We get asked questions like this all the time and I guess most would say yes that will be fine, some might say it depends on your load requirements, few would say no. The answer lies within the maths and calculations of your power consumption which is subtracted from your power generation. To work this out you take the power of each device and how long it's used for to get a daily demand. Now there are also peak loads and hours of minimal usage but for now, let's just work with a total figure for the day then divide that by 24 hours. 

We recently investigated a system where the customer had spent over $6500 on multiple replacement batteries & upgrades to the charging and solar system on top of the base installation the motorhome came with to resolve their shortfall. Only a few years ago a 150W panel and a 220Ah or 260Ah battery were all your motorhome came with which would be considered only a mid-sized occasional off-grid solution now. Many still don't have anything more, especially those vehicles out of Europe built for the rental market unless upgraded locally.

Their new setup included 540W (3 x 180W) Solar panels and 2 x 270Ah AGM batteries, DC charging from the alternator but it turns out they don't actually travel that much. The panels are connected in parallel into a 20A solar controller and they have 50A of AC Battery Chargers they run off a generator if the batteries get low. As it turns out that means they run the generator every second day to ensure they keep the batteries above 12.5 volts so they will get 5 years cycle from them.

With a moderate 5A discharge per hour, 24 hours a day the performance figures of their new system look like this:

We didn't want to alter the chart in any way to correct the errors we can see but we will explain something to clarify. The last column is headed LOLP meaning Likelihood Of Lost Power. in the months of May through to July there is Zero % of the system working and Battery State of Charge will be 100% discharged. But you will notice in Jan - Mar then Oct - Dec there is less than 1% chance of the system failing. April it's 50/50 and in August an 80% chance based on the last few years sun hours in the Coromandel when the panels are mounted flat on the roof of your motorhome. The reason the chart is displaying errors is nobody with this level of information would actually design-build, or implement a system that is shown to fail. The system would require over 1kW of solar to be able to maintain a 5A continuous load (120Ah but 133.3Ah when losses are included daily). You can see days carry over for the batteries is only 3 days without sun and that in summer months in the southern hemisphere you'd have plenty of excess power.

That might lead some people to figure out that 540W of solar far exceeds the 20A controller included. But over the summer months, the battery will fully recharge from the max 50A discharge from overnight in the first 3 hours of sun in the morning so will be in float mode only supplying discharge load at midday in the full summer sun. With the large array size, you have the benefit of collecting more of the low-level power for longer over the winter months. It's just not enough to meet demand.

We have been designing Marine & Motorhome charging systems for over 12 years and have seen many changes in the industry and new inventive solutions are developed to address known shortcomings of traditional solutions.

The biggest being the inclusion of DC charging from the vehicles alternators, programable regulators, MPPT as a standard for solar and arrays getting larger to meet today's power-hungry demands.

One of the most common questions is how much power do I need? Then how much solar do I need to support that amount of power? Given every person's needs are different maybe it's best to just give two examples of what's common for a Marine or Motorhome installation and you guys find out where you fit on the consumption scale.

In this first example, we investigate how much solar is required when we have a very modest 30W (2.34A) current draw 24 hours a day like what a small fridge would consume with 300W of solar mounted flat on your roof or deck with a latitude across the country in Christchurch which has average sun hours for the country.

You can see that in February with an array/load ratio (which means the amount of excess power available) you are going to start to have issues if the weather isn't sunny every day. Depending on the size of your battery bank it will change how many days it is until your bank is 100% discharged. But just important is it will take 5 days of full sun to recharge a completely flat battery bank is sized as we had with a small 100Ah battery. By March you will remain in deficit till September and your batteries will be severely damaged and not able to function correctly throughout summer again if it was not for the arrays power running devices during the day.

In the second example we increase the power drain to 40W per hour continuously (24 hours per day) which might reflect a fridge freezer, or LED lighting, USB chargers, TV or radio, recharging a notebook, occasional use of a water pump. We used a 200Ah battery bank and kept the same 300W solar array using the same MPPT solar controller.

