wood in storage nice, humidity,heat, cool all that jazz.

I also admit I am not sold on it for my use. I am out in my shop darn near every day, at least 4 hours after work, and usually most weekends. It is my man cave. I just have normal heat. Up stairs it gets quite warm with the heat running that you need to turn it off even with two fans moving stuff around. I thought about heating the slab, but not sure if my use would warrant it.
Oh and both don't use "water" it is some kind of antifreeze type stuff, just incase something fails it will not bust up the concrete. Not sure I said that before.
OP, any chance you can use wood to heat the circulating water?
Peco is the power company.

Yes, it's a 50/50 mix with water and RV anti-freeze, water is just a bit of an oversimplification.

I heat my house with wood. It's a bit of a pain. Felling tree's, cutting them up, splitting the wood, seasoning the wood. I'm looking for less maintenance and less reliance on outside elements.

I know a guy in nebraska that heats his shop with radiant floor heat with electric hot water for between $40-60/mo. However, he has massive amounts of insulation. Which I will have eventually. But not yet. My walls are 8" thick which means I can get R27 in there, and R40-50 in the roof. The garage doors and man doors are insulated as well. The idea is once there's heat there, it'll stay.
One idea that isn't mentioned much is to hook up solar panels outside and wire them up to solar panels inside. Panels are basically just large surface area diodes that can dissipate heat, and you can more easily keep them near the max power point of the outside panel and get a bit more energy than just directly hooking up a heating element to a solar panel. You could also use diodes in series to dissipate heat, but you already have panels.

The guy I heard about this from used two outdoor panels to run into 2/3rds of an indoor panel, and it heats his internal panel up to a good temp, but less than a typical solar panel may get on a hot roof in summer. Dacian from electrodacus.com has pictures on his forum/website in the gmail section. Not everyone wants solar panels in their homes, but for this case it could do some good without spending a lot of money.
Lots of math about how much energy to heat the slab to a certain temp. What is really missing to help size the system is the temp the slab needs to be at to keep the air temp at a certain point (if that is something desired beyond warming up the floor some for when doing work). Siegenthaler has some excellent webinars and blogs about low temp hydronics, more specifically in this case, the math to figure out what surface temp a floor needs to be at to maintain a certain room temp assuming you know the BTU load of the space.