About Roger Perry

Roger Perry manages all of SES’s solar production and installation crews, including commercial solar thermal design and implementation, as he transfers his extensive knowledge and experience to up-and-coming solar installer/designers. Roger is the Mid-Atlantic’s foremost expert on Solar Thermal applications. He has spent a lifetime in the solar industry installing and designing thousands of commercial and residential solar systems.

Solar Energy Solar Panel Washington DC Solar
Written by Roger Perry

Practical Considerations for a Battery System

As long as I’ve been doing solar, people have been asking about batteries. The response has always been “yes, we can do them, but it will cost a lot”. That usually ended the conversation.

One question to ask is “why do you want batteries?” Do you lose power frequently? If not, a portable generator will keep your refrigerator, freezer, computers, tv and some lights going. Downsides are; noise, fumes, refueling, having to run extension cords to where needed and putting everything away when power comes back. Not too bad once in a while and it’s very cost effective. Just hope you aren’t out of town when power goes out. There is nothing automatic about this set-up.

If you lose power often and don’t want to do the portable generator dance every few months, you can get a permanently installed generator with automatic start. These are close to $5000 installed for the ones that will run most of your house when the power goes out (a larger one can be installed for a few thousand more that will run everything). This is what hospitals and critical buildings use. Power goes out, the generator starts automatically and powers the house with only a momentary loss of electricity.

Downsides? Noisy, they need maintenance and, if propane or diesel powered, they need the fuel tanks to be kept filled.

What if you lose power often, don’t want the noise, maintenance and fuel expense of a generator? What if you want a system that is environmentally friendly, will turn on automatically, is silent, will run pretty much nonstop without refueling? You should look into installing a battery back-up system connected with a solar system.

What are the downsides? Well, cost is one. While not as expensive as in years past, battery back-up systems are still costly. Compared to a permanently installed generator, battery backups tend to run few thousand more. Ask your accountant but you may be able to take the 30% solar tax credit on the additional cost of the batteries. This brings the price in line with a generator.

Another downside is you can’t run everything in your house. Things a battery cannot run for any length of time are air conditioners or heat pumps, electric water heaters, electric dryer or electric ranges. What they can run are gas or solar water heaters, gas or oil boilers, refrigerators, freezers, lights, tvs, computers, fans and pretty much everything else. Well pumps are on the edge depending on how efficient they are. While running a modern variable speed well pump is not an issue, older well pumps require a large startup current which can be too much for the battery to handle. We are about to install a “soft start” control to try and reduce a well pumps surge demand for one of our customers but the jury is still out.

We are now installing LG Chem lithium Ion batteries. LI batteries have a lot of advantages over the old lead acid batteries. The big advantage they have is they are not damaged by running them dead, whereas lead acid batteries do not like being discharged to less than 50% of charge, a Lithium Ion battery will give you it’s full rating. The one we use is rated for 10 Kilowatthour (KWH). A lead acid battery would need to be rated at 20 KHW to achieve the same capacity. Lithium batteries are also much lighter (not that the customer will have to move them) and can be charged much faster.

The big downsize of LI batteries is the upfront cost compared to LA but they will last much, much longer and they are maintenance free.

In a future blog I will discuss the two different ways to interface a solar system with the batteries, AC coupled and DC coupled and the pros and cons of each as well as what a 10 KWH battery will give you as far as run times for various appliances. I’ll also talk about how a battery system can be retrofitted to your existing solar system.

Commercial Solar Service ,Annapolis MD
Written by Roger Perry

On-Demand as Back-up for Solar Water Heating?

Solar veteran Roger Perry gives his two penneth about on-demand water heaters as viable solar water heating back-up candidates.


Commercial Solar Service ,Annapolis MDAll solar water heaters in Maryland, D.C., and the Mid-Atlantic provide a family of four with around 75% of their annual hot water load. Most customers use their existing electric or gas sources for the remaining 25% backup (usually needed dead winter.) On-demand water heaters have a couple of advantages over tank-type heaters, but they are somewhat mitigated with a solar system.