You can see above power might start to become something to savour throughout January but you won't see enough again till October and that month will really be all about trying to maintain a full charge even if you did charge on a generator or mains power on the 1st Oct. Your battery would have had an average State of Charge of just 76% and would most definitely be sulphated. These deficit charging won't fully be noticed till February in the second year when things all start going wrong and you start complaining about how poor the batteries performance has been. Of course, the opposite is true, the solar arrays ability to collect the required amount of power required has been lacking, resulting in battery failure.


Batteries are built with a specific purpose in mind. This encompasses the materials used in the manufacture of the battery, the glues/adhesives used. The types of plastics used in the case and many other physical attributes. But there are things that can't be seen or measured easily like, the thickness of plates, Specific Gravity (SG) of the acid which has a huge impact on the battery's ability to give a high CCA when new, but if a high SG also limits the batteries longevity.

The next is true of all batteries and brands but we'll generalise one brands product offerings to give an overview of how their models within the range differ. And we'll be even more specific and limit this article to just AGM batteries within a single brand to give a better overview of the variations.

The first and cheapest entry-level AGM would be a 3-5 year design life battery (which means typically a 1 year warranty) it also means traditionally a battery manufactured with cheap labour and high volumes of turn over. Design life does not mean it will last 3-5 years either, the product will typically last 2-3 years in its desired application. Please note if you use it in the wrong application it might only last 6 months to a year. These batteries are also always referred to as float life batteries as they are meant to be connected to a charger or power supply full time like used in an alarm or UPS system.

Next would be a float life battery with a 10-year design (meaning it might last up to approx 8 years if conditions are suitable). These are occasionally used as an entry point deep cycle battery as they will give approx 250 cycles to 80% which would be similar to use of an occasional user in a cyclic application like a weekend motorhome or boat enthusiast. They might advertise 400-500 cycles at 50% but this can rarely be proven.

Then you'd enter step up to the next design of thicker plated batteries with a lower SG level to give the batteries a better reserve performance over a longer duration. This is really where true deep cycle batteries start. They might have an advertised performance of 800-1200 cycles at 50% depth of discharge. They are also flat plate construction and in a general statement we'd say are the high end of Chinese production. These batteries will however all look the same as the above 5 and 10 year design float life batteries so you really need to know what to look for and test yourself to prove the claims of capacity and longevity.

If you were to try comparing one brand with another you must first match the above 3 characteristics before reviewing and matching apples with apples the following technical specs: Amp hour, C rate (number of hours discharged to calculate the Amp hour rate), end voltage of discharge rate, temperature of discharge rate, start temp of discharge rate test, weight of the battery. Again, like for like as in AGM with AGM, not AGM with Lithium or AGM with Lead Carbon). Cycle life vs. depth of discharge. Once you have worked your way through that nightmare, brand, warranty or number of pages and exclusions, after sale service, advice given up front vs. retrospective, cost, location, distribution, ability to diagnose or test if there is ever an issue. All these need to be reflected to get a sense of value or worth in a battery.

That typically brings us to the end of the upgrade path by a single brand as the technology doesn't change for a 6 volt or 2 volt cell if its AGM. There are other chemistries available from the same said manufacturer like OPzV which are typically a tubular gel and in many cases far superior to the 12 volt AGM deep cycle batteries with cycle life charts normally starting at 1000+ for 80% Depth of Discharge. Do not even confuse these with traction batteries, they are used very differently from a reserve capacity battery.

As far as OPzV or traction batteries are concerned do not think they are better for you though, it's all about the balance of charge time, current and discharge rate. Mostly there is a cost difference and which point you might consider Lithium. This is where expert advice is required to match your use with the intended purpose of the battery and your available charging systems.

Returning to traditional 12 volt batteries or the slightly higher capacity 6 volt cells we then move to high-end global brands where the reputation of the brand is world-renowned. Like Odyssey, SunXtender, Lifeline, Optima, Trojan, Sonnenschein. These batteries typically use industry-standard systems for measuring performance, life cycle and are well proven and offered by distributors like ourselves because of the brand's reputation and the requirement to service and support with adequate ability.