the problem[s] with 1:1 solar charging

this is a draft

1:1 rule of thumb

There is a rule of thumb that says a 1:1 ratio is the minimum amount of solar needed to charge a deep cycle bank.  The long form is 1w:1ah, which means 1 watt of solar panel for every amp-hour of lead chemistry deep cycle battery capacity under good insolation.  So if you had 150w of panel and 100ah of battery it would be 150w:100ah, or 1.5-to-1.  Yes, I know the case on the unit abbreviations is wrong but it’s easier to read.

As with all rules of thumb, it serves as a way to “sanity test” a flooded bank configuration.  If it’s not at least 1:1 then the solar is obviously insufficient as a sole charging source.  Many of use have been uneasy with the rule of thumb, fearing it was hiding problems.  I think I’ve figured out what those problems are.

Note:  although the ratio refers to Ah, for clarity battery capacity will be expressed in Wh below,  using 12.5v as a constant.

hours in the day

Epiphany #1.  there aren’t enough hours of Full Sun Equivalent (FSE)  anywhere in the U.S. at any time to fully charge

  • lead deep-cycle batteries

  • drained to to 50% depth of discharge

  • using 1:1 setup

  • flat-mounted (non-tilted) panels.

To make things simple, we will use a 100w:100Ah lead configuration, assume excellent conditions and MPPT controller.

After considering ~15% charging inefficiencies, it will take 718.75Wh to fully charge the battery the next day.    Under near-perfect conditions (5% dc-dc losses, 10% temperature derating) an mppt controller will extract 85w from the panel.   It will take 8.46 hours of FSE (718.76Wh/85W) to replace those amps.  There is nowhere in the US with that kind of sun, not even Las Vegas in the summer (7.69hrs), which is an extreme outlier.

Note that FSE is a measurement of solar harvest, not literal charging time.

So what do we do?


The most obvious solution would be to add additional panel to make the 718.75W in one’s location, and we will discuss that in a bit.

But what if we are stubborn and want to make 1:1 work?

cycle less deeply

Cycling to only 75% DoD halves the amount of Amps that have to be replaced, which halves the hours of FSE required.   Are there places in the U.S. with 4.225 hours of FSE?  Sure, most places during the summer, and a bunch of places average that across the year (Little Rock, San Francisco, Phoenix, Birmingham, Atlanta, New Orleans, Albuquerque, Tulsa, Las Vegas, San Antonio, Houston, Salt Lake City, and Casper Wyoming).

tilt the panels

FSE is expressed in terms of a flat area.  Carefully and consistently aligning the panels throughout the day yields ~30% more power from the panels in locations/times when the sun is hitting the panel at an oblique angle.

Example:  tilt can bring FSE in the Seattle winter up from 0.9hrs of FSE to 1.17, a meaningful change.  It can’t bring Vegas in summer from 7.69 to 9.99hrs.

add in another form of charging

Alternator is the most likely source for off-grid folks in vans.  Generator is likely for off-grid cabin folk, and shore charging is possible for folks who weekend and can boost the bank just before leaving the house.

Alternator charging will be difficult to quantify without a coulomb counting monitor.  We do know it can provide massive amounts of current for the dollar.

There are several factors at play:

  • battery chemistry / internal resistance - lithium is capable of accepting power >=  1C, although C/2 or lower is best for longevity and balance.  AGM accepts up to ~C/3.  Flooded p to C/5.

  • different alternator charging methods will charge at different voltages.  Isolators will charge at alternator voltage (13.8v for the purposes of this discussion).  DC-DC chargers will provide Absorption voltage (Vabs, 14.4v for the purposes of this discussion)

  • Isolators pass current in huge amounts, often maxxing out the ability of the battery bank to accept it.  DC-DC charger output is dictated by the electrical components inside.  Common outputs are 20A-50A with the most popular bing 20A.   Note that isolator and DC-DC charging can be paralleled, as CTEK does with the SmartPass product.

  • charging approaches vary;  a commuter might be driving for 30 minutes.  A road-tripper might drive for 3 hours.  A stationary camper might idle for 5 minutes in the morning.

  • in a combined solar-alternator charing system, we are not concerned with fully charging the bank from the alternator – we only want to get a solid contribution from the alternator so  our solar can finish the job.

Luckily for us, in Bulk charge lead batts charge at almost 100% efficiency.   Alternator charging the battery for 30mins in the morning (driving or idling) will put about 162.5w into AGM or **106.25w into flooded **batts due to their different resistances.

  • AGM now requires  6.54 hours of FSE instead of the unassisted 8.46 hours.  (718.75w - 162.5w)/85w)

  • Flooded now requires 7.20 hours of FSE instead of the unassisted 8.46 hours.  (718.75w - 106.25w)/85w)

2:1 overpaneling

Doubling our panels to 200w:100Ah drops hours of FSE required to 4.23, the same as halving our cycling to 75.

Let’s do the alternator charging math again.  The alternator puts the same power out.

  • AGM now requires  a reasonable 3.27 hours of FSE instead of the unassisted 4.23 hours.  (718.75w - 162.5w)/170w)

  • Flooded now requires 3.4 hours of FSE instead of the unassisted 4.23 hours.  (718.75w - 106.25w)/170w).  This might be counterintuitive, but flooded accepts less current from the alternator so solar has to carry more of the load.

3:1 serious overpaneling

Tripling our panels to 300w:100Ah drops to 2.82 hours of FSE required, remembering that actual charging time will be more than that.

  • AGM now requires  2.18 hours of FSE instead of the unassisted 2.82 hours.  (718.75w - 162.5w)/255w)

  • Flooded now requires 2.4 hours of FSE instead of the unassisted 2.82 hours.  (718.75w - 106.25w)/255w)

Battery health

Epiphany #2: 1:1 charging is insufficient to meet minimum charge rates for lead batteries.

Flooded batteries require a minimum of C/10 bulk charge rate to stay healthy.  To do this with an mppt charger requires a 1.25-to-1 ratio. PWM requires 1.5-to-1.

AGM requires a minimum of C/5, twice what flooded needs.  To do this with an mppt charger requires a 2.5-to-1 ratio. PWM requires 3-to-1.


My own system is 570w:220Ah, or 2.59-to-1 charging a bank of flooded golf cart batts.

Since I camp in two-week outings, the alternator is rarely spinning.  Solar-only charging does a great job charging.

I typically discharge to 12.3v-12.4 by morning (60-70% state of charge).  I hit 50% state of charge if I run an electric blanket all night or similar, or if I play Minecraft too late.  :-)

  • On normal days absorption is complete (endAmps == C/133) by 2pm .  Sometimes earlier depending on sunlight.  This is bulk + ~4.5 hours of absorption.

  • In challenging insolation like today (light rain, full overcast), it will be late afternoon before it’s complete.

  • under terrible insolation conditions (snow, thunderstorms) it may only reach float voltage without alternator assistance


I think the 1:1 minimum stands as a sanity check for flooded batteries charged under excellent insolation/FSE, or batteries partially cycled.

2:1 would be a safer default for those running normally cycled (50% DoD) flooded, or for running AGM with alternator assistance.

3:1 would be a good default for those running AGM without alternator charging.

In tabular and more granular form:

W:Ah ratio flooded / MPPT flooded / PWM AGM / MPPT AGM/PWM
good insolation (Phoenix, AZ) 1.25 1.5 2.5 3
average insolation (Salt Lake City, UT) 2.5 3 5 6
poor insolation (Seattle, WA) 3.75 4.5 7.5 9