My 28 ft round above ground pool is at a weekend home. We are not there during the week to take off solar heater during the day. I was advised not to get a solar heater due to possible algae buildup since a blanket would impair gas exchange.

I want to heat the water some (water can be a little chilly for my taste). The house has no gas or propane. It is all electric. Which would you choose: solar or electric heat pump after considering the following:

1. I do not really get swimming till may and will end by September when the kids start school again.
2. Only weekend use of the pool
3. No solar blanket

If solar, which one? I have looked at Vortex but they are quite expensive and need more panels (per their calculator). If fafco, they recommend less panels but only heat up to 10 degrees. I would prefer to mount on the roof.

Any help/suggestion that you all can offer is much appreciated.

I think the hardest thing for you is to keep your chlorine levels up without any care for 5 days each week. I would invest in both an SWG and a solar blanket. I'm not sure why a solar blanket would not work. It retains heat, but you still continue to get light through the blanket. I would think most of your issues with algae buildup will be due to the fact that your FC levels will drop without regular dosing.

i have a pump infusing bleach slowly into the pool daily. It kept up with FC last season without any issues.

Whichever panels you decide on should be fine. Also you need to figure more panels than the mfg's recommend. If you really want to generate heat you should be close to the square footage of the surface of your pool in solar panels. In a completely optimum installation you can get by with a little less but it's a good idea to have a 1:1 ratio.

A solar cover will be fine to use as long as you can keep the FC up. Since you're going to be using it on weekends you can remove the cover for a while every weekend and it'll give any CC's that's built up a chance to dissapate.

+1 with Docs911 comment. Keep forgetting there are other things other than an SWG. Bottom line - you need something that keeps feeding the chlorine into your pool during those 5 day periods. Otherwise, your FC will be 0 at the beginning of each weekend which is NOT GOOD!

A 1:1 ratio for a 28 ft round pool? The surface area for 28 ft round is 615 sq feet. I am looking at 4x12 ft panels. I would need 13 panels on the roof.

Would that be too much weight?
Will a 1.5 HP pump be sufficient?

Where is the pool located? Climate makes a huge difference in panel area. The usual calculations are for swimming while the sun is at lower angles. If it's too cold to swim then, you can get by with much less area.

I am in Virginia and I will be cutting trees down to get full sun onto the pool and roof of the house

I'm in Indiana, and heat the pool in my sig with 6 4X8 panels. I can keep the pool 85 easily from mid-May through Labor Day with that amount of panels. If you want to be able to swim outside the normal pool season you might want a bunch of panels, but for supplemental heat in season with a little stretching, you can get by with less.

How many panels can a typical roof safely hold?

Weight isn't much of an issue. They don't really hold very much water.

The dealer who sold me the pool and set it up highly suggests the heat pump. He is telling me that the solar panels won't get me much more than 10 degrees. If I want temps in mid 80's , he is telling me that heat pump is the way to go.

That is simply not true if you are looking at a reasonable number of solar panels and you also use a solar cover. In my pool between a relatively thin opaque pool cover and solar panels (about 78% of the pool surface area since 2 panels are mostly in shade) I easily get over 20ºF higher pool water temperature than average day/night temperature. If I had a more insulating bubble-type cover, this would be more than 25ºF higher. A cover alone gets around 10-15ºF depending on the cover (clear ones allow some sunlight through). For the most heating, you would get a more clear bubble-type cover so that sun can heat the pool directly as well.

The sun's energy output (insolation) in Virginia is around 80% of where I live near San Francisco so instead of getting over 25ºF higher than average day/night temps with a bubble-type solar cover plus solar panels combo, you might get over 20ºF average day/night temperature. I'm not sure where you live in Virginia, but using the temps in Richmond, VA I get the following for avg. day/night (rounded up) temperature, the insolation (for panels tilted at 15 degrees less than latitude so tilted 22.5º towards south), and expected pool temp:

MONTH: ......... Jan .. Feb .. Mar .. Apr .. May .. Jun .. Jul .. Aug .. Sep .. Oct .. Nov .. Dec
INSOLATION: .. 3.2 .. 3.9 .. 4.8 ... 5.5 .. 5.8 ... 6.1 .. 5.9 .. 5.7 .. 5.1 ... 4.4 ... 3.5 .. 2.9
AVG. TEMP: .... 37 .... 40 ... 48 ... 58 .... 66 ... 74 ... 78 ... 77 .... 71 .... 59 ... 50 .... 41
POOL TEMP: ... 48 .... 53 ... 64 ... 76 .... 85 ... 94 ... 98 ... 96 .... 88 .... 74 ... 62 .... 51

In practice, you wouldn't bother using the solar in the off months since it just uses more pump electricity and you won't be using the pool. You also wouldn't heat it as hot as I indicate in June through August -- a solar cover alone would be sufficient for those months (especially July and August). The solar panels really are there to extend the season into May and September (maybe late April and early October if you swim in lower 80's). My numbers might be too conservative if you use a good mostly clear bubble-type cover (i.e. you can get more heat retention in that case as well as some direct solar heating) but for solar panel heating I assume a fairly large panel area of around 80% of the pool's surface area.

If you do not use a solar blanket, then you won't be able to get the pool warm enough when it gets really cold at night in late April and early October and even May will be difficult. With no solar blanket and no solar panels or other heat source other than the pool hitting the pool your pool temperature will probably get to no more than 5ºF warmer than the average day/night temp shown so in the low 80's in July and August, but that's about it (this assumes a white plaster pool; with a dark vinyl or painted pool it could get perhaps 7-8ºF warmer if there isn't wind). You can get your pool warmer in those months (if you wanted to) with either a solar cover or solar panels but both together are certainly not needed in those months nor in June.