The touted advantages of on-demand heaters:

Endless hot water– In a solar system with a properly sized back-up you will, pretty much, have endless hot water in the summer and plenty of hot water the rest of the time.

Large savings because of no stand-by losses– The claims of “up to 40% savings” are a bit exaggerated in all but the oldest water heaters. Our solar tanks are extremely well insulated and their stand-by losses are minimal. Also, in a solar water heater, the tank is an integral part of the system so the small stand-by losses are a fact of life.

Then there are the disadvantages with on-demand heaters:

On-demand heaters can get very confused when fed water that is already hot– This can result in temperature fluctuations as the heater tries to adjust or, in extreme cases, the heater shutting down on high limit.  One way we have gotten around this is to use a motorized 3-way valve that bypasses the on-demand heater.  This works well but adds to the complexity of an alredy complex piece of equipment, which leads me to…

They are very complex pieces of equipment– A standard water heater, gas or electric, has a bimetallic spring that moves to make an electrical connection when it cools, and moves to break that connection when it heats up. It doesn’t get a lot simpler than that. An on-demand heater must sense the flow of water (demand), measure the outgoing water temperature, and adjust the amount of gas to feed the burners so as to maintain a constant temperature at the tap. It also has to do this very quickly so as to not deliver water that is too cold when another tap is turned on or too hot when another tap is turned off. After a couple of years, scale begins to build up in the water valve; the unit needs to be descaled or it will stop working. For safety reasons, the default for most problems in any of these systems is for the unit to shut down.

All of this said, believe it or not, I do like these systems and would recommend one to anyone who could not get a solar system for some reason. I just don’t think they are worth the added headaches as a back-up for a solar water heating system.

Commercial Solar Energy Residential Solar Panels
Written by Roger Perry

Would that be One Tank or Two, madam?

Solar veteran Roger Perry discusses the pros and cons of One Tank Vs Two for Solar Back-up

Commercial Solar Energy ,Residential Solar PanelsAll solar water heaters in Maryland, D.C. and the mid-atlantic provide a family of four with around 75% of their annual hot water load. Most customers use their existing electric or gas sources for the remaining 25% backup (usually needed dead winter). When installing a solar system one question that will come up is “Should I use my existing tank and have it fed by the solar system?” or “Should I remove my existing tank and use the electric back-up that comes in the solar tank?”. A couple of situations make this an easy decision;

  1. An existing electric water heater with no room for another tank. This is especially satisfying and cost effective if the existing tank is leaking or on it’s last legs. It’s like getting $1500 off the cost of a solar system because that money would have needed to be spent anyway. In this case you would definitely chose a single tank system.
  2. The other is if you have a gas water heater. While not as cut and dry as the example above, using the electric element would mean using a higher cost fuel for back-up (not so much with propane). Much of this extra cost would be mitigated because the single tank back-up would not run as much because it would be affected by solar input without running a faucet. You would need to run an electric circuit (30 amp, double pole breaker). This may be difficult or very easy depending on the breaker box location and available space in it. Most jurisdictions will also require a master electrician and a permit for this to be done. In this case, I think, most people would shy away from a single tank system unless there was just no room for another tank.

Let’s look at the pros and cons of each system;


  • Smaller footprint
  • Less heat loss
  • Electric element can sense the solar output and not come on from stand-by losses
  • Can go “all solar” just by turning off the element (essentially flipping a switch).


  • Less back-up in cloudy weather (can be compensated with a larger solar tank which will have a larger back-up capacity).
  • Less solar storage when the element is on (can be compensated for with a larger solar tank which will have more solar storage).


  • Usually greater back-up capacity


  • Greater heat loss
  • Larger footprint

The secondary backup tank can’t sense the primary solar tank temperature unless a faucet is turned on, sending the water through the two-tank system.  This causes the second tank to turn on from stand-by losses when the primary solar tank is already plenty hot.

Going all solar requires operating valves as well as turning off back-up.

Personally I’m a fan of single tank systems. For the most part their two main drawbacks can be compensated for by installing a larger tank. An upgrade from a 80 to a 120 gallon solar tank is only a few hundred dollars. For a 50% increase in solar storage it is a small price to pay